COMPOSITION OF VAPORS FROM BOILING NITRIC ACID SOLUTIONS

B A T T E L L E M E M O R I A L I N S T I T U T E

DISCLAIMER

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Report No. BMI-978 Chemis t ry -Genera l

(M-3679, 15th E d . )

M Contract No. W-7405-eng-92

COMPOSITION OF VAPORS FROM BOILING NITRIC ACID SOLUTIONS

by

R. C. Crooks R. Q. Wilson A. E . Bear se R. B. F i lber t , J r .

F e b r u a r y 9, 1955

BATTELLE MEMORIAL INSTITUTE 505 King Avenue

Columbus 1, Ohio

K TABLE OF CONTENTS

Page

ABSTRACT 1

INTRODUCTION 1

PART I. COMPOSITION OF VAPORS FROM AQUEOUS SOLUTIONS OF NITRIC ACID BOILING AT 200 MM MERCURY PRESSURE 2

Exper imenta l Work 2 Resul ts and Discussion 4 Conclusions 15

PART II. VAPORIZATION OF CHLORIDE FROM AQUEOUS NITRIC ACID SOLUTIONS 18

Exper imenta l Work 18 Resul ts and Discussion 20 Conclusions . . . . . . . . . . . . . . . . . 26

PART III. PHOTOMETRIC DETERMINATION OF CHLORIDE IN AQUEOUS NITRIC ACID SOLUTIONS 27

Apparatus and Reagents 27 Determinat ion of Chloride 28 Charac te r i s t i c s of Silver Chloride Suspensions 30 Conclusions 33

SUMMARY 36

REFERENCES 36

- / -

COMPOSITION OF VAPORS FROM BOILING NITRIC ACID SOLUTIONS

R. C. Crooks, R, Q. Wilson, A. E , Bea r se , and R. B. F i lber t , J r .

The composition of vapors from aqueous nitric acid solutions boiling at 200 mm mercury total pressure is established for solutions containing between 0 and 67,5 tv/o nitric acid. The volatility character­istics of low concentrations of chloride in the same concentration range of nitric acid have been measured in solutions boiling at 200 mm mercury. The effects of chloride concentration and pressure of boiling are evalu­ated. A spectrophotometric method for the determination of chloride in nitric acid solutions is described.

. - ]

INTRODUCTION

The cur ren t technology of ma te r i a l s process ing has resul ted in in­c reased use of ni t r ic acid as a solubilizing agent in the process ing of nuc l ea r -fuel m a t e r i a l s . Recovery of ni t r ic acid for recycl ing to m a t e r i a l s - p r o c e s s i n g operat ions has become increasingly important . The presen t investigation was undertaken as a par t of the p r o g r a m of plant a ss i s t ance to the Feed Mater ia ls Product ion Center operated by the National Lead Company of Ohio.

The technology of ni t r ic acid process ing is somewhat de termined by the cor ros ion res i s t ance of available m a t e r i a l s of construct ion. Although the recovered ni t r ic acid is usually concentrated by rectif ication at a t m o s ­pheric p r e s s u r e , advantages of lower cor ros ion r a t e s in equipment a re found in the lower t empe ra tu r e s of dist i l lat ion at subatmospheric p r e s s u r e s . Therefore , the objective of this work was to examine the existing values '^) of the vapor composition in boiling ni t r ic acid solutions and to extend the exper imenta l data to lower concentrat ions of ni t r ic acid for solutions boiling at 200 mm of m e r c u r y total p r e s s u r e .

In the concentrat ion of n i t r ic acid solutions which contain some chlor ide, the chloride tends to accumulate in the fractionation column and somet imes p resen t s a severe .corrosion problem. Although severa l nickel-containing alloys a re prac t ica l ly inert to at tack by ni t r ic acid solutions, they a re often rapidly corroded by ni t r ic acid solutions in which chloride is p resen t . Chlo­ride ma te r i a l s tend to accumulate in fractionating equipment because hydrogen chloride is rapidly str ipped from solutions which a r e high in ni t r ic acid con­centrat ion, and is quickly absorbed in the reflvix liquid which is low in acid concentrat ion.

* References at end of this report.

2

Haggerty and Hixon(2) have measu red the volatility cha rac t e r i s t i c s of chloride in ni t r ic acid solutions boiling at a tmospher ic p r e s s u r e . This r e ­port p resen t s work on the determinat ion of chlor ide-dis t r ibut ion ra t ios in ni t r ic acid solutions boiling at 200 mnn m e r c u r y total p r e s s u r e .

In the course of this work, a spectrophotometr ic procedure was devel ­oped for the determinat ion of smal l concentrat ions of chloride in ni t r ic acid solutions.

PART I. COMPOSITION OF VAPORS FROM AQUEOUS SOLUTIONS OF NITRIC ACID BOILING

AT 200 MM MERCURY PRESSURE

The l iquid-vapor equi l ibr ium composit ions of aqueous n i t r ic acid solu­tions were measu red in solutions boiling at 200 m m m e r c u r y total p r e s s u r e . The composit ions and boiling t e m p e r a t u r e s were determined in solutions which contained between 0. 0165 and 0. 385 mole fract ion HNO3. Published da ta ( ' ) on par t i a l p r e s s u r e s of n i t r ic acid over aqueous solutions show values for ni t r ic acid concentrat ions for 20 w/o (0. 0665 mole fraction) n i t r ic acid and above. Since dilute ni t r ic acid solutions do not follow Henry ' s law, the l iquid-vapor equi l ibr ium re la t ionship for the more dilute n i t r ic acid solutions was uncer ta in . Therefore , the a ims of this work were (1) to m e a s u r e the l iquid-vapor equi l ibr ium composit ions for dilute n i t r ic acid solutions, and (2) to tes t the exper imenta l data for thermodynamic consis tency.

The equi l ibr ium-composi t ion values obtained in this work a re in a g r e e ­ment with those of Taylor ( l ) , but slightly lower boiling t e m p e r a t u r e s were observed. Both this work and Tay lo r ' s values were checked by the rmody­namic consistency by graphical integrat ion of the Gibbs-Duhem equation. . The equil ibrium re la t ionships for the ni t r ic ac id-water sys tem a re readi ly extrapolated to zero acid concentrat ion when the data a r e plotted as ra t ios of the vapor and liquid composi t ions . The extrapolat ion provides the informa­tion n e c e s s a r y for the construct ion of l iquid-vapor equi l ibr ium d iag rams in the di lute-acid region as used in the engineering design of n i t r ic acid d i s t i l l a ­tion columns.

Exper imenta l Work

Apparatus

The composit ions of liquid and vapor f rom ni t r ic acid solutions boiling at 200 m m m e r c u r y total p r e s s u r e were obtained by use of an Othmer- type st i l lw) of improved design. Vacuum was provided by a water a sp i r a to r ; a Car tes ian manostat controlled the sys tem p r e s s u r e to within about 0. 5 nnm

3

of the 200 m e r c u r y operating p r e s s u r e . A tower filled with soda l ime and a 1. 5-l i ter flask were connected between the manosta t and the equi l ibr ium stil l to absorb ni t r ic acid vapors and reduce the p r e s s u r e fluctuations in the sti l l sys tem. A capi l lary tube drawn to a very fine d iameter at the tip provided a minute s t r e a m of a i r to promote smooth boiling action at the 200 m m m e r -cury p r e s s u r e . Without such a boiling aid, severe bumping was tinavoidable. A res i s t ance wire of 25 ohms, wrapped about an external reboi ler U-tube at the bottom of the sti l l , supplied the heat for boiling. The upper surfaces of the sti l l unit were insulated by asbes tos and a cloth wrapping.

A the rmomete r with i ts bulb located in the vapor tube of the st i l l m e a s ­ured the vapor tennperature . The the rmomete r was graduated in 0. 2 C divi ­sions, was cal ibrated against another t he rmomete r which had been cal ibra ted by the National Bureau of Standards, and was checked in the st i l l against the boiling t empera tu re of dist i l led water at var ious p r e s s u r e s . Correc t ions for the exposed the rmomete r s tem were applied to al l r ead ings . A U-tube mercury- f i l l ed absolute manometer was used to m e a s u r e the absolute p r e s ­sure in the equi l ibr ium st i l l .

Analytical Method

Samples of the liquid and condensed vapor were analyzed for total acidity by t i t ra t ion with carbonate-f ree 0, 1487 N NaOH to a phenolphthalein end point. The sodiunn hydroxide solution was s tandardized against po t a s ­sium acid phthalate .

P rocedure

Approximately 250 ml of a solution of ni t r ic acid in water was charged to the sti l l and the sys tem evacuated to an absolute p r e s s u r e of 200 mnn m e r c u r y . With the des i red p r e s s u r e in the sys tem, the by-pass stopcock on the manostat p r e s s u r e control ler was closed to maintain the p r e s s u r e at that value. Power was then applied to the heater winding of the sti l l to heat the solution to boiling, and the boiling action was continued for 1.5 hr to bring the composit ions of the sti l l liquid and vapor condensate to s teady-state va lues . Steady t empera tu re readings were obtained within 0.75 hr after the boiling s ta r ted . During the boiling period, approximately 5 volumes of condensate flowed through the condensate r ece ive r , thereby assur ing s teady-s ta te composit ions in both the rece ive r and the s t i l l .

Samples were taken from the draw-off connections on the st i l l and the condensate r ece ive r d i rec t ly into 60-ml g lass - s toppered dropper bo t t l es . Check exper iments showed that no significant change in composit ion by s e l e c ­tive vaporization occur red with this method of sampling.

The heating ra te of 160 w was selected as providing the best ra te of boiling for this par t icu lar equil ibrium st i l l . Lower heating r a t e s resu l ted

4

in some fractionation of the vapor, and higher heating r a t e s led to en t ra in -ment of pa r t i c les of liquid in the vapor . The effect of heating ra te on the condensation t empera tu re of the vapor while boiling a 20 w/o ni t r ic acid solution is shown in F igure 1. The t empera tu re curve shows that the f r a c ­tionation effect becomes negligible at heating r a t e s over 140 w.

System p r e s s u r e s were recorded to within 0. 1 m m m e r c u r y . T e m ­pera tu re readings after s tem cor rec t ion were further adjusted to a p r e s s u r e of 200. 0 m m m e r c u r y .

Resul t s and Discussion

Liquid-Vapor Equi l ibr ium Data

The observed values for the l iquid-vapor equil ibrium composit ions of n i t r ic acid solutions boiling at 200 m m m e r c u r y p r e s s u r e a r e shown in Table 1 and on the t empera tu re -compos i t ion graph for the HNO3-H2O sys tem in F igure 2. In the t empera tu re -compos i t ion d iagram of F igure 2, the conn-posit ions of liquid and vapor obtained in this work agree well with those of Taylor, but the observed boiling t e m p e r a t u r e s a r e somewhat lower . The activity coefficients of n i t r ic acid vary over an exceedingly wide range, fronn a nninimunn of 0. 00272 in a dilute solution to a nnaximum of 0. 287 near the azeot rope .

The observed activi ty coefficients of n i t r ic acid and water a r e plotted in Figure 3, as

y i ^ ^ y2 p •y = —= , and 7 = — 1 X p« 2 X P °

1 1 2 2

>

where

-y = activity coefficient x = mole fract ion in liquid y = mole fraction in vapor P = total p r e s s u r e P " = vapor p r e s s u r e of pure component

Subscript 1 = H2O Subscript 2 = HNO3.

The par t ia l p r e s s u r e s for 100 per cent HNO3 from Taylor were plotted against t empera tu re to obtain values for n i t r ic acid vapor p r e s s u r e , P2° •

Smoothed values a r e shown in Table 2 for the boiling t e m p e r a t u r e s , the liquid and vapor composit ions, the activity coefficients, and the ni t r ic acid distr ibution r a t io . The ni t r ic acid distr ibution ra t io , K = y /x , is plotted

5

120 Heat Input,w

240

FIGURE EFFECT OF HEATING RATE ON VAPOR TEMPERATURE IN THE RE­CIRCULATING STILL

A- I 3 6 88

4 TABLE 1. EXPERIMENTAL RESULTS FOR LIQUID-VAPOR EQUILIBRIA BETWEEN

NITRIC ACID AND WATER AT 200 MM MERCURY PRESSURE

Concentrat ion of HNO3 Tennperature, x^. Mole Frac t ion y-y^ Mole F rac t i on

C in Liquid in Vapor

66.46 67.13 67.96 68.76 68.84 69.77 70.98 72.69 74.91 76.62 77.88 80.88 82.95 84.73 86.18 36.38

0 0.0166 0.0315 0.0466 0.0488 0.0631 0.0820 0.1075 0.1374 0.1606 0.1830 0.2212 0.2554 0.2979 0. 3440 0.3850

0 0.000095 0.000228 0.000699 0.000940 0.001061 0.002180 0.00540 0.01167 0.0201 0.0335 0.0644 0.1082 0.1831 0.2928 0.4122

1.000 0.986 0.966 0.947 0.941 0.922 0.892 0.850 0.796 0.755 0.726 0.653 0.599 0.542 0.475 0.418

—

0.00272 0.00338 0.00682 0.00875 0.00737 0.01119 0.0203 0.0316 0.0440 0.0621 0.0909 0.1248 0.1721 0.228 0.287

Activity Coefficient

for for HoO HNO3

0.20 HNO , mole fraction

030 0.40

FIGURE 2. TEMPERATURE-COMPOSITION DIAGRAM FOR THE SYSTEM HNO-H 0 AT 200 MM MERCURY ' A - l 3 6 8 9

8

\J0

a. a. x"

II

c 0)

0) o o

o 0.010

p-̂ JC too4v.

/

«

41'

^ r

\^

/ f

P<x

4 /

^

/

/ ^ f

K_ rS:

/

H,^ 2

" - 0

, /=«

HNC

\ _

/ ^

'3

0.001 0.0 0.1 0.2 0.3 0.4

Mole Fraction HNOg in L iqu id , x g

FIGURE 3. ACTIVITY COEFFICIENTS OF HNO3 AND HgO IN SOLUTIONS BOILING AT 200MM MERCURY

A - I 3 6 9 0

1 T A B L E 2 . S M O O T H E D VALUES F O R LIQUID-VAPOR EQUILIBRIA B E T W E E N

NITRIC ACID AND WATER AT 200 MM MERCURY PRESSURE

M o l a r Ac t iv i ty Coeff icient D i s t r i b u t i o n

C o n c e n t r a t i o n of HNO3 7 j , 7 2 , R a t i o , K, for T e m p e r a t u r e , X T . Mole F r a c t i o n y^^ Mole F r a c t i o n for for HNO^,

C in L iqu id in Vapor H2O HNO3 (K = y2 /X2)

66.46 67.4 68.4 69.6 70.8 72.2 73.6 75.1 76.6 78.0 79.5 82.6 85.0 85.6 86.15 86.35 86.45 86.5 86.4 86.1

0.000 0.020 0.040 0.060 0.080 0.100 0.120 0.140 0.160 0.180 0.200 0.250 0.300 0.320 0.340 0.350 0.360 0.3726 0.380 0.400

0.00000 0.000128 0.000433 0.00105 0.00221 0.00421 0.00752 0.01273 0.02023 0.03098 0.04529 0.0998 0.1881 0.2337 0.2841 0.3103 0.3393 0.3758 0.3982 0.4595

1.000 0.981 0.956 0.927 0.894 0.863 0.829 0.795 0.761 0.727 0.693 0.611 0.534 0.504 0.475 0.461 0.448 0.431 0.420 0.394

(0,00182) 0.00306 0.00497 0.00776 0.01169 0.01708 0.02428 0.03351 0.04511 0.0588 0.0743 0.1191 0.1737 0.1981 0.2238 0.2377 0.2506 0.2678 0.2783 0.3069

(0.00370) 0.00650 0.01075 0.0175 0.0276 0.0421 0.0626 0.0910 0.1268 0.1720 0.226 0.399 0.627 0.730 0.835 0.887 0.944 1.009 1.050 1.149

10

in Figure 4 as a function of n i t r ic acid concentrat ion. The smooth curve for the distr ibution ra t io is readi ly extrapolated to ze ro n i t r ic acid concentrat ion and should be par t i cu la r ly useful in dist i l lat ion calculations on dilute solu­tions of ni t r ic acid.

F igure 5 shows the l iquid-vapor equi l ibr ium curve for solutions of ni t r ic acid in water at 200 m m m e r c u r y . The curvature of the equi l ibr ium curve in dilute ni t r ic acid solutions is i l lus t ra ted in F igure 6, which was computed f rom the extrapolated values for the K distr ibution r a t io .

The boiling t e m p e r a t u r e s were read f rom the t empera tu re -compos i t ion plot. Smoothed values for the activity coefficients were taken from the activity function curves shown in F igure 7. Since the activity of n i t r ic acid va r i e s over a la rge range , the data were plotted in the fo rm of

log 7 , iSi * ^ ,

( l - x j )

as suggested by Darken and Gurry , ('*) F r o m the smoothed values of j3 were calculated the corresponding smoothed values for the activity coefficients and the vapor composit ions for the chosen liquid composi t ions .

Evaluation of Data

Both the equi l ibr ium data of this work and those of Taylor were found to be thermodynamical ly consistent when checked by the Gibbs-Duhem equa­tion. The usual form of the equation.

= x i

In -Y, = - / —^— d In 7 i = - / -—- ^ in 72,

desc r ibes the manner in which the activity coefficients of the water (sub­scr ipt 1) and n i t r ic acid (subscr ipt 2) a r e re la ted under equi l ibr ium condi­t ions . For example, the Gibbs-Duhem rela t ion r equ i r e s , for equi l ibr ium conditions, that each inc rease in the activity coefficient of n i t r ic acid r e s u l t ­ing f rom a change in the acid concentrat ion be accompanied by a co r re spond­ing dec rease in the activi ty coefficient of the wa te r . Though this equation is s t r ic t ly t rue only for conditions of constant p r e s s u r e and t e m p e r a t u r e , the unavoidable e r r o r s a r e not grea t over smal l changes of p r e s s u r e or t e m p e r a ­t u r e . In a b inary sys tem, such as ni t r ic acid and water , boiling under con­stant p r e s s u r e , the changes in composit ion cause changes in the boiling t e m p e r a t u r e .

The thermodynamical ly consistent curve for the activi ty coefficient of water a s computed f rom smoothed values of the n i t r ic acid activi ty

II

10.0

1.0

^ | x ~ II

c o

CO

O

OJ o

n O Z X

0.1

001

0.001

1

c o

yo

/

A

/ < ^

,y ; ^

(

A^ X ^

urve from

D Experimental v

A Calculated fron

f^

r ^

smoothe

alues ~1

1 values c

jy V^ a

d values

if Taylor

^^•"^

0.05 0.10 0.15 0.20 0.25 0.30 Mole Fraction HNO, In Liquid,x.

035 0.40

FIGURE 4. DISTRIBUTION RATIO OF HNO3 IN BOILING AQUEOUS NITRIC ACID SOLUTIONS AT 200 MM MERCURY

A- I 3 691

12

1.0

Ol

CM

>«

o o. o >

O Z X c o u o

o

aoi

0.001

0.000!

0 0 0 0 0 !

1

/

/ Y/ L _ ^

\

/ /

/

F Curve

_y V /^

from sm

X ^

/ ^

oothed v(

k̂

nlues

^ ^ r^

ao5 0.10 0.35 0.40 0.15 0 2 0 025 0.30 Mole Fraction HNO^in Liquid, Xg

FIGURE 5. VAPOR COMPOSITIONS OVER BOILING AQUEOUS NITRIC ACID SOLUTIONS AT 2 0 0 MM MERCURY

A - 1 3 6 9 2

13

0.0005

0.0000 0.01 0.02 0.03

Mole Fraction HNO3 in Liquid,Xg 0X)4

FIGURE 6. EQUILIBRIUM DIAGRAM FOR DILUTE SOLUTIONS OF NITRIC ACID IN WATER AT 200 MM MERCURY

A - 1 3 6 9 3

14

-2.8

-08

I

II

car

X

c o

3 UL

<

- 0 4 -

-02

-QC-

± ± _L 0 OIO 0 2 0 0 3 0

Mole Fraction HNOjin Liquid, Xg

FIGURE 7 )S ACTIVITY FUNCTIONS FOR HgO AND HNO3

A - 1 3 6 9 4

0.40

-2.4

o

-2.0 g«

I

o •*-c o

OJ X

.M

-1.6 u c U.

• • -u <

-1.2

-as

15

coefficient is shown in F igure 8, Fo r these calculat ions, the activity coeffi­cients were obtained by graphical ly integrating the Gibbs-Duhem equation, s tar t ing at the azeotropic composition and integrating toward the compos i ­tion pure H2O, Similar computations were made to tes t the thermodynamic consistency of the ni t r ic ac id-water values compiled by Taylor, and the r e ­sults a re shown in F igure 9. Both the data of this work and the values of Taylor conapare favorably with the computed thermodynamical ly consistent curves for water ac t iv i t ies . The maximum deviation in the computed water activity for the data of this work is 5, 8 per cent, occurr ing at 8 mole per cent ni t r ic acid. The s imi lar maximum deviation for the Taylor data is 4, 3 per cent, occurr ing at about 19 mole per cent n i t r ic acid. In both ca se s , the graphical integrations f rom the ni t r ic ac id-water azeotrope composition give calculated activity coefficients for pure water which a r e within 2 per cent of the t rue activity coefficient of 1 for pure wa te r .

To facili tate the integrat ion of the Gibbs-Duhem equation, the method descr ibed by Darken and Gurry''*) was used. In this method, the activity coefficient -y is incorporated in a new function a, defined as

In a.-> =

\

Upon substituting ao in the Gibbs-Duhem equation, the activity coefficient 7 j of water was then expressed in the equation

/̂ ^l"^ r ] f^r' ^°^\—J =-[''2 ^1 ^2 -°^i ^1 ^2 J - J x i ' °^2^^1-

In this work, the " p r i m e " quantities refer to the azeotrope composition, and the "double -pr ime" quantit ies re fer to n i t r ic acid concentrat ions between the azeotrope and pure water .

Conclusions

The l iquid-vapor equil ibrium compositions of ni t r ic acid solutions boiling at 200-mm m e r c u r y p r e s s u r e have been es tabl ished for al l concentra­tions of ni t r ic acid between pure water and the azeotrope (37 mole per cent HNO,), This has been done by extrapolation of the exper imenta l ly d e t e r ­mined distr ibution r a t ios , K, for ni t r ic acid to zero acid concentrat ions on a semilogar i thmic char t .

The data of this work and the values repor ted by Taylor a r e both fair ly consistent from a thermodynamic standpoint as shown by compar i son of the data with the values computed f rom the Gibbs-Duhem equat ion.

IL

1.0

w- 0.9 0)

i 0>

o «^ 9i O O >\

>

u <

0.8-

07

06

OS

0.4

^Thermodynamically consistent activity coefficient of water obtained by graphical integration of Gibbs-Duhem equation from composition of azeotrope

Experimental water activity coefficient curve from smoothed data

a. o

0.10 0.20 030 Mole Fraction Nitric Acid in Liquid ,Xg

FIGURE 8. ACTIVITY COEFFICIENT OF HgO IN NITRIC ACID SOLUTIONS AT 200 MM MERCURY A- 1 3 6 9 5

0.40

I i

17

o

c 0)

0) o O >s

• * -

'> • * -

U <

.2 0.7 -

0.!0 0.20 Mole Fraction Nitric Acid in L iqu id, Xg

0.30 0.40

FIGURE 9. ACTIVITY COEFFICIENT OF HgO IN NITRIC ACID SOLUTIONS AT 200MM MERCURY,FR0M DATA OF TAYLOR(')

A - l 3 6 9 6

18

PART II, VAPORIZATION OF CHLORIDE FROM AQUEOUS NITRIC ACID SOLUTIONS

Chlorides in the ni t r ic acid solution feeds to ni t r ic acid dist i l lat ion columns under ce r ta in operating conditions sonnetimes resu l t in ser ious cor ros ion p r o b l e m s . Chlorides tend to accumulate in the column ra the r than to pass through in a s t ra ightforward path.

Operating conditions which favor the build-up of chloride concentrat ion in a column can occur when dilute n i t r ic acid is concentrated toward the higher boiling azeotropic composit ion (67 w/o HNO3) and the lower boiling water is rectif ied f rom the top of the column. Under these c i r cums tances , the chloride is rapidly vaporized f rom the solutions in the lower pa r t of the column, and quickly absorbed by the dilute n i t r ic acid refliix in the upper par t of the column. Chlorides originally p resen t in low concentrat ions in the feed accumulate to much higher concentrat ions within the column. At this point, the par t icu la r cor ros ive p roper t i e s of chlorides become evident.

Aqueous solutions of strong inorganic acids a r e highly nonideal in b e ­havior, and thei r vaporizat ion cha rac t e r i s t i c s a r e difficult to pred ic t . In concentrated ni t r ic acid solutions, chloride is oxidized to free chlor ine , Haggerty and Hixon \^) investigated the volatility of chloride in ni t r ic acid solutions boiling under a tmospher ic p r e s s u r e . In this work, the effects of p r e s s u r e and concentrat ion level of chloride on the vaporizat ion c h a r a c t e r ­i s t ics of chloride in ni t r ic acid solution were studied.

Exper imenta l Work

Apparatus

A rec i rcu la t ion st i l l constructed according to the specifications of Othmerw) was used to obtain equi l ibr ium liquid and vapor samples of n i t r ic acid solutions of chlor ide , A Car tes ian manostat in the vacuvim line was used to control the p r e s s u r e in the sys t em. P a r t I of this r epor t gives additional details on the construct ion and a r rangement of the appara tus .

Analytical Methods

Chlorides were determined from the absorbance of light by the s i lver chloride suspensions formed upon react ion with s i lver n i t ra te , A Beckman Model DU spectrophotometer with 1-cm-wide Corex cel ls was used to m e a s ­ure the light absorbance of the s i lver chloride suspensions at a wavelength of 450 mju. Since the s i lver chloride suspensions were not s table, they were p repa red and examined under controlled condit ions. The suspensions were p repa red in an aqueous media which contained 25 volume per cent

19

ethyl alcohol and ni t r ic acid in the range between 0, 5 and 2, 0 N HNO3, Absorbance readings were taken at 20 min after the s i lver chloride was p r e ­cipitated. Chloride concentrat ions in the suspensions were then obtained from a cal ibrat ion curve .

To p repare a sample for chloride analys is , a sample of known volume and containing between 0, 05 and 1, 5 mg of chloride was pipetted into a 50-ml volumetr ic f lask. Then 2, 5 ml of 13 N HNO3, 12 ml of 200-proof ethyl alcohol, and siofficient dist i l led water to br ing the total volume to about 40 ml were added to the flask and the contents mixed. Following this , 2, 5 ml of 0. 05 N AgN03 was added dropwise to the flask while swirling the contents, then dist i l led water was added to the 50-ml m a r k and the suspen­sion thoroughly mixed, A portion of the suspension was poured di rect ly into the Corex cell , and the covered cell positioned in the spectrophotometer compar tment . Twenty min after the suspension was formed, the light absorbance of the suspension at 450 naju was me a s u re d . Distil led water in a s imi la r Corex cell was used as a reference at 100 per cent t r ansmi s s ion . Additional information on the spect rophotometr ic method for the de t e rm i na ­tion is presented in P a r t III,

The acid contents of the liquid and vapor were determined by t i t ra t ion with 0, 1487 N NaOH to a phenolphthalein end point. Since comparat ively smal l quantities of HCl were p resen t in the samples , the total acidi t ies were recorded as n i t r ic acid concentra t ions .

P rocedure

Approximately 250 ml of a solution of ni t r ic acid containing a measu red quantity of hydrochloric acid was charged to the equi l ibr ium s t i l l . Known volumes of a s tandardized hydrochloric acid solution provided the des i red chloride concentra t ions . After evacuation to a subatmospher ic p r e s s u r e of 200 mm mercu ry , the st i l l was heated at a ra te of 160 w to give a boiling period of about 1 h r . During the boiling period, about th ree r ece ive r vol ­umes of condensate flowed through the condensate r ece ive r to provide s teady-state composit ions in the sti l l pot and the r e c e i v e r . Samples were taken at a tmospher ic p r e s s u r e , f rom draw-off connections on the st i l l and condensate r ece ive r , d i rect ly into 60-ml g lass - s toppered dropper bo t t l es .

• Caution: Concentrated nitric acid may react with ethanol to form ethyl nitrate which is highly explosive.

20

Resul ts and Discuss ion

Data on Vaporization of Chloride

The vaporization cha rac t e r i s t i c s of chloride in aqueous solutions con­taining between 0 and 67, 5 w/o of ni t r ic acid were me a s u re d . In these solu­t ions , chloride was p resen t in concentrat ions between 17 and 2180 ppm as chlor ide . The m e a s u r e d re la t ive volat i l i t ies of chloride var ied widely in magnitude, from very low values in ni t r ic ac id-f ree solutions to exceedingly high values in concentrated ni t r ic acid. F o r example, in a n i t r ic ac id-f ree solution of dilute hydrochloric acid, the measu red molar distr ibution rat io between the vapor and the liquid for chloride was K = 0, 0046, In a solution containing 58 w/o ni t r ic acid, the distr ibution ra t io was K = 91 .9 .

The exper imenta l r e su l t s on the vaporizat ion of chloride f rom ni t r ic acid solutions a r e shown in Table 3, Acidi t ies of the liquid and vapor phases a r e expressed as total acidity in t e r m s of mole fract ion of n i t r ic acid, b e ­cause the hydrochloric acid concentrat ions a r e comparat ively very low. The tendency for the chloride to vaporize is expressed as the molar distr ibution ra t io

^ci-K = ,

X ci-

where

y ^_ = mole fract ion of chloride (Cl~) in the vapor C l

x _ , - = mole fract ion of chloride (CI") in the liquid.

Since the p resence of n i t r ic acid resu l ted in the loss of chloride f rom the dist i l lat ion sys tem, the durat ions of boiling and amounts of chloride r e ­covered a re repor ted in the data. In the solution which contained 68, 5 w/o n i t r ic acid (Run 8), the chloride was rapidly oxidized to free chlorine as the solution was heated to its boiling t e m p e r a t u r e . Over 99 per cent of the chloride was lost from the concentrated ni t r ic acid solution during the 1-hr boiling per iod. The p resence of a smal l quantity of free chlorine was noted in the vapors f rom a solution containing 58, 15 w/o n i t r ic acid (Run 10), but no free chlorine was observed during dist i l lat ion of solutions containing less than 50 w/o ni t r ic acid.

The chloride concentrat ions repor ted in Table 3 r e p r e s e n t the total chlorine content of the liquid and vapor s ample s . Analyses in which sodium ni t r i te was used to reduce free chlorine that may have been p resen t gave the same resu l t s as when the samples were not t r ea ted with sodium n i t r i t e .

al TABLE 3 . VAPORIZATION OF CHLORIDE FROM NITRIC ACID SOLUTIONS

Total Chloride in Distillation

Run

1 2 3 4 5

6 7 8 9

10 11 12

13 14 15

16

Temperature, C

66.2 66.9 68.6 72.3 75.9

79.0 83.6 85.6 72.0

83.1 83.7 76.3

75.9

75.9 69.0 66.4

Pressure, mm mercury

199.0 200.0 198.7 198.5 200.2

197.0 198.0 199.0 197.0 199.0 198.0 202.0

200,0

200.0 200.0 .200.4

Concentration of HNO3

''HNOS Mole Fraction

in Liquid

0.0000 0.0292 0.0588 0.1038 0.1528

0.2020 0.283 0.362 0.1063 0.284 0.285 0.1565

0.1528

0.1510 0.0545 0.0011

yHN03 Mole Fraction

in Vapor

0.0000

0.000273 0.000945 0.00312 0.01860

0.0489 0.1580 0.394 0.00515 0.1670 0.1620 0.0211

0.0173 0.0204

0.00069 0.000005

Concentration of Chloride

' 'C1-. Mole Fraction

in Liquid x 10^

181. 193. 187. 198. 184.

98.5 15.2

2.6(^) 212.

54.3 6.84

1270.

29.4

12.3 1160. 1110.

Vci-. Mole Fraction

in Vapor x 10^

0,83

1.08 5.50

33.2 215. 563. 746.

1.6(a) 32.2

4990. 312.

1052.

40.0 16.6 25.6

5.48

Molar Distribution

Ratio of _, Chloride,

\ C l

0.0046 0.0056 0.0294 0.167 1.17 5.72

49.1

Duration of Boiling, r

2.2 1.3 1.0 1.0 0.8 0.9 1.0

- - (^) 1.0 0.152

91.9 45.6

0.829 1.36

1.35 0.0222 0.0049

0.8 0.5 0.5

0.8

0.9 1.5 0.8 0.8

Apparatus

Charged

Before Boiling,

mg

75.0 78.6 82.3 86.5

91.1 96.0

102.0 105.0

86.4 678.

20.4

607.

6.08 6.0

550. 507,

Found

After Boiling,

mg

75.7 79.5 76.9 81.0

85.1 77.2 56.2

0.96 84.9

347. 23.7

550.

14.0 5.8

477. 462,

Chloride Recovered

Per Cent of Cl- in Charge

101 101

94 94

93 80 55

0.9 98 51

116 (b)

90

230 (b) 97 87 91

(a) Chloride was rapidly oxidized to free chlorine.

(b) High chloride recovery due to presence of some chloride in the still from a previous run.

M

22

Evidently the oxidation-reduction react ion between concentrated ni t r ic acid and the chloride, as

3HC1 + HNO3 = CI2 + NOCl + 2H2O,

may have been r e v e r s e d upon dilution of the samples for ana lys i s . Although there is the possibi l i ty that the ethyl alcohol used to stabilize the si lver chloride suspension may have reduced some free chlorine to the chloride, other exper iments show that such a reduction was not l ikely. Fo r example, chloric acid, HCIO3, tes ted by the same analyt ical procedure showed p r a c ­t ical ly no reduction by the ethyl alcohol to the chloride, but was rapidly r e ­duced by sodium n i t r i t e .

Distr ibution Ratio of Chloride in Boiling Acid Solutions

The volatil i ty of chloride in boiling n i t r ic acid solutions was found to depend p r ima r i l y upon the total acidity. Values for the mola r -d i s t r ibu t ion ra t ios for chlor ides observed in the boiling n i t r ic acid solutions a r e plotted as a function of the ni t r ic acid acidity in F igure 10, Over the range of chlo­ride concentrat ions investigated, the distr ibution ra t io was independent of the chloride concentrat ion. The curve drawn on the char t probably r e p r e ­sents all the data within the range of exper imenta l accuracy . Since some chloride was slowly lost f rom the dist i l lat ion sys tem, most likely through oxidation to free chlorine by the ni t r ic acid, it was not possible to obtain a t rue physical equi l ibr ium in some of the exper imen t s .

The chloride volatili ty is de termined p r i m a r i l y by the acidity of the solution. To a sma l l e r extent the chloride volatil i ty is a l t e red by changes in the chloride concentrat ion and by the p r e s s u r e a t empera tu re at which boiling takes p lace . As seen in F igure 10, the dis tr ibut ion ra t io , K, for chloride va r i e s f rom a value of 0, 002 in very dilute acid solutions to values over 70 in solutions containing over 0, 30 nnole fraction (60 w/o) n i t r ic acid. Therefore a range of over 35, 000 to 1 in the value of K may occur in a d i s ­ti l lation p rocess for concentrat ing ni t r ic acid.

When the boiling p r e s s u r e is inc reased f rom 200 m m m e r c u r y total p r e s s u r e to a tmospher ic p r e s s u r e , the dis tr ibut ion ra t io , K, for chloride in boiling ni t r ic acid solutions i n c r e a s e s by a factor of f rom 2 to 4, The chloride distr ibution ra t ios calculated f rom the data of Haggerty and Hixon a re plotted against n i t r ic acid acidity in F igure 11, The smooth curve drawn through the a t m o s p h e r i c - p r e s s u r e K values l ies about 300 per cent above the 200-mm curve for K,

In ni t r ic acid solutions which contain l e s s than about 2 w/o of chlor ide, the chlor ide-dis t r ibut ion ra t io , K, is essent ia l ly independent of the chloride concentrat ion. F igure 12 shows the K values for chloride in boiling hydro­chloric acid plotted as a function of the HCl acidity, as computed f rom

23

o o > » | K

II

_o x: O •«-o o

o

3 j Q

(A

b

o

0 0 0 ! 0 0.05 010

Acidity, mole fraction HNO3 FIGURE 10. DISTRIBUTION RATIO OF CHLORIDE IN NITRIC ACID SOLUTIONS

BOILING AT 200 MM MERCURY A- 1 3 6 9 7

24

100

u o

II

<U

w O

o <*• o o

Q:

§

v> O

o o

0.001 0.05 0.30 0.10 0.15 0.20 0.25

Acidi ty , mole fraction HNO3

FIGURE II. EFFECT OF PRESSURE ON THE DISTRIBUTION RATIO OF CHLORIDE IN BOILING NITRIC ACID SOLUTIONS

A-l 3 6 9 6

25

too

10

X o X

X

II

CJ X ••-o o

c o 3

• -.2 Q

o o

0.01

0.001

/ 6 / / / / / '

/ / oA 2 / /

0 0.0

/ /

/ /

/ /

/ /

/ /

1 3 0 ^ - + ^ ^̂ 2,V / " ^ >

/ /

^ / to/

^ ' ! «

# / -^/'V. #+/^

^ / /

/ /

/

y /

***^ K-».|atx = l

3.127 mole fraction HCl =22.8 Vo HCl. r20/4?A°"0^''0P®

/c^V

S*^

/ Am— Curve from

/ /

f

5 0.

Figure 10

#

1

+ Points calculated from partial.

0 0.

pressure values of Zeisberg»'. Figures indicate weight per cent HCl in 1

5 0

Iquid.

20 0. 25 0. 30 Acidity, mole fraction HCl or HNO3

FIGURE 12. DISTRIBUTION RATIO OF CHLORIDE AS HCl IN BOILING AQUEOUS HYDROCHLORIC ACID SOLUTIONS AT SELECTED^OILING TEMPERATURES Boiling temperature has some value as for nitric acid solutions of equivalent acidity and boiling under 200 mm mercury total pressure.

A - 1 3 6 9 9

26

published va lues ' ^ ' on pa r t i a l p r e s s u r e s over hydrochloric acid solut ions. In the calculation of the K values , the boiling t e m p e r a t u r e s of the hydrochlo­r i c acid solutions were selected to match the boiling t e m p e r a t u r e s of n i t r ic acid solutions of equivalent acidity and boiling under 200 m m nnercury total p r e s s u r e , A compar ison of the values for the distr ibution ra t io , K, of chlo­r ide in hydrochloric acid solutions with the chloride K values at equivalent acidi t ies in ni t r ic acid solutions shows that they do not differ grea t ly over a wide range of acidity. In solutions containing less than 0,2 mole fract ion of HCl or HNO3, the l a rges t deviation is about 300 per cent between the ni t r ic acid solutions and the hydrochloric acid solut ions. Therefore , the K values for chloride should vary not more than about plus or minus 15 per cent in ni t r ic acid solutions which have an HC1:HN03 mole ra t io of l ess than 0, 1, However, high concentrat ions of ni t r ic acid no doubt would inc rease the volatil i ty of chloride by oxidation of the chloride to f ree chlor ine .

The dec rease in value of the distr ibution ra t io of chloride in concen­t ra ted hydrochloric acid solutions occurs because HCl and H2O constitute a binary sys tem, in which the distr ibution ra t io , K, must approach unity as the sys tem composit ion approaches pure HCl, In aqueous n i t r ic acid solu­tions of HCl, a t e r n a r y sys tem HNO3-HCI-H2O is formed, therefore , there is no theore t ica l l imit to the values which K may a s s u m e .

Conclusions

The molar dis tr ibut ion ra t io , K, of chloride in boiling ni t r ic acid solu­tions is de termined p r i m a r i l y by the total acidity of the solutions, and var ies over an ex t remely wide range . In the range between 0, 00 and 0, 30 mole fraction (60 w/o) of HNO3, the values for K increase f rom 0, 002 to 70, or a range of 35, 000 to 1,

Lowering of the boiling p r e s s u r e r e su l t s in a lower volatility of chlo­r ide . At 200 m m m e r c u r y p r e s s u r e , the volatil i ty of chloride is about one-third that of chloride in ni t r ic acid solutions boiling at a tmospher ic p r e s s u r e .

In dilute solutions of chloride in ni t r ic acid, the dis tr ibut ion ra t io of chloride is near ly independent of the chloride concentrat ion.

In ni t r ic acid solutions of chloride boiling at 200 m m m e r c u r y p r e s ­sure , chloride is rapidly oxidized by 67, 5 w/o HNO3, but only slowly oxi ­dized by 58 w/o HNO3.

27

PART n i . PHOTOMETRIC DETERMINATION OF CHLORIDE IN AQUEOUS NITRIC ACID SOLUTIONS

The very low solubility of s i lver chloride in aqueous solutions has been for seve ra l y e a r s a convenient bas i s for the determinat ion of chloride in the pa r t s per mill ion range of concentra t ions . By the formation of a precipi ta te of s i lver chloride under controlled conditions, the turbidity of the resul t ing suspension can be measu red by one of severa l optical methodsV") and the chloride concentrat ion then obtained from a cal ibrat ion curve . However, the si lver chloride suspensions a r e not completely stable and their stabili ty cha rac t e r i s t i c s a r e often affected by interfering ma te r i a l s p resen t in solution. In the determinat ion of smal l amovints of chloride in ni t r ic acid solutions, comparat ively large quantities of ni t r ic acid a re p r e s ­ent in the si lver chloride suspensions and adverse ly affect the stabil i ty of the suspensions .

By adding ethanol to the sample as suggested by the work of Lamb, Carl ton, and Meldr t im ' ' ) the stability of the si lver chloride suspension was grea t ly inc reased . Within a ce r ta in range of n i t r ic acid concentrat ions , the optical density of s i lver chloride suspensions was found to be substantial ly independent of the acid concentrat ion. By using alcohol as a stabilizing agent, and controlling the acidity of the sample, the s i lver chloride suspen­sions were sufficiently stable and reproducible to pe rmi t the determinat ion of chloride in dilute or concentrated ni t r ic acid solutions,

Kolthoff and Yutzyv°) descr ibe the stabilizing action of ethanol upon s i lver chloride suspensions, and d iscuss the effects of other fac tors in the prepara t ion of s amples . Luce, Denice, and Akerlund(9) desc r ibe a t u rb id i -me t r i c method for the determinat ion of chloride in which the sample is f i r s t neutral ized by addition of alkali (or acid) and the suspension stabil ized by the use of ethanol.

Apparatus and Reagents

A Beckman Model DU spectrophotometer was used to m e a s u r e the absorbance of the si lver chloride suspension. Corex sample cel ls having a l ight-path length of 1,0 cm contained the samples of s i lver chloride suspen­sion and dist i l led water , which was used as a re ference m a t e r i a l . Light at a wavelength of 450 na/i was used in the measu remen t of absorbance for the determinat ion of chlor ide .

The s i lver n i t ra te solution was 0, 05 N AgN03 and 0, 2 N HNO3, con­taining 8, 5 g of si lver ni t ra te and 12, 5 ml concentrated ni t r ic acid per l i te r of solution in dist i l led water . The n i t r ic acid reagent was 13 N HNO3, p r e ­pared by dilution of concentrated ni t r ic acid with dist i l led water . The

28

ethanol used in the prepara t ion of samples was 200 proof, A stock solution of hydrochloric acid which contained 100 mg chloride per l i ter of solution was used in the prepara t ion of the cal ibrat ion curve .

Determinat ion of Chloride

Calibrat ion Curve

A cal ibrat ion curve was p repa red by measur ing the absorbancies of suspensions containing known quantities of chlor ide . Aliquot quantities of the hydrochloric acid stock solution to give concentrat ions of 1, 2, 5, 10, 20, 30, 40, and 60 mg of chloride per l i ter of dilution were used in the p r e p ­ara t ion of the cal ibrat ion curve . The s i lver chloride suspensions were p r e ­pared by the procedure descr ibed in the following section for the de t e rmina ­tion of chlor ide . The suspensions were 0, 65 N HNO3, contained 25 volume per cent ethanol, and were measu red at 450 m/i at 20 min zifter the formation of the s i lver chloride prec ip i ta te .

The cal ibrat ion curve in F igure 13 shows the absorbancies of the s u s ­pensions plotted against the chloride concentra t ions , A s t raight line p a s s e s through the cal ibrat ion points for chloride concentrat ions between 2 and 30 mg chloride per l i ter concentra t ions . At concentrat ions below 2 mg per l i ter chloride the curve is no longer s t ra ight . Above 30 mg per l i ter chloride, the suspensions were too unstable and e r r a t i c readings were obtained.

The optical p roper t i e s of the suspension deviated slightly from the L a m b e r t - B e e r relat ionship, which r equ i r e s in normal solutions that the absorbancy be d i rec t ly proport ional to the f i r s t power of the concentrat ion. In the si lver chloride suspensions containing between 2 and 30 mg per l i ter chloride, the absorbance curve for the 1-cm light path length is descr ibed by the equation

A 1 ^o n ^ i Q / - O . 8 4 7 A = log = 0, 418 C ,

where,

A = absorbance I = intensity of incident light (450 mjj.) I = intensity of t r ansmi t ted light C = concentrat ion of chloride in mg per l i t e r .

The absorbance of the reagents in the sample was very smal l , amount­ing to an absorbance value of 0, 004 when no chloride was added to the sample in blank de te rmina t ions . Therefore the absorbance of the reagent was in ­cluded in the values used for the cal ibrat ion curve , •

29

i.O

a> u c o

ja k. o (A

<

0.1

0.01

^

Y

/

/ ^

/

/

/ V

/

/ f ^

^ i /

y^ Age of suspension 20 min

1 Concentration of ethanol 25 volume per cen1

Cone

L'gh

Slit

Refe trc

entrationof HNO3 0.65 N

f wavelength 4 5 0 m

width 0.28rr

rence at lOOper cent Distille msmission

IL 1

m 1 d water

I 10 Concentration of Chloride, mg chloride per liter

100

FIGURE 13. CALIBRATION CURVE FOR CHLORIDE IN SUSPENSION OF SILVER CHLORIDE

A - 1 3 7 0 0

30

Analytical P rocedure

A known volunne of sample containing between 0, 05 and 1, 5 mg of chloride was t r ans f e r r ed to a 50-ml volumetr ic f lask by use of a pipet te . To the sample were added 2, 5 ml of 13 N HNO3, 12 ml of ethanol, and sufficient dist i l led water to bring the total volume to about 40 ml , and the contents mixed. With samples which contained over about 20 w/o HNO^* a. port ion of the dist i l led water was added before the ethanol to avoid exposure of ethanol to high concentrat ions of n i t r ic acid, * Then 2, 5 ml of 0, 05 N AgN03 reagent was added dropwise to the flask while swirling the contents, dist i l led water added to the 50-ml mark , and the suspension thoroughly mixed, A port ion of the suspension was poured into the Corex sample cell , the cel l cover replaced, and the sample cell positioned in the spectrophotom­eter compar tment . The light absorbance of the suspension was measu red at 450 m/i at a t ime 20 min after the s i lver chloride was precipi ta ted . D i s ­ti l led water was used as a reference at 100 per cent t r ansmi s s ion . The concentration of chloride in the suspension was then read f rom the ca l i b r a ­tion curve, and the chloride concentrat ion in the or iginal sample calculated from the dilution factor and density of the sample, as

/ ^1-/1 1 \ (mg Cl~/ l i te r suspension) ( 50 (mg CI /kg sample) = *—a : £ L I (density of sample) \ m l sample

Charac te r i s t i c s of Silver Chloride Suspensions

Spectra l T ransmis s ion

Silver chloride suspensions in water have thei r g rea tes t light a b s o r b ­ancies at the lower wavelengths. F igure 14 shows that the re a r e no peaks in a spec t ra l -absorbance curve between 350 and 900 m/Li, but the absorbance dec rease s rapidly as the wavelength is inc reased . The absorbance of the si lver chloride suspension is probably more a cha rac t e r i s t i c of the size of par t ic les r a the r than the spec t ra l color of the individual p a r t i c l e s .

Stability of Silver Chloride Suspensions

In Aqueous Nitr ic Acid Solutions, Suspensions of s i lver chloride were found to be much more stable in ex t remely dilute or very concentrated solu­tions of n i t r ic acid than they were in acid solutions of in termediate concentra­tion. F igure 15 shows the effect of n i t r ic acid concentrat ion upon the s t ab i l ­ity of the s i lver chloride suspens ions . The stabili ty is seen to be at a minimum at an acid concentrat ion of 0,20 N HNO3, However, the stabili ty

* Caution: Concentrated nitric acid may react with ethanol to form ethyl nitrate which is highly explosive.

31

04

0.3

u c o

o «A

<

0.2

0.1

0.0

Chloride, 9-6 mg per liter Acidity, 0.0064 NHNO3

± ± ± ± _L ± 300 400 800 9 0 0 500 600 700

Wavelength, m/i FIGURE 14. EFFECT OF WAVELENGTH UPON ABSORBANCE OF A SILVER

CHLORIDE SUSPENSION A - I 3701

32

0.60

0.50

0)

u c o o

a4o

030 -

0.20

o o <

n > A V

0.003 N HNO3 0.10 N HNO3 020 N HNO3 0.50 N HNO3

2.0 N HNO3 5.0 N HNO3 11.7 N HNO3

aoo3 N

Chloride content, 10 mg per liter Wavelength, 375 m/i,

J. J. 10 20 30 40

Time After Precipitation of Silver Chloride,min 50

FIGURE 15. EFFECT OF NITRIC ACID CONCENTRATION UPON THE AGING CHARAC­TERISTICS OF SILVER CHLORIDE SUSPENSIONS IN AQUEOUS NITRIC ACID SOLUTIONS

A- 1 3 7 0 2

33

of the si lver chloride suspension was re la t ively independent of the actual acid concentrat ion in the range between about 0,20 and 2, 0 N HNO3,

The s imi la r i ty of stabili ty cha rac te r i s t i c over a comparat ively wide range of acid concentrat ions becomes a useful feature in a nnethod for the determinat ion of chloride in ni t r ic acid solut ions. By adjusting the ni t r ic acid concentrat ion to within a suitable range , by ei ther dilution with water or addition of concentrated ni t r ic acid, it was possible to form si lver ch lo­ride prec ip i ta tes of reproducible stabili ty c h a r a c t e r i s t i c s .

During the formation of the precipi ta te the par t i c les f i r s t grow in size and number, and the optical absorbance of the suspension i n c r e a s e s . Upon aging, the individual pa r t i c les tend to agglomerate and settle out of suspen­sion. Ei ther action r e su l t s in lower absorbance .

In Alcoholic Nitr ic Acid Solutions, Ethanol in the suspending medium was quite effective in stabilizing the s i lver chloride suspensions for a short period of t ime . In aqueous solutions containing ni t r ic acid, the optical a b s o r b ­ance of the suspension decreased rapidly with t ime from the ea r l i e s t reading, which was about 4 min after the s i lver chloride was prec ip i ta ted . The p r e s ­ence of ethanol resu l ted in a gradual initial inc rease in the absorbance , which reached a maximum within 20 to 30 min after the suspension was formed. Figure 16 shows the effect of ethanol upon the stabil i ty of the suspension in solutions with 0, 50 N and 0, 65 N HNO3 ac id i t i es . Lower initial optical absorbancies were obtained with suspensions in which ethanol was p re sen t .

The presence of ethanol grea t ly improved the stabili ty of the s i lver chloride suspensions in the p resence of ni t r ic acid . F igure 17 shows that suspensions which contained 25 volume per cent ethanol gradually increased to a maximum in absorbance . Fo r the acidity range between 0, 65 and 2, 5 N HNO3, it is seen that a var iat ion in acidity would resu l t in no more than a 4 per cent change in the optical absorbancy. The stabil ization effect of ethanol and the very smal l var ia t ion in absorbance over a range of n i t r ic acid acidity provided a bas i s for spect rophotometr ic determinat ion of ch lo ­ride in ni t r ic acid solutions.

Conclusions

By using ethanol to stabilize the si lver chloride suspension and con­troll ing the ni t r ic acid acidity, low concentrat ions of chloride in ni t r ic acid solutions can readi ly be determined by a spect rophotometr ic p rocedure .

34

0 5 0

0.40

0* u c o o w

< 0.30

0.20'--

O Ovoiume percent ethyl alcohol, 0 5 0 N HN03,375m/i 25volume percent ethyl alcohol, 0.65 N HN03,450m/x 50 volume percent ethyl alcohol, a65NHN03, 450 m/I

Chloride content, 10 mg per liter

0 percent alcohol

10 20 30 40 Time After Precipitation of Silver Chloride,min

FIGURE 16. EFFECT OF ETHYL ALCOHOL UPON THE AGING CHARACTERISTICS OF SILVER CHLORIDE SUSPENSIONS IN DILUTE NITRIC ACID SOLUTIONS

A - l 3 7 0 3

36

aeo

u c 0) •̂ 0.50 o M

<

0.40

Chloride content,20 mg per liter Ethanol, 25 volume per cent 450 m^

0.1 N HNO3

0.65JiHN03 2.5 N HNO3

5.ONHNO3

± ± I ± 0 10 20 30 40

Time After Precipitation of Silver Chloride, min

FIGURE 17 EFFECT OF NITRIC ACID CONCENTRATION UPON THE AGING CHARACTERISTICS OF SILVER CHLORIDE SUSPENSIONS IN DILUTE ETHYL ALCOHOL SOLUTIONS

A - I 3 7 0 4

36

SUMMARY

The composit ions of liquid and vapor in aqueous solutions of ni t r ic acid boiling at 200 m m m e r c u r y total p r e s s u r e have been es tabl ished for the range of concentrat ions between pure water and the the ni t r ic ac id-water azeotrope (67, 5 w/o HNO3), The molar dis tr ibut ion ra t io of ni t r ic acid con­centrat ions in the vapor and liquid phases r eaches a minimum of about 0,004 in very dilute solutions of ni t r ic acid.

The l iquid-vapor equi l ibr ium measu remen t s a r e in agreement with p a r t i a l - p r e s s u r e data published for ni t r ic acid concentrat ions over 20 w / o . The thermodynamic act ivi t ies of the water component a r e shown to be thermodynamical ly consistent with the ni t r ic acid act iv i t ies , using the Gibbs-Duhem equation as a c r i t e r ion .

The vaporizat ion cha rac t e r i s t i c s of chloride in boiling ni t r ic acid solutions vary over a wide range and a r e de termined p r ima r i l y by the total acidity of the solutions. Over a n i t r ic acid concentrat ion range between 0 and 60 w/o HNO3, the chloride molar distr ibution ra t io i nc r ea se s f rom 0, 002 to 70, a range of 35, 000 to 1, in n i t r ic acid solutions boiling at 200 m m m e r c u r y . The data of Haggerty and Hixon indicate that the relat ive volatility of chloride is about 300 per cent g rea te r in s imi la r solutions at a tmospher ic p r e s s u r e . In ni t r ic acid solutions which contain l e s s than about 2 or 3 per cent of chloride, the chloride dis tr ibut ion ra t io is prac t ica l ly independent of the chloride concentrat ion.

Chloride in a 58 w/o n i t r ic acid solution boiling at 200-mm m e r c u r y is slowly oxidized to free chlorine, and rapidly oxidized in a 67, 5 w/o ni t r ic acid solution.

Chloride concentrat ions as low as 1 pa r t per mil l ion were de termined in ni t r ic acid solutions by measu remen t of the light absorbance of s i lver chloride suspensions at 450 m/i. The suspensions were stabil ized by the addition of ethanol to the diluted ni t r ic acid solutions p r io r to the addition of a s i lver ni t ra te reagent . Consistent measu remen t s were obtained by adjusting the acidity and reading the light absorbance of the suspensions at a definite t ime after precipi ta t ion.

REFERENCES

(1) International Cr i t ica l Tables , Vol. Ill, (G. B. Taylor) , McGraw-Hill Book Company, New York (1928), 304,

(2) Haggerty, P . F , , and Hixon, A, N , , P r iva te Communication, January, 1953, Universi ty of Pennsylvania, Philadelphia, Pennsylvania ,

^y 37 and 38

(3) Othmer, D, F . , Analytical Chemis t ry , ^ , 763 (1948).

(4) Darken, L, S , , and Gurry, R, W., Physica l Chemis t ry of Metals , pp, 262 to 266, McGraw-Hill Book Company, New York (1953).

(5) International Cr i t ica l Tables , Vol, III, p , 301 (Fred C, Zeisberg) , McGraw-Hill Book Company, New York (1928).

(6) Wells, P , V, , Chemical Reviews, 3, 331 (1927).

(7) Lamb, A. B . , Carlton, P . W., and Meldrum, W. B . , J. Am. Chem. S o c , 42, 251 (1920).

(8) Kolthoff, I. M. , and Yutzy, H . , J . Am. Chem, S o c , ^ , 1915 (1933).

(9) Luce, E , N , , Denice, E , C , , and Akerlund, F , E . , Ind, Eng. Chem, , Anal. E d , , 15, 365 (1943),

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