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732

A N A L Y T I C A L C H E M I S T R Y

sponsorship of the Bureau of Ships, Navy Departmen t, which

initiated this investigation. Thanks ar e expressed to James

McCambridge and Leonard Zoole, under whose supervision this

work was conducted, for their continued interest.

The views expressed by the authors are their own and are not

to be construed as representing the official views of t he Navy

Department. RECEIVEDeptember

15

1948

LITERATURE CITED

1)

Bureau

of Ships,

Navy Department,

~ p e c i f i c a t i o n 1-S-47(IXTT)

(2)

Hanawalt,

J. D.,

Rinn,

H. W.,

and Frevel,

L.

K., ISD. ENG.

(Oct .

1, 1947).

C H E Y . ,ANAL.

ED.,10

467-513 (1938).

Spect rophotomet r ic e terminat ion

o f

Hydrogen Sulf ide

Methylene Blue Method

J A J l E S

K . FOGO AND MILTON POPOWSKY

Southern California Gas Company, Los Angeles, Calif.

Hydrogen sulfide is absorbed from gases and precipitated as zinc sulfide. The

precipitate is then redissolved and allowed to react with p-aminod imethy laniline

in

the presenceof ferric chloride. The optical density of the resulting methylen e

blue solution is measured at 670-millimicron wave length and the corresponding

quantity of sulfide is read from a previously prepared calibration curve. The

method is sensitive to about 3 micrograms and the range up to about

500

micro-

grams.

The procedure is convenient for occasional as well as frequent use.

HE determination of hydrogen sulfide in gases has usually

T een accomplished by iodometric methods 2 , 6 ) . These give

accurate results on appropriate samples but ar e often too insensi-

tive for samples containing very lit tle hydrogen sulfide. A much

more sensitive method is th at of Field and Oldach S ) , n which the

sulfide is converted to bismuth sulfide which is determined photo-

metrically while in suspension. This method, although very

sensitive (1.4 micrograms), is not well suited for the occasional

user, because very rigid control of technique is said to be neces-

sary and all solutions must be protected against oxygen.

The method described herein is a refinement of the methylene

blue method 1, 5,

7 ) .

The technique has been improved by use

of opt imum conditions for the principal reaction and by applying

modern spectrophotomet ry to t he measurement of concentration.

The manipulation is simple and the results are not affected by

minor variations. Th e method has been in successful use in the

form given for several pears.

The hydrogen sulfide is absorbed from

a

stre am of gas in a

sus-

pension formed by adding sodium hydroxide to a solution of zinc

acetate. The stripped gas is then suitably metered. The suspen-

sion then containing the absorbed sulfide as zinc sulfide is treate d

with a n acid solution of p-aminodimethylaniline, followed by the

addit ion of a small amo unt of ferric chloride solution. Bfter time

has been allowed for the formation of the methylene blue, the

solution is diluted in

a

volumetric flask and an aliquot is trans-

ferred to the spectrophotometer for measurement. The corre-

spondin quan tity of sulfide is then determined from a previously

preparef calibration curve, plotted from similar measurements

on methylene blue solutions prepared in the same manncr with

known amounts of sodium sulfide

or

hydrogen sulfide.

The method is sensitive to abou t 3.5 micrograms of sulfide when

used as given. Greater sensitivity could be obtained fairly easily

by appropriate reductions in the volumes of solutions used. The

upper limit of the method as given is abou t 500 micrograms. The

1 Present addreas, Chemistry De part men t, University of Southern Cali-

fornia, Los Angeles, Calif.

precision at such high concentration is somewhat poorer than a t

abo ut 100 to 200 micrograms, where it is *3%.

APPARATUS

The list of appar atus includes the items necessary for taking

two samples simultaneously and thereafter treating them con-

secutively.

Two 250-ml. coarse sintered-glass type gas washing bottles.

(Those made by Corning Glass Works are suggested.)

Two test meters, either wet

or

dry type.

One pipet, 25-ml.

Two pipets, 5-ml.

One graduated cylinder, 250-ml.

One glass tubing cross, 8-mm.

Three tubing clamps, screw type .

Ten mete rs of 7-mm. Tygon tubing.

Three volumetric flasks, 250-ml.

One spectrophotometer or filter photometer.

REAGENTS

KO special care need be take n in the preparat ion of th e rea-

gents. Deviations up to 5y0 in the concentrations given are al-

lowable. If the diamine used produces a dark colored solution, a

fresh supply should be obtained.

Zinc acetate, c.P.

1%

solution in distilled Kater.

Sodium hydroxide, c.P. 12y0 olution in distilled water.

Ferric chloride, c.P. 0.023 molar solution in 1.2 molar hydro-

chloric acid.

gram in

500

ml. of

5.5

molar hydrochloric acid.

p-Aminodimethylaniline sulfate, Eastman white label, 0.5

SAMPLING

A dual sampling procedure in which two samples are obtained

simultaneously is recommended.

The absorption should be done

if

possible directly at the source. Gas samples brough t into the

laboratory in metal or rubber vessels usually give low results due

to th e reaction of hydrogen sulfide with the metal

or

its oxide or to

its solubility in rubber. The pressure at the source must be at

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V O L U M E 2 1 N O . 6, J U N E 1 9 4 9

least 50 mm. of mercury above atmospheric

or

a pump must be

used t o draw the sample through th e absorption bottle.

One arm of a glass cross is connected to t he source with Tygon

tubing. A 5-cm. length of t ubing is attached t o another ar m and

a screw clamp is placed on th e tubing. Each absorber is then

charged with 130 ml. of 1% zinc acetate solution an d 5 ml. of 12%

sodium hydroxide solution, and the solutions are mixed by swirl-

ing. The parts of the bottles are assembled with petrolatum and

fastened with rubber bands. The inlets of the bottles are con-

nected t o the remaining arms of the cross and screw clamps are

placed on the connecting tubing. A test meter is then connected

to t he outlet of each absorber.

M'ith all screw clamps open, the gas is turne d on a t th e source

a t a rate considerably in excess of the sampling rate. Then the

clamp on t he bleeder arm is slowly closed until gas passes through

the gas washing bottles at a ra te of about 170 liters per hour ( 6

cubic feet per hour). The rates through the two bottles may be

equalized by adjusting the screw clamps on the connecting tub-

ing. The amoun t of sample to be take n should be th at which

will conta in betiveen 35 and 350 micrograms. Where it is neces-

sary to use samples smaller than 50 liters, the sampling rate

should be reduced correspondingly.

If the method is being used to determine the amount of hydr o-

gen sulfide resulting from th e conversion of oth er sulfur com-

pounds to hydrogen sulfide for analysis 4),nly one gas washing

bottle is used and the test meter can be replaced by a simple

flowmeter.

PROCEDURE

After the sample has been passed through the gas-washing bot-

tle, the inlet and outle t of t he bottle a re closed by slipping the

ends of a 25-em. length of tub ing over them. Jus t before begin-

ning the methylene blue reaction the temperatu re of the bottle

and contents is adjusted to 24 3 C.; the temperature of the

diamine reagent should be similarly adjusted. Then the top of

the gas-washing bottle is rai;ed and

25

ml. of diamine reagent are

pipetted int o the bottle. The bottle is closed quickly and the

contents are sni rled until all the precipitate is dissolved. Then

by alternately applying slight pressure and suction on the inlet,

a

small amount of the solution is forced back and forth through the

sinter in order to dissolve any zinc sulfide that may have concen-

tra ted there. When all the sulfide is dissolved, the top is again

raised and 5 ml. of ferric chloride reagent are pipetted into the

bott le, followed by mixing as before. The use

of

pipets designed

forsho rt delivery time rat her t han great accuracy is recommended.

733

Table

I .

Specimen Calibration Data

(Coleman Universal spectrophotometer, wave length

650

millimicrons)

Micrograms/2SO

M I .

Optical Density

Sulfide Inserted,

6 4 . 8

6 4 . 5

129.6

129.6

2 59

239

389

359

0 2 1

0 .22

0.41

0.43

0.80

0 .82

1 .12

1 .16

After the closed bottle is allowed to stand for

10

minutes the blue

solution is transferred to a 250-ml. volumetric flask and diluted t o

the mark with distilled water. Before the optical density is meas-

ured, the solution should be allowed to stand at least 20 minutes

but not more th an 20 hours in a place out of direct sunlight.

A blank solution is made by mixing the same amoun ts of th e

four solutions used above in a 250-ml. volumetric flask and dilut-

ing to 250 ml. with distilled water. This solution should be al-

lowed to age for about 30 minutes before use in the spectropho-

tome ter; the solution may be stored for several days in a dark

or

dimly lighted place.

The optical density

or

transmit tanc e of th e test solution is de-

termined by making the initial adjust ment of the instrum ent

while the cell is filled with the blank solution. Sorma lly, an d for

highest sensi tivi ty, the measurements are made with light of 670-

millimicron nave length. Light of 750-millimicron wave length

may be used if t he solution is unusually opaque.

CA LIBR 4TION

If

the measurem ent of th e optical density of th e test solution is

to be useful, a calibration curve must be prepared by making up

several standards in the manner described above but using care-

fully measured quan titi es of sodium sulfide solution

or

hydrogen

sulfide in place of th e sample.

A

solution of sodium sulfide con-

taining about 20 micrograms of sulfur per milliliter is sati sfacto ry.

The lumps of sodium sulfide should be thoroughly washed imme-

diately before making the solution, in order to remove any

so-

dium sulfite. Oxvnen-free distilled water should be used in mak-

W VE

LENGTH-MILLIMICRONS

Figure 1.

Transmittancy of Methylene Blue Solution

157

microgram. of

sulfur

i n 250 m l .

Spectral band width, 2 to 3 millimicrons.

Cell thickneae, 1.00 cm

I

ing the solution. The solution is standa rdized

iodometrically. Care must be taken throughout

the preparation of the standa rds

t o

protect the

sodium sulfide solution from more than a mini-

mum am ount of contact with oxygen.

The calibration is completed bv measuring

the optical densities of the standard methylene

blue solutions and plotting the values obtained

against the corresponding mass

of

sulfide used

in preparing the 250-ml. solution. The result-

ing curve should be nearly linear in the lower

half of the useful range of concentrations. Speci-

men calibration data are given in Table

I.

Once

made the calibration may be used indefinitely.

Data should be obtained at 670 millimicrons and

also if possible at

710

and 750 millimicrons. The

apparent peak absorption wave-length may vary

somewhat from 670 millimicrons when instru-

ments

of

low spectral purity are used. For

ex-

ample, with the Coleman Universal spectro-

photometer the apparent peak is at 650 milli-

microns; this is apparently due to this instru-

ment's band width

of

about 35 millimicrons.

The absorption spectrum for a methylene blue

solution compared to a blank solution with

a

Beckman Model

DU

spectrophotometer using a

to 3

millimicron band width is shown in Figure

1.

EXPERIMENTAL

The amount

of

methylene blue finally formed

in the reactions involved is a function of the tem-

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934

A N A L Y T I C A L C H E M I S T R Y

Table 11. Effect of Temperature on Yield of Methylene

Temp erat ure, C. 5 20 25 30 40 5 5 75

Relative yield, 76 99 100 98 79 64 43

Blue

Table

111.

Effect

of

Acid Concentration of Diamine

Reagent on Optical Density of Methylene Blue Solutions

Mola rity (HC1) of

Optical density at

diamine reagent' 2 4 5 5 . 5 6 7

8 10

660 millimicrons 0.33

0 . 66

0.69 0 .71 0 .70 0 . 66

0 . 6 3

0.57

a -411solutions contained 222 micrograms of sulfide per 250 ml.

perature and other variables.

A t

higher temperatures the reac-

tion is rapid but greater amount s of hydrogen sulfide escape from

the acid solution into th e vapor space of the gas-washing bo ttle

before reacting; a t low temperatu res little hydrogen sulfide

escapes bu t th e methylene blue reaction becomes

so slow

tha t side

reactions occur to a greater extent . Th e over-all effect of tempera-

ture on the relative yields of methy lene blue fromide ntical reaction

mixtures is shown in Table

I1

Fortunately, the maximum yield

occurs at ab out 24 C. and a reasonable tolerance may be allowed.

Th e effect of final acid concentration on the optica l density of

a methylene blue solution formed from a given amo unt of hydro-

gen sulfide

was

investigated

by

preparing the solutions

as

de-

scribed above bu t with diamine reagents of various acid concen-

trations. All the solutions contained 222 micrograms of sulfide

per 280 ml.; the results are given in Table

111.

Th e effect is be-

lieved to be due largely t o the influence of acidity on the absorp-

tion spectrum of methylene blue rather tha n to influence on the

yield of the reaction.

When the di amine reagent is added to the suspension contain-

ing zinc sulfide, hydrogen sulfide is formed. Some of i t escapes

into the vapor space of th e bottle and is lost. The amount which

escapes is a function of t he solubili ty and the to tal amount pres-

ent . When only small amount s of sulfide were present no hydro-

gen sulfide was detectable over the solution an d this was arbitrar-

ily assumed to indicate complete conversion

t o

methylene blue.

Then solutions were prepared with greater amounts of sulfide and

these solutions were diluted with blank solution sufficiently so

that the diluted solution should have corresponded to the one in

which complete conversion was assumed.

Invariably the optical

densities of the dilu ted solutions were found to be less than t hat

of

the reference solution, indicating a loss.

The results of these ex-

periments are given in Table

\-.

Because corresponding losses

occur in preparing the calibration curve, this effect is not consid-

ered to be important for methylene blue solutions representing

less than

470

micrograms of sulfide in 250 ml. of solution.

This

effect accoun ts for the deviation of the calibration curve from

Beer's law.

Table

IV.

Recovery of Hydrogen Sulfide as Rlethylene

Blue

Sulfide inserted,

microgr ams 35 122 243 366 487 610 730 855

Recovery, 100 99 98 97 96 94 88 80

The reaction time of

30

minutes allowed in the procedure in-

cludes a considerable safety factor. Periodic determinations of

the opt ical density of a solution during the reaction period indi-

cated tha t the reaction

was

just completed after

10

minutes-that

is,

no further increase in the opt ical density was detected afte r 10

minutes. After about

20

hours a decrease due

t o

fading may be-

gin to be measurable.

LITERATURE

CITED

(1)

Almy,

J . Am . Chem.

Soc.,

47, 1381 (1925).

(2) Calif. Natural Gasoline Assoc.. Los Angeles, Calif., Determina-

tion of

Hydrogen Sulfide in Natural Gas, Bull. TS

413, 1943.

(3)

Field and Oldach, IND.KG. H E X . ,

N ~ L .

D.,

8,

665

(1946).

4)

Fog0 and Popowsky,

ATAL.C H E M . 1,734 (1949).

5 ) Mecklenburger and Rosenkranzer,

2 . anorg. Chem. 86, 143

(6) Shaw, ISD.ENG.HEM

N A L .

ED.,

2,

668

1940).

(7)

Sheppard and Hudson,

I b id . 2, 73 (1930).

(1914).

RECEIVE D ugust 30, 1948.

onversion of Sulfur ompounds t Hydrogen

Sulfide

n Ai r, Fuel

G a s , or

Mixtures

JAMES K. FOG01

AND

MILTON POPOWSKY

Southern California Gas Com pany,

Los

Angeles, Calif.

HE

sulfur content of fuel gases is usually determined either

T

y oxidation

or

by hydrogenation. Oxidation methods

5,

8

I O 11)

are capable of accurate results on gases conta ining

as lit tle as 114 micrograms of sulfur per cubic meter (0.005 grain

per

100

cubic feet), but the technique is cumbersome and the

apparatus is likely to be capricious.

Hydrogenation methods

2 ,

4,

, 9)

have some advantages but are limited in scope by the

interference of oxygen, which is plentiful in certain types of gases.

Even the oxidation methods cannot be applied to explosive

mixtures

or

nonflammable gases.

A

method that can be used

on any mixture

of

air and fuel gas became necessary

for

this labo-

ratory in order

t o

determine whether the natu ral gas present in

1 Present address, Chemistry ~ ~ ~ ~ ~ t ~ ~ ~ t ,niversity of southern tali-

Soil gases

was

th at normally present in the soil of certa in areas

or leakage from gas distribution lines which carry natural gas

ornia,

Los

Angeles, Calif.