VOL. 8 (1953) ANALI’TICA CHIMICA ACTA 339

ALIZARXN BLUE AS r\ SPECTI~IC REA(;13S’I ITOR THE

GRAVIRIlXRIC

l7ETERMINATION OF COPPER

Tn a previous paper’ it was pointed out that the dye alizarin blue, for which, in view of its insolubility in aquous acids and alkali hydroxides, was assumed the chclate formula (I), reacts specifically with cupric ions. In strongly acid solutions, only this ion forms a blue crystalline precipitate when some drops of a pyridine solution of alizarin blue arc added*. The copper nlizarin blue formed, for which the chclate formula (II) was proposed, is extrcmcly insoluble, as shown by its resistance against highly concentrated mineral acids as well as by its forma- tion from highly diluted solutions. In line with this, the copper alizarin blue is not altcrccl by sulfide ions (hydrogen sulfide or ammonia sulfide) and is also inso- luble in potassium cyanide solutions which dissolve almost all wntcr insoluble normal and complex copper salts.

* On addition of a pyridinc solution of the dye to ammomacal solutions of Zn-_, Cd- and Ni’ salts, blue or blue green prcclpitates arc formed, which are easily destroyed by diluted mmeral acids.

l?efeue?rccs p. 343.

340 F. HEIGL, A. CALDAS VOL. 8 (vs)

Based on the extreme insolubility of copper alizarin blue, and by means of the previous formation and extraction of copper cupferronate2, a method for detection of traces of copper in water or metallic nickel could be elaborated. In view of the specific behaviour of alizarin blue toward copper ions and of the fact that copper alizarin blue has only a content of about IoO/~ of copper, it seems desirable to develop a method for the gravimetric determination of copper. In the following a proccdurc is described which allows an exact determination of mg amounts of copper. It is highly probable that by the same way or with appropriate alterations smaller quantities of this metal may be determined microgravimctrically.

In the attempt to elaborate a gravimetric method based on the copper alizarin blue reaction, the handicap has to bc overcome that the dye is only soluble to the extent of about o.zOk, in pyridine. Thcrcfore, when this solution is added to an aqueous acid copper solution, not only the blue copper salt is precipitated, but also the cxccss of the blue red dye. At first glance, the danger of a permanent occlusion of the dye by the precipitate could be expected. However, it was observed that, probably owing to the insolubility of the copper salt and its crystalline structure, an occlusion does not occur, anyway not to such an extent as to hinder the complete purification of the precipitate. When the precipitate containing copper alizarin blue and an excess of the dye is treated first with acetic anhydride and then with pyridine, the copper salt remains unaltered, whereas the dye is completely removed *. In this way the specific precipitation action of the dye is maintained and chemically pure precipitates are obtained.

In view of the peat volume of precipitated copper alizarin blue, the amount of copper to be determined should not be greater than z mg. According to our experiments, the following procedure can be recommended when cupric ions are present .

Xcngents

I. Saturated solution of ulizarin blue in pyridine. The stable solution is obtained by dissolving an excess of alizarin blue in hot pyridine and filtering after cooling to room temperature.

2. Acetic anhydride. 3. Pyridinc.

Pvoced~rre

To about 20 ml of aqueous solution containing maximally z mg of copper, are added 20 ml of 6 N sulfuric acid. The solution is heated and then the reagent solution is added dropwise. Initially the reaction is slow but, once in course, ---.-_ _.. l ‘l’he solubility of the dye in acetic anhydride probably is clue to an acetylation of phcnolic groups. ‘l’his assumption is m line with the fact that the solution in acetic anh dride does not react with copper ions, but when evaporated and treated with pyre me the reactivity is re-cstahllshcd. *c?

References p. 343.

VOL. 8 (Igs3) GRAVIMETRIC DETERMINATION OF COPPER 341

continues rapidly. When the’precipitation is completed, the supcmatant solution must show a reddish-blue color and remain distinctly acid. After standing 30 minutes to I hour, the precipitate is collected, with the aid of 3 N sulfuric acid, in a filter crucible of fine porous plate.

To purify the precipitate, the following procedure is advisable: The crucible is placed in a small beaker filled to half the height of the crucible with acetic anhydri- de and heated (60 -80”) for half an hour. It is recommendable to cover the beaker with a watch glass. Then the hot acetic anhydride is filtered through the crucible and the precipitate is washed with hot pyridine, until the filtrate shows no more blue colour. After removing the pyridine by washing with alcohol, the precipitate is dried at IOOO C during 30 minutes, and weighed. The theoretical factor is o.oggr, and the results of determinations on standard samples arc rather accurate as shown by a maximum error of 4Ok, for the amount of metal present.

Dcterminution of copper in the presence of alkali cyanide

It is well known that, due to the presence of stable complex cyanocuprous anions, practically all precipitation and colour reactions of cupric ions in solutions containing excess of alkali cyanide are masked. For the detection of copper in such solutions complete destruction of alkali cyanide is necessary. As a rule this is achieved by prolonged heating with mineral acid, a procedure not only time-consuming, but also rather disagreeable in view of the formation of volatile poisonous hydrocyanic acid. The clcctrolytic deposition of copper from alkali cyanide solution has the disadvantage that, together with copper, also silver, nickel, cobalt, etc., may be deposited. Furthermore, the determination of copper in amounts smaller than z mg is not accura@.

In view of the fact that only a minimal cupric ion concentration is necessary for the precipitation of copper alizarin blue, the behaviour of white cuprous cyanide, or more correctly, the cupric-cuprous system, towards alizarin blue was csamincd. It was stated that copper alizarin blue is immediately formed when a sulfuric acid or a pyridinc solution of alizarin blue is added to a suspension of cuprous cyanide in acids. The transformation obviously is the result of the following steps :

The cuprous ions coming from the equilibrium

Cu,(CN) 2 ” Cua+2 + z CN- . . . o . . . . (I)

undergo autoxidation according to

cua+2 + 0 +- 2 11+ 3 2 cu+2 -+ l-1,0 . . . . - (2)

The cupric ions formed by (2) react immediately with alizarin blue, disturbing the ccluilibrium (I) which is immediately re-established. In this way, all the

Refer1 ?tccs p. 343.

342 F. FEIGL, A. CALDAS VOL. 8 (1953)

copper of the cuprous cyanide is transformed into insoluble copper alizarin blue. Cuprous cyanide can easily be formed from’ solutions containing complex cyan0 cuprous anions by addition of formaldehyde. Demasking occurs owing to the formation of glycolic acid nitril, according to:

CCU,(CN),]-~ = Cu,(CN), + 2 CN-

<

0- CN- + CH,O 3 CH,

CN

The dcmasking effect by means of formaldehyde has already been used4 to obtain nickel ions from complex pi(CN)J2 ions.

According to our experiments, the following procedure can be recommended for the determination of copper in alkali cyanide solutions. The procedure is also valid in presence of other metal cyanide complexes.

Reagents

I. Saturated solution of alizarin blue in pyridine. 2. Formaldehyde. 3. Acetic anhydride. 4. Pyridine.

Procedure

To about 20 ml of aqueous solution containing no more than 2 rng of copper in the form of complex cyano cuprous anion, a few drops of formaldehyde are added. The solution is acidified with 20 ml of G N sulfuric acid and heated to boiling until the excess of formaldehyde is completely expelled. Then the solution of alizarin blue in pyridine is added dropwise, and the precipitate formed is treated as mentioned in the procedure above.

A set of results is shown in the Table I, in which each figure is the average of duplicate determinations.

TABLE I

DETPRMINATION OF COPPER

(in presence of zoo mg of NI+~ and great excess of CN-) .---- -. ._ -__.- _.__ ._. _._ ____. .-. _.. .~___._____ .-

Copper present %sr

Difference 0

mg mg 111’6 _. __ . _.-_ . . .._ _ . . _ ._._ - __ __ _. --.- -_-- _ - - .--

2.00 2.06 0.00 I .oo I .04 0.04 0.50 0.53 0.03

__- . _ _ _ _. . _. _ - _ _ . . _ - 1?cfcrc11ccs p * 343,

VOL. 8 (1953) GRAVIMETRlC DETERMINATION OF COPPER 343

ACKNO\VLEDGEMENT

The authors arc indebted to the Conselho Nncional cle Pesquisas (Brazil) for material assistance.

SUMMARY

A new method for the gravimctric determination of copper was <eveloped, based on the specific precipltatlon of copper alizarin blue from highly acid solutions. The determination can be carried out m very dilute solutions up to an amount of z mg of Cu. Cop er can also be determined in alkali cyanide solution after demasking with form s dehyde.

line nouvclle m&hode de dosave gravimdtrique du cuivrc est proposde; elle est basde sur la pr&ipitation spdclfique du compose cuivrc - bleu d’alizarme, en sollltion fortemcnt acide. Ce dosage pcut s’cffcctucr cn solution tr& dilude (jusqu’h 2 mg Cu). Le cuivre pcut tigalement etre dosd, en prdscncc dc cyanure alcalin, aprc?s dCmasquage au moyen dc formald&hydc.

ZUSAMMENFASSUNG

Auf Grund der spezifischen Fiillbarkcit von T<upfcr Alizarinblau aus stark saurer T,iisung wurdc eine neue Methode zur cravimetrische l%estimmung von l<u gearbcltct. Die Bestimmung kann in sehr verdiinnter Lijsung (bis 2 mg e

fer aus u) aus-

gefuhrt wcrden. Die Kupferbestimmung kann such in alkalicyanidhaltigcn Losungen nach Demaskierung mlt Formaldehyd vorgcnommcn wcrden.

REFERENCES

1 L;. FEIGL AND A. CALDAS, Anal. Clbi~rr. Aclu, 8 (1953) 117. s 0. BAUDISCH, Circr~. 3 D. E. H. FRISAR. Ind. 4 F. FEIGL AND H. J. xgso, p. 128;

Z.anal.Chcm., 82 (xg30)417.Comp.also 17. FEIGL AXD D. GOLDSTUN, ~kfi/rroc~bewarc, 40 (1952) 46.

Received January 13th, 1953