Titration of zinc with potassium ferrocyanide: Use of diphenylamine or

until the colorchanged to yellowish green (false end point). After waitingthen for 2 to 3 minutes, a characteristic purple color developed, whereupon ...
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Titration of Zinc with Potassium Ferrocyanide Use of Diphenylamine or Diphenylbenzidine as Internal Indicators I. M. KOLTHOFF AND E. A. PEARSON, University of Minnesota, Minneapolis, Minn. HE general application of

Commercial and c. P. potasT H E M E T H O D of the titration of zinc with the well-known zinc tis i u m ferricyanide s a l t s were potassium ferrocyanide using diphenylamine tration with potassium recrystallized, from w a t e r and or diphenylbenzidine as a n indicator has been ferrocyanide as a reagent has indried in the air. submitted io a systematic examination. The recreased considerably since Cone Ammonium sulfate and other sults obtained are mainly dependent upon the and Cady (2) proposed the use salts, c. P., were recrystallized of diphenylbenzidine as an inonce or more from water. conditions of titration (speed of adding ferroternal indicator. D i p h e n y l STANDARD SOLUTIONS.Zinc cyanide, temperature, direct or back titration) ; amine may also be used, but it sulfate, 0.05 M (if the strength the most favorable acidity and concentration of has the slight disadvantage that was s o m e w h a t different, the ammonium sulfate being 0.5 to 1.5 N a n d at least the amount of indicator affects figures were r e c a l c u l a t e d in 1 to 2 per cent are of minor importance. I n the results. I n t e s t i n g Cone order t o r e l a t e t h e m + t o the and Cady's procedure, Kolthoff strength of the standard soluagreement with Cone and Cady, it was found ( 3 ) found that different factors, tion), and 0.025 M potassium that with the use of diphenylbenzidine no insuch as the acidity, kind of acid f e r r o c y a n i d e (10.5580 grams dicator correction has to be applied, whereas present, concentration of zinc, of K4Fe(CN),.3H,O per liter), with the use of 1 per cent diphenylamine 0.033 presence of neutral salts, speed containing 0.15 gram of &Fecc. of 0.025 M ferrocyanide has to be added as a of and temperature during the (CN), and 0.2 gram of sodium titration and ferricyanide concorrection per 1 drop of indicator (0.023 nl.) carbonate per liter, the latter tent of the reagent, affect the added in o r d e r to s t a b i l i z e to the final titration figure. The authors prefer l o c a t i o n of t h e e n d p o i n t . t h e s o l u t i o n , were used. In the use of diphenylamine to diphenylbenzidine, Kolthoff used diphenylamine t h e f i n a l experiments the adas the latter sometimes gives much less pronounced as a n i n d i c a t o r and recomd i t i o n of ferricyanide t o t h e color changes. mended that t h e t i t r a t i o n be standard solution was omitted. performed at a temperature of According t o t h e e q u a t i o n around 50" C. Under these conditions a fairly rapid direct titration to the end point can be made and a back titration of the excess ferrocyanide with a zinc solution of known theoretically, 25 cc. of the zinc solution requires 33.33 cc. content is not necessary. For the determination of small of the ferrocyanide solution. A 1 per cent solution of diamounts of zinc, Bradley ( 1 ) recommends titrating the solu- phenylamine or diphenylbenzidine in concentrated sulfuric tion after acidifying with hydrochloric acid and the addition acid (indicated by D. A. or D. B.) was used as indicator. of 4 t o 6 drops of an aqueous solution of 20 per cent potassium ferricyanide at a temperature of 60" to 70" C. I n a study RESULTS OF TITRATIONS (4) on the eo-precipitation of zinc sulfide with copper sulfide If the reagent is added too rapidly, the equivalence point the authors had to determine the zinc content of solutions under varying conditions, and it appeared necessary to make is easily passed. Large amounts of ammonium sulfate have a systematic investigation on the influence of different fac- a pronounced retarding effect, as may be inferred from the tors upon the titration in order to find a uniform procedure results in Table I. In each case the titration was carried applicable to varying conditions. An outline of the more out rapidly until near the end point where the reagent was than 1000 titrations which have been made will be given in added drop by drop. After the end point had been reached, the yellowish green suspension was titrated back slowly this account. with zinc solution, and a sharp color change to purple-blue EXPBRIMENTAL PROCEDURE was observed. I n all cases a correction of 0.033 cc. of 0.025 MATERIALS.Zinc sulfate (c. P.) of different origin was M ferrocyanide for each drop of diphenylamine was added recrystallized two to three times from water and dried in to the titration figure. The experiments reported in Table I1 were carried out in desiccators a t the proper vapor tension (relative water v. t., 0.65 to 0.70), until constant weight was obtained. such a way that the reagent was added slowly to within 2 The salt was analyzed and shown to have the composition ml. from the theoretical end point, and then drop by drop ZnSOr.7Ht0. Moreover, solutions of spectroscopically pure until the color changed to yellowish green (false end point). zinc (kindly supplied by New Jersey Zinc Company) in After waiting then for 2 to 3 minutes, a characteristic purple dilute sulfuric acid were prepared. It may be mentioned color developed, whereupon the titration could be finished that solutions of Kahlbaum's zinc were also made, but this by dropwise titration to the definite end point. As the rezinc is not so pure as the product of the New Jersey Zinc sult of a large number of experiments, it can be definitely Company, and leaves, after dissolving the metal in dilute stated that the speed of titration is of great influence only sulfuric acid, a black, spongy residue. near the equivalence point. In order to obtain reproducible Samples of c. P. potassium ferrocyanide were recrystallized results, careful dropwise titration must be made to the from water twice and dried in a desiccator over deliquescent appearance of the false end point, and after waiting for the sodium bromide until constant weight was obtained. A appearance of the blue or purple color the titration is finished water determination showed the composition K4Fe(CN)s.- by slow dropwise titration to the final end point. The appearance of the false end point is very characteristic. 3Hz0 (drying at 110" C.). 147

148

ANALYTICAL EDITION

The light blue solution fades out to a pale green or almost colorless solution which gradually becomes purple. The longer the period of standing the more brilliant becomes the purple color. The solution should be vigorously stirred or shaken during the entire titration and especially while waiting for the purple color to develop. If the titration is carried out too rapidly near the end point, the false end point, which develops 0.2 to 0.4 cc. short of the true end point, will be missed and high results will be obtained (as shown in Table I). AND DIFFERENT TABLEI. EFFECTOF RAPIDTITRATION AMOUNTS OF AMMONIUM SULFATE

(25 00. of 0.05 M ZnSOd; 10 00. of 4 N HzSOI: and 2 drops of D. A., temp., 24' C.) (NH&SO( DIRECT DEVN.FROM BACK DEVN.FROM ADDED TITRATION THEORY TITRATION THEORY Urams % % 0.5 33.11 -0.7 33.08 -0.8 -0.3 33.21 1.0 33.24 -0.4 -0.2 33.21 1.0 33.27 -0.4 6.0 34.67 $4.0 33.47 +0.4 5.0 34.77 +4.3 33.41 +0.2

TABLE11. VERYSLOWTITRATION (Presence of ammonium sulfate: other oonditions as described in Table I) (NHdzSO4 DIRECT TITRATIONDEVN.FROM THEORY Grams % 1 33.04 -0.9 1 33.03 -0.9 8 33.13 -0.6 S 33.14 -0.6

If the titration is carried out very slowly under these conditions, even very large quantities of ammonium sulfate only slightly affect the location of the permanent end point. The results given in Table I11 show that within wide limits, the concentration of sulfuric acid and of ammonium sulfate and the dilution are without influence upon the results. Large amounts of ammonium sulfate seem to bleach the color to some extent, possibly because the salt decreases the hydrogen-ion concentration and consequently the oxidation potential of the ferri-ferrocyanide solution. I n all the titrations, the ferrocyanide was added fairly rapidly under thorough shaking of the titration flask to about 1 cc. from the theoretical end point, and from there on slowly. After development of the purple color the reagent was added very slowly, drop by drop, until the permanent color change was reached. Especially a t high acidity (about 1 to 2 N ) very sharp color changes were observed. The results are reproducible within about 0.5 per cent or closer. The speed of titration, even when the reagent is added drop by drop, has an appreciable influence in the direct titration, especially near the end point. de Koninck and Prost (7) assume that the zinc ferrocyanide first formed is slowly transformed into the potassium zinc ferrocyanide. However, as many other authors notice, on standing, near the end point the appearance of the precipitate seems to change entirely. At the false end point the ferrocyanide concentration is large enough to decrease the oxidation potential to such an extent that the quinoid form of the indicator is reduced to the colorless form. On standing, the ferrocyanide concentration decreases, owing to the transformation of the precipitate into the stable and less soluble double salt, the oxidation potential increases, and the indicator is oxidized again. Kolthoff and Verzyl (6)have studied this change of the oxidation potential near the equivalence point. The direct titration a t room temperature, although it yields reproducible results, is not recommended for routine work, as the manipulations require much attention and time. The results obtained are about 0.6 to 1 per cent low. If, after such a slow titration, an excess of reagent (5 to 25 per cent) is added which is titrated back with standard zinc solution, the results are raised only 0.3 to 0.4 per cent.

Vol. 4. No. 1

TABLE111. TITRATION OF ZINC SULFATEWITH POTASSIUM< FERROCYANIDE (25 co of 0 05 M ZnSOd: 0.025 M &Fe(CN)a oontainin 0.15 gram of KaFe(CN)s per liter: and 6 drops of D . A. oorrected for iniicator: temp., 200 C J FERRODEVN. COMMENT ("4);WATER CYANIDE FROM ON END 4NHnsO4 so4 ADDED USED THEORY pnrw, Cc. Urams Cc. cc % 1 10 90 33.01 -1.0 Fairly good 2 10 90 33.08 -0.7 Fairly good 2 10 150 33.09 -0.7 10 4 33.09 90 -0.7 Fairly good 10 8 33.22 90 -0.4 20 1 90 32.97 -1.1 Rairl good 2 20 90 33.11 -0.7 Good 4 20 90 33.15 -0.6 Very good 20 8 90 33.24 -0.3 Good 30 1 90 32.97 -1.1 Goo8 2 90 30 33.06 -0.8 Very good 2 30 150 33.06 -0.8 Very good 4 30 90 33.19 -0.4 Very good 30 33.28 8 90 -0.2 Good 1 90 50 33.05 -0.8 Very good 2 50 90 33.08 -0.7 Very good 2 50 150 33.04 -0.8 Very good 4 50 90 33.15 -0.6 Very good 50 50 33.26 8 -0.2 Very good 50 90 33.34 8 0.0 200 8 50 33.37 $0.1 Very good

--_.-

.

BACKTITRATION AT ROOMTEMPERATURE The procedure is more satisfactory if the reagent is added fairly rapidly until an excess of 5 to 15 per cent is present. The mixture in the flask has a creamy light yellowish green color. By back titration with standard zinc solution, a change to purple is obtained a t the end point which coincides within 0.3 per cent with the equivalence point. Here the change from purple to yellowish green is completely reversible and most data are based on the latter end point. Some results, reproducible within 0.2 per cent, are given in Tables IV and V. TABLEIV. RESULTS OF BACK TITRATION (25 cc, of 0.05 M ZnSO4' 10 oc. of water: 10 oc. of 4 N HzSOc: 1 gram of ( N H ~ ) Z S O2~drops I of 1% D. A.: temp., 24O C.) Excnss REAQENT BACK DEVN.FROM ADDED~ TITRATION THEORY

.

. 1.0 cc

%

33.30 -0.1 1.0 33.35 +o. 1 2.0 33.35 +0.1 2.0 33.35 $0.1 5.0 33.40 +0.2 5.0 33.38 $0.2 0 Added fairly rapidly (70to 90 secon'da). Near the end point the speed of addition of ferrocyamde was decreased.

TABLE V. RESULTS OF FTJRTHER BACKTITRATIONS (25 0 0 . of 0.05 M ZnSOr. 90 cc. of water. 30 00. of 4 N HzSO4; 2 grttms of ( N H I ) z S O I idrops ; of 1% D.'A.; temp., 24' C.) EXCESSR E A Q ~ N T BACX DBVN.FROM ADDED TITRATION THEORY

cc.

0.2 1.0 1.0 2.0 6.0

% 33.32 33.32 33.34 33.37 33.36

0.0 0.0 0.0 +o. 1

$0.1

If the direct titration method was applied (slow titration and waiting for the appearance of the false end point), the results again were 0.9 to 1.1 per cent low. In agreement with Cone and Cady (a), it was found that with diphenylbenzidine the amount of indicator is without influence upon the results, whereas increasing the concentrations of diphenylamine lowers the titration figures. Nevertheless, in most of the experiments diphenylamine was used because under some conditions a less sharp color change was observed with diphenylbenzidine (6). Each drop (0.023 cc.) of 1 per cent diphenylamine lowers the result by 0.033 cc. of 0.025 M ferrocyanide (TableVI). If necessary a correction may be applied for the indicator effect. The use of 2 drops of 1 per cent diphenylamine willao for a volume of 50 cc., and for a larger volume a correspondingly larger quantity of indicator is used. Many of the experi-

January 15, 1932

INDUSTRIAL AND ENGINEERING CHEMISTRY

ment8 reported in Tables I to V were repeated with diphenylbenzidine as an indicator and the same phenomena were observed as with diphenylamine.

149

since no correction has to be applied for the oxidation of the indicator. I n most of the experiments 2 drops of 1 per cent diphenylamine were used and, in this case, a correction of 0.066 cc. of 0.025 M ferrocyanide had to be added to TABLEVI. EFFECTOF DIFFERENT AMOUNTSOF DIPHENYL- the titration figure in order to account for the indicator effect. BENZIDINE AND DIPHENYLAMINE This was done in all experiments listed in this paper. (25 cc. of 0.05 M Zn804; 10 00. of water; 1 gram of ("4)&04. 10 cc. of The following procedure is shown to give satisfactory 4 N His04 + 2 drops of 1% ferricyanide. tem 24O C.; back' titration; results for the titration of 3 mg. or larger quantities of zinc. 1 drop of 1% indicator' = 0.&3 0 0 . ) To a measured volume of the 0.05 M zinc solution, 10 cc. of FERROCYANIDE DEVN.FROM 1%~INDICATOE TAXEN TSBORY water, 10 cc. of 4 N sulfuric acid, 1 gram of ammonium sulDTOpS cc. % fate, 2 drops of 1 per cent potassium ferricyanide, and 2 DIPHENYL BENZIDINE drops of 1 per cent diphenylamine are added, whereupon 2 33.44 $0.3 the reagent is added fairly rapidly until a distinct excess 4 33.40 +0.2 8 33.44 4-0.3 is present. The ferrocyanide is titrated back with a standard 12 33.44 3.0.3 zinc solution, the end point being marked by a sharp color DIPHENYLAMINE change from yellowish green to purple. I n the titration of 2 33.37 +o. 1 4 33.31 -0.1 very small amounts of zinc the use of a microburet is recom33.16 -0.5 a 12 33.04 -0.9 mended. I n Table VI1 some of the results are recorded.' The INFLUENCE OF CONCENTRATION OF FERRICYANIDE ON TITRA- third and fourth columns give the figures without correcting for the indicator effect, the fifth and sixth after correction. TIONS AT ROOM TEMPERATURE With 2 drops of 1 per cent diphenylbenzidine, about the From the results of experiments described in Tables I same results are obtained as given in the fifth and sixth to VI, it is concluded that a t room temperature the direct columns. As a whole, the procedure gives. satisfactory retitration to the end point is quite a tedious procedure. If sults for amounts of zinc varying within wide limits. I n only very small amounts of zinc are present in the solution, the application of the method, standardization of the ferroit is extremely hard to obtain accurate and concordant re- cyanide solution against a zinc solution of known content sults according to this method, whereas a sharp end point under the same conditions used in the titration is recomis observed if an excess of reagent is added which is titrated mended. As 2 drops of ferricyanide are added separately back with standardized zinc solution. to the solution, it is not necessary to add it to the standard The standard ferrocyanide solution used in the titra- ferrocyanide solution. tions contained only 0.15 gram of potassium ferricyanide per liter, I n the determination of small amounts of zinc, TABLEVII. TITRATION OF VARYING AMOUNTS OF ZINCAT ROOM the ferricyanide content of the mixture during the titration TEMPERATURE (22' C.) is so small that no distinct blue color is developed. There(Diphenylamine as indieator) fore, it is necessary to add some ferricyanide to the solution. NOTCOR.FOR COR.F O R EXCEBS INDICATOR INDICATOR Too large amounts should be avoided, as seriously low reFERRODevn. Devn. PERFORMANCE OF sults are obtained. The oxidation potential is increased too 0.05 M ZINC CYANIDE Zinc from Zino from TAKEN ADDED found theory found theory TITRATION much, and therefore the color change may take place before CC. % cc. % cc. % the equivalence point has been reached. Moreover, it is 1.90 50 1.88 -1.0 1.92 +1.0 Slow 3.68 3.60 30 -2.2 3.64 -1.1 Slow possibie that part of the ferricyanide oxidizes the indicator 4.30 20 4.28 -0.5 4.32 +0.5 Slow 4.50 40 4.48 -0.5 4.52 + 0 . 5 Slow in an irreversible way, whereby the ferricyanide is reduced 9.96 9.90 -1.3 0.7 9.94 -0.2 Slow to ferrocyanide. 10.20 10 10.10 -1.0 10.14 -0.6 Slow 15.06 3.3 15.07 0.0 15.11 +0.3 Rapid A large number of titrations, in which the concentration 15.20 10 15.10 -0.7 15.14 -0.4 Slow of sulfuric acid, ammonium sulfate, and ferricyanide was 19.95 1.6 19.81 -0.7 19.85 -0.7 Slow 25.00 0 24.72 -1.1 24.76 -0.9 Veryslow; divaried within wide limits, were performed in order to arrive reot 25.00 3 24.85 -0.6 24.89 -0.4 Slow at a procedure which would hold for the titration of different 25.00 3 25.00 0.0 25.04 $0.15 Rapid amounts of zinc. It was found that the acidity and the 26.00 6 25.02 +0.1 25.06 +0.35 Rapid 25.00 5 24.77 -0.9 24.83 -0.7 No(NHd),SO! ammonium sulfate concentration are of minor importance, added: rapid whereas the addition of 1 to 4 drops of a 1 per cent solution of potassium ferricyanide is the most favorable amount, TITRATION AT HIGHER TEMPERATURE larger quantities being harmful especially a t low zinc conKolthoff ( 3 ) previously pointed out some of the difficulties centrations. With very large amounts of ferricyanide (1 cc. of 10 per cent solution or more) the solution does not met with in the direct titration of zinc a t room temperature. turn purple a t all, but dirty green, and the end point is marked He recommended titration at 50" to 60" C. and observed by a change from dark to bright green (with diphenylamine a false end point before the definite color change was obas indicator). The amount of ferricyanide Bradley (1) tained. About 0.5 cc. before the final end point the dark recommended, 2 to 3 drops of 20 per cent KaFe(CN)6, is blue color goes suddenly over to yellowish green. If one undoubtedly too large, especially when the titration is per- waits 15 to 30 seconds, the change reverses itself and the formed a t 60" to 70" C., as Bradley suggests. It should liquid takes on a characteristic bright blue-violet color. be mentioned here that a ferricyanide solution is not very The titration is continued drop by drop until the color no stable. It has to be prepared in water of good quality, longer turns back t o blue-violet after 20 seconds. The color the addition of a trace of (0.03 per cent) sodium carbonate change is not so brilliant as at room temperature, but, on having a stabilizing effect. It is also best to prepare a fresh the other hand, the titration can be carried out a t 50' to solution every 4 or 5 days and to keep it protected from 60" C. much more quickly than a t room temperature and the action of sunlight. No special study has been made of the results are quite reproducible. The authors could confirm these statements if the indithe stability of ferricyanide solutions. In the titration of very small amounts of zinc, the use 1 The experimental conditions are described more fully in the thesis of of diphenylbenzidine has advantages over diphenylamine, E. A. Pearson (Minnesota, 1931).

150

ANAI>YTICAL EDITION

cator and 2 to 4 drops of 1 per cent potassium ferricyanide were added after heating the solution and just before the titration was started. The method was tested under different conditions a t 50" to 60" C., and it was found that the same procedure can be followed as given for the experiments in Table VII. Again, a large excess of ferricyanide is harmful. Especially for the determination of small quantities of zinc (25 mg. or less), the titration in warm solution is advantageous. I n the latter case it is well to add a small excess of ferrocyanide followed by back titration, since the direct titration takes time. Some of the results are reported in Table VIII. The experiments were made under the same conditions as described for those in Table VII.

Vol. 4. No. 1

Aluminum. The potentiometric method gives no end point in the presence of even traces of aluminum salts. With the indicator method, 0.5 gram of aluminum chloride or less had practically no effect (results were raised about 0.5 per cent); with much larger amounts of aluminum salts no end point could be detected.

DIRECTTITRATION ENDPOINTAT ROOMTEMPERATURE. The reagent can be added fairly rapidly until about 1 cc. before the equivalence point. Upon further careful addition of reagent, a false end point is found (from purplish blue to yellowish green). If one waits 1 to 2 minutes, a characteristic blueviolet color develops; the titration is continued; the reTABLEVIII. TITRATION OF VARYING AMOUNTSOF ZINC AT agent is added drop by drop with continuous shaking of the 50' TO 60" C. titration flask until the permanent end point is reached. (Ferrioyanide and indioator added after heating solution and immediately This procedure cannot be recommended for general use, before addition of ferrooyanide) as the method is time-consuming and the false end point COR.F O R EXCESS NOTCOR.FOR 0025 M INDICATOR INDICATOR may be passed if the addition of reagent is not regulated 0.05 M FERRODevN. DevN. PERFORMANCE ZINC CYANIDE Zino from Zino from OF very carefully. TAKENADDED found theory found theory TITRATION ENDPOINT AT 50" TO 60" C. The titration can be carried cc. % cc % cc. % out under usual conditions until the color changes from 0.99 23 0.99 0.0 1.03 +4 Slow 3.48 -0.6 Slow 10 3 45 -1.4 3.50 purplish blue to yellowish green (false end point). Gfter 4.94 -1.6 Slow 7 4.90 -2.4 5.02 waiting for 15 to 30 seconds, the characteristic bluish color 10.14 7 10.18 +0.3 10.22 +O.S Fairlyrapid 15 04 0 14.91 -0 9 14.95 -0.6 Direat; after dedevelops and the titration can be finished. The end point veloDment of color' at false is reached when the blue color does not come back after 20 end point, proceeded slowly seconds' waiting. The procedure can be used for the titra-0.3 -0.6 14.95 Rapid 14.91 2.7 15.00 tion of 25 mg. of zinc, or larger quantities. For the determi19.92 -0.5 Slow -0.7 19.88 5 20,03 24.76 -1.0 -1.1 Direct 24.72 nation of quantities smaller than 25 mg., the back titration 0 25.00 24.78 -0.9 Direct -1.0 24.74 0 25.00 method a t room temperature or a t 50" to 60" C. is recom-0.9 24.78 Rapid -1.0 7 . 5 24.74 25.00 -0.2 24.94 Very rapid -0.4 25,OO 1 2 . 8 24.90 mended. INFLUEWE OF ADDITION OF OTHERSUBSTANCES Hydrochloric acid has no interfering effect. It can be used instead of sulfuric acid if the solution contains a t peast The most 0.5 to 1 gram of ammonium acidity is again 0.5 to 1 N . Ammonium &,loride, if present in large amounts, retards the development of the color as anlmonium sulfate does. It has PracticallY no influence upon the results, yielding about 0.2 per cent lower results than ammonium sulfate. at least if am&'odium Sulfate without any monium sulfate is present. Earth alkali salts in small quantities (less than 0.5 gram) in the presence of ammonium sulfate do not exert any influence; if present in larger amounts they have a tendency t o lower the results. The interference, however, is not serious. Ferric iron and other cations which form insoluble ferrocyanides interfere very strongly and have t o be removed. Kolthoff and Verayl (6), in their potentiometric work, used fluorides t o render ferric iron harmless. This substance was used in this work also. The stability of the ferric fluoride complex decreases with increasing acidity, and therefore it is necessary to regulate the acidity very carefully. At too low acidity the oxidation potential is not high enough to produce the oxidation of the indicator. The following method, though not very practical, could be used for the titration of 30 mg. of zinc in the presence of less than 20 mg. of ferric iron: Ten cubic centimeters of 0.05 M zinc sulfate were treated with 7 grams of potassium fluoride, 50 to 60 cc. of 48 per cent hydrofluoric acid solution, and about 50 cc. of water. The mixture was heated to 60" t o 70' C., 2 drops of 1 per cent ferricyanide and 4 t o 6 drops of diphenylamine were added, and the titration started. The color change was not so sharp as under ordinary conditions (reproducibility 1 per cent).

DISCUSSION O F

General Procedure. Add to a measured volume of the zinc solution 1 gram of ammonium sulfate, sufficient sulfuric acid t o make the acidity about 1 N , 2 drops of 1 per cent potassium ferricyanide, and 2 drops of 1 per cent diphenylamine (or 1 per cent diphenylhenzidine). If the titration is carried out at 50" to 60' C . the ferricyanideand indicator are added after heating the solution and immediately before adding the reagent. The ferrocyanide solution is added with normal speed until an excess is present. During the addition of the reagent the titration flask is continuously shaken. After 1 t o 2 minutes' waiting, the excess of reagent is titrated back with a standard zinc solution; the end point is reached when the color changes from yellowish green to a characteristic blueviolet. The color change is very sharp (to 1 drop of 0.025 M ferrocyanide). If diphenylamine is used as an indicator, 0.07 cc. is added a8 a correction to the consumed volume of 0.025 M ferrocyanide.

One cubic centimeter of 0.025 M ferrocyanide corresponds to 2.45 mg. of zinc. At room temperature approximately theoretical results are found; a t 50" to 60" C. they run about 0.5 to 1 per cent low. If quantities of zinc smaller than 20 mg. have to be titrated, it is recommended to add the ferrocyanide slowly. Small amounts of alkali and earth a l l d i salts and aluminum do not influence the results. Ferric iron interferes, but amounts smaller than 20 mg. can be made harmless by titrating a t 60" to 70" C. in a solution containing a large quantity of potassium fluoride and hydrofluoric acid. LITERATURE CITED Bradley, Chemist-Analyst, 17, 14 (1928). Cone and Cady, J. Am. Chem. soc., 49,356 (1927). Kolthoff, Chem. Veekblad, 24, 203 (1927). . Kolthoff, Mensel, and Furman, "Volumetrio Analysis," Vol. 11, 255, Wiles, 1929. (5) Kolthoff and Sarver, J.Am. Chem. SOC.,52,4179 (1930). (6) Kolthoff and Verayl, Rec. trav. chim., 43,380 (1924). (7) Koninck, de, and Prost, 2. angezu. Chem., 15,460, 564 (1896). RECEIVED May 29, 1931. Part of a thesis submitted by E. A. Pearson to the Graduate School of the University of Minnesota in partial fulfilment of the requirements for the degree of doctor of philosophy.