Determination of Iodate and Other Oxidizing Agents in the Presence of Cupric Salts Use of the Iodine Monochloride End Point ERNEST H. SWIFT AND THOMAS S. LEE, California Institute of Technology, Pasadena, Calif.
I
N ORDER to determine iodate in the presence of cupric copper, Kapur and Verma (6)have recently suggested that the iodate be allowed to react with an excess of iodide and the iodine thus produced be titrated with a standard thiosulfate solution; by the addition of pyrophosphate the copper is held in the cupric state because of the formation of a stable complex ion. This method has the disadvantage
from a buret until iodine was present, and the solution was backtitrated with iodate. A series of experiments made under various conditions showed that the iodate thus found was from 0.5 to 5 per cent low, and chlorine or hypochlorous acid was detected above the solutions. The method was therefore modified by adding an excess of the standard iodide to a neutral solution, then adding the concentrated hydrochloric acid, and back-titrating the iodine present with standard iodate. Table I gives results of these titrations. The error in titrations 1 to 3 and 6 was subsequently found to have been caused by making the iodate back-titrations too rapidly. In titration 9 the iodate-iodide mixture was acidified with 1 ml. of 6 N sulfuric acid before the hydrochloric acid was added.
that iodine is liberated very slowly by the iodate-iodide reaction under the conditions necessary for the formation of a stable cupric pyrophosphate complex ion. If, however, the iodate is titrated with standard iodide solution to a n iodine monochloride end point, cupric copper is not appreciably reduced at the equivalence point. The experiments described below show the conditions under which this reaction can be made the basis for accurate volumetric methods for determining iodate and certain other oxidizing agents in the presence of cupric salts.
TABLE11. TITRATIOW O F PEFWANQANATE-COPPER MIXTURES WITH IODIDE
Substances and Solutions The chemicals used were of "analytical chemical" grade and were proved to be free from interfering substances. Potassium iodide and iodate solutions were prepared from the dried salts, and the iodide solution was checked by titration against the iodate solution, using the iodine monochloride end point as recommended by Swift (6). A sodium thiosulfate solution was standardized against the iodate solution and a copper sulfate solution was standardized against the thiosulfate under conditions recommended by Foote and Vance (1). The carbon dioxide used was from a cylinder, and was proved to be free from significant amounts of reducing agents. Standard potassium bromate and otassium chlorate solutions were prepared directly from the drie8salts. A ermanganate solution was standardized against the potassium i o i d e solution, using the iodine monochloride end point.
21 22 23
ExDt.
T1TR.4T10N
OF loDATE-CoPPER T-7.77.-
TITRATIoN
0.1 F
Expt.
Taken
HT Final Normality
M1. 0 10
M1. 0
12.5 50
Final Normalitv 3.0 3.0 3.0
KBrOa Used MI. 25.32 25.22 25.22
OF PERMANGANATE-COPPER
KBrOi Calcd.
Error
M1.
%
25.37 25.27 25.28
-0.2 -0.2
-0.2
wITa
Io-
chloride and the cupric solution added, and then iodide was added o.2 ml* was present in excess. from a buret This mixture was acidified with hydrochloric acid and backtitrated with Dermaneanate. Table I1 lists the results. The mixturk of titration 19 was allowed to stand 15 minutes prior to acidification with hydrochloric acid in order to note if the manganese dioxide formed in the neutral solution caused catalytic-decomposition of the permanganate. In order to prevent the formation of manganese dioxide, titrations 21 to 23 were made by adding the iodide to a permanganate solution which was 0.6 N in sulfuric acid, then acidifying with hydrochloric acid and back-titrating. TITRATION OF BROMATE-COPPER MIXTURES WITH IODIDE. To the bromate-copper solution were added carbon tetrachloride and then iodide until approximately 0.2 ml. of iodide was present in excess. The mixture was acidified with hydrochloric acid and back-titrated with bromate. Table I11 shows the results. TITR.4TION OF CHLORATE-COPPER MIXTURESWITH IODIDE. The procedure used was similar to that used for the iodide titrations of the bromate-copper mixtures. Table IV lists the results. A pronounced tendency toward oxidation of iodide by air is found, since in experiments given treatment a no precautions were taken to exclude the air; in treatment b the chlorate and hydrochloric acid solutions were saturated with carbon dioxide; in treatment c the neutral mixture of chlorate and iodide was saturated with carbon dioxide and the hydrochloric acid saturated with CO,.
Error i/C
0.0997 -0 3 0,0998 -0.2 a 3 0 2.9 0.0997 -0.3 4 0 3.0 0.1000 0.0 5 0 3.0 0.1000 0.0 6 0 3.0 0.0998 -0.2 7 0 3.3' 0.09998 -0.02 8 0 3.4s 0.10006 +0.06 0 3.4 0.10006 +0.06 9 10 0 3.4 0.10006 +0.06 11 0 3.4 0.09996 -0.04 12 0 3.4 0.10002 f0.02 13 12.5 3.7 0.10008 f0.08 14 25 3.6 0.10006 +0.06 15 50 3.5 0.10006 +0.06 0 Solution was first aoidified with HClO,, allowed to stand for 3 minutes, then su5cient NaCl added to make the chloride concentration 3.4 formal. 1
-0.1 -0.2
For experiments 16 to 23 a 25-ml.portion of rmanganate wm transferred by pipet to an iodine flask, 4 ml, o c a r b o n tetra-
MIXTuREs 'ITH
KIO: (0.1000 N ) , Normality Found
-0.1
DIDE.
.L"UlUW
cuso4
CuSOc
Added
24 25 26
TITRA~ION OF IODATE-COPPER MIXTURESWITH IODIDE. In the first procedure, 25 ml. of the iodate solution and 10 ml. of the cupric sulfate were transferred to a stoppered flmk. Carbon tetrachloride was added, followed by the volume of concentrated hydrochloric acid required to give the desired acid concentration. Standard iodide was then added I'
0.1067 0.1067 0.1066
TABLE111. TITRATION OF BROMATE-COPPER MIXTVRESWITH IODIDE 0.1 F n +,
Determination of Oxidizing Agents in Presence of Cupric Salts
TaBLE
4.3 4.0 4.3
0 12.5 50
3.2 3.2
466
ANALYTICAL EDITION
June 15, 1942
T.IRLEIT. Expt
2i
29 30
CHLORITE-COPPER MIXTURESW I T H IODIDE
H +, Final Normality
0.1 F CUSO, .4dded 'W. 0 0
Treatmrnt a h
i.0
7.2 7.2 7.2
12..j 25 T.\BLE
a
TITRATION O F
TITRATlON
Y.
H +, Final Normality 2 !1 3.4
Expt 31-35 36-43 Average.
C
C
KClOs Takm
IiClos Calcd.
ME.
M1.
%
25.32 25.11 25.18 25.18
25.21 25.08 25.19 25.18
f0.4 fO.l
O F IODIDE WITH
MIXTURES
Expt
n
1
I. I.
Treatments: with CO:.
TABLEVII.
Error
0.0994" 0,0995a
+0.4a
Treatment
U
Time Min. n ;. " 17
T I T R l T I O N O F I O D I D E WITH TURES B
H +,
CuSO4:KIOs Ratio of Volumes 2:l 1:l 1:2 2:l 1:2 2:1 1:l 1:1 1:2
Experiments 44 to 51 (Table VII) were then made. Those experiments indicated as having been given treatment a were made as were experiments 31 to 43; in those indicated by treatment b the flask was filled with carbon dioxide and the acidified iodide solution saturated with carbon dioxide; in those indicated by treatment c both titrating and titrated solutions mere saturated with carbon dioxide before the titration and the flask was filled with carbon dioxide. I n each experiment 10 ml. of iodide solution were taken. Thus iodate can be determined accurately in a n iodatecopper solution by this method if the solutions are saturated with carbon dioxide previous to titration.
f0.3a
THE PRESENCE
0.01/4 F KIOs Csed
M1, n ns
h li 0024 0.012 U 17 0.012 b 17 a, flask open; b , solution saturated with COz and flask filled
Final Normality 3.4 3.2 4.8 46 3.4 4i 4.8 3.4 48 40 3.4 50 3.4 51 4.8 a Average.
Expt. 44 31-43 4B
7%
KIOs (0,0991 S ) Normality Found
a
0024
0.0 0.0
IODATE-COPPER
TABLE VI. OXIDATION O F I O D I D E BY OXYGEN I N OF COPPER CUSO? Formality
Error
467
IODATE-COPPER h'lIX-
KIOs (00991 N ) , Normality Found 0.1011 0.0995 0.0995 0.0993 0.0992 0.0982 0.0991 0,0990 0.0991
Yo TreatError ment f2.0 a +0.45 a +0.4 u +0.2 b +0.1 b +O.l c 0.0 c -0.1 c 0.0
c
TITRITIOSOF IODIDE w r H IODATE-COPPER MIXTURES. In this procedure (which was developed before satisfactory conditions for the direct titration of the iodate-copper mixture had been found) the iodat'e-copper solution was diluted to an exact volume and a portion of the solution transferred t o a buret. An exact volumc of the standard iodide solution was then pipetted into a flask. Carbon tetrachloride and concentrated hydrochloric acid were added and this mixture, after being cooled, was titrated with the iodate-copper mixture. I n the experiments shoan i n Table V exactly 10 ml. of the 0.04991 formal potassium iodide solution were taken; and the iodate-cop er solution was composed of equal volumes of 0.0991 N (0.0991h formal) iodate solution and 0.1 formal cupric solution. It seemed probable that the positive error was due to air oxidation of iodide. To investigate this, similar solutions of iodide and hydrochloric acid at the concentrations prevailing in a pretitration mixture were allowed to react with the same volume of cupric sulfate In the presence of air and under carbon dioxide. The resulting solutions (approximately 40 ml. in volume) were shaken with 4 ml. of carbon tetrachloride, the carbon tetrachloride was separated in a separating funnel and washed with small amounts of water, hydrochloric acid was added, and the iodine was titrated with 0.01 -V iodate solution to an iodine monochloride end point. The results are listed in Table VI. The iodine found in experiment 1 was due entirely to uncatalyzed air oxidation of iodide. The iodine of the experiments given treatment a was caused by oxidation by cupric ion and air oxidation of iodide, while that of the experiments given treatment b was due to oxidation of iodide by cupric ion solely. The oxidation of iodide by oxygen increased with increase of the copper concentration. This oxidation of iodide by oxygen under these conditions is of interest, since i t is much larger than that usually experienced in the conventional iodometric determination of copper.
Determination of Total Copper and Oxidizing Agent TITR.4TION O F TOTAL COPPER AND IODATE WITH THIOSULFATE. These titrations were made as though the solution? being titrated contained only a cupric salt. Iodate and cupric solutions were transferred by pipet to a flask and 1.5 ml. of 6 N sulfuric acid were added. Three grams of pulverized potassium iodide were added, the solution was swirled, then titrated immediately with thiosulfate solution. Starch was added as the end point was approached, and finally 2 grams of potassium thiocyanate. Table VI11 lists the results. Attempts were also made to titrate with thiosulfate the copper-iodine monochloride solutions obtained from the iodate determinations. These were not satisfactory because of the large volume resulting from the neutralization of the acid solution, the high salt concentration, and the stable emulsion formed by the cuprous iodide and the carbon tetrachloride.
TITRATIOS OF TOTAL COPPERAND PERMAKGANATE WITH THIOSULFATE. The procedure used was the same as that for the determination of total copper and iodate except that 1.0 instead of 1.5 ml. of 6 iV sulfuric acid were used. Table IX shows the results. TITRATION O F TOTAL COPPER AND BROMATE WITH THIOSULFATE. The procedure was that used for the determina-
TABLEVIII. TITRATION O F IODATE-COPPER MIXTURES THIOSULFATE Expt. 52 53 54
TVITH
0.0943 iV cuso4 Taken
0.1487 N KIOs Taken
NazSzOs Used
MI.
M1.
M1.
M1.
%
20 10 10
10 20 20
28.95 29.40 29.41
28.94 29.43 29,43
0 0 -0 1 -0.1
?ia&Oa Calcd
Error
TABLEIx. TITRATION OF TOTAL COPPER AXD PERMANG.4NATE WITH THIOSULFATE Expt. 55 56 57
0.0943 N CuSO4 Taken
0.1068 N KMnO4 Taken
NazSzOa Used
NazSzOJ Calcd.
Error
MZ.
MI.
M1.
M1.
%
20 10 10
10 20 20
29.78 31.05 31.03
29.77 31.03 31.03
0.0 fO.l 0 0
TABLEx. TITRATION O F TOTALCOPPERAND BROMATE WITH THIOSULFATE Expt.
0.0943 N CuSO4 Taken M1.
0.1000 N KBrOs Taken
M1.
SaaSnOa Used .MI.
NazSnOs Calcd. M1.
Error
%
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
468
tion of the-total of copper and iodate. Table X gives the results. The titrations of Table X have a sulfuric acid concentration of 0.15 formal and, where copper is present, no consistent error is noted. Since this acid concentration is somewhat lower than that usually specified for the bromate-iodide reaction ($, 4) especially in view of the results of experiment 60, it is suggested that copper may catalyze this reaction. TITRATION OF TOTAL CHLORATE AND COPPERWITH THIOSULFATE. The chlorate-iodide reaction requires such a high hydrochloric acid concentration that the iodometric determination of copper under these conditions was not found feasible.
Summary Iodate, permanganate, bromate, and chlorate in the presence of cupric salts can be determined by adding an excess of
Vol. 14, No. 6
a standard iodide solution to neutral solutions of the mixtures, acidifying, and back-titrating with standard oxidizing solutions. An alternative method, investigated in the case of iodate-copper mixtures, consists in titrating a standard iodide solution with the mixture, all solutions having been saturated with carbon dioxide previous to titration. The total amount of iodate and copper, of copper and permanganate, and of copper and bromate can be determined iodometrically with thiosulfate.
Literature Cited (1) Foote and Vance, J. Am. Chem. SOC.,57, 845 (1935). (2) Kapur and Verma, IND.ENG.CHEM.,ANAL.ED.,13, 338 (1941). (3) Kolthoff, Pharm. Weekblad., 56, 391-3 (1919). (4) Kolthoff and Furman, “Volumetric Analysis”, p. 387,New York, John Wiley & Sons, 1929. ( 5 ) Swift, J. Am. Chem. SOC.,5 2 , 894 (1930).
Determination of Small Amounts of Gold with Stannous Chloride COLIN G. FINK
AND
GARTH L. PUTNAM, Columbia University, Kew York, N. Y.
Previously known modifications of the widely accepted test for gold, the stannous chloride test, are shown to be qualitatively unreliable because of failure to take into account the factor of acid concentration. So-called colloidal gold may exist in two distinct forms: the yellow form produced in solutions of low acidity, and the purple form produced in solutions of high acidity. The low acidstannous chloride test developed by the authors is more reproducible, sensitive, and reliable than the high acid test.
Factors Influencing Stannous Chloride Test
EFFECT OF CONCENTR-QTIONOF ACID ON T I N T O F COLOR. The alleged variation in tint of color with variation in the gold concentration is the basis of the colorimetric determination as given in standard analytical texts (3, 6, 7 , 10, l a ) . TABLEI. EFFECTOF ACID CONCENTRATION ON TINTOF COLOR
70
N
T
HE purpose of this paper is to present a reliable method for the qualitative detection and estimation of gold.
Notwithstanding the fact that of all metals gold is second only to iron in economic importance ( I l ) , experience has indicated that a rapid and reliable chemical method for the qualitative detection and colorimetric determination of traces of gold has not yet been reported in the standard analytical texts. Heretofore, the most widely accepted colorimetric test for gold has been the stannous chloride “purple of Cassius” test (3, 6, 7 , 10, I,$’), which was discovered by Andreas Cassius in 1663 (8, 9). The authors find that previous descriptions of the test are qualitatively erroneous, as the factor of acid concentration has not been taken into account, and report below a reliable method for the detection and estimation of traces of gold. The following reagents for the detection of gold have been considered, tried, and rejected: hydrogen peroxide, hydrogen peroxide-potassium hydroxide, potassium iodide, potassium iodide-potassium hydroxide, potassium iodide-potassium mercuric iodide-potassium hydroxide, resorcinol, resorcinolpotassium hydroxide, formaldehyde, formaldehyde-potassium hydroxide, benzaldehyde, benzaldehyde-potassium hydroxide, potassium formate-potassium hydroxide, acetylenepotassium hydroxide, sodium thiosulfate, ferrous sulfate, sodium bisulfite, sodium sulfite-potassium hydroxide, hydrogen sulfide, and glucose-potassium hydroxide. When used under the proper conditions, stannous chloride was found to give a more sensitive and more reproducible test than any of the above reagents.
0.002
Yellow 10 8.4 Light brown 0.08 0.16 20 12.9 T a n with faint purple tinge 0.32 10 10.2 T a n with distinct purple tinge 5 0.64 10 13.6 Purple a Includes acid present in stannous chloride reagent. b These values have little bearing on the reproducibility of the test under ordinar analytical conditions, as i t is very d i 5 c u l t t o prepare standards which gave exactly the kame ages and compositions as the unknowns. Values are included to show order of magnitude of experimental error. 1 2
20
10.6
3 4
In order to determine the effect of acid concentration on the tint of color, 40 ml. of a solution containing 0.20 mg. of gold, as bromide, were added to each of ten 50-ml. Nessler tubes, and the solutions were made up to the mark with distilled water and h drochloric acid. The solutions therefore contained 4 mg. of goyd per liter. The Nessler tubes were closed with a rubber stopper (that had been soaked in chlorine water), and inverted twice. To each tube waa then added 1 ml. of 0.25 M stannous chloride (see Reagents), and the tube was again closed with a rubber stopper and inverted twice. One or two groups of ten trials each were made for each acid concentration. After standing for 15 minutes, one of the tubes of each series was taken as the standard and the rest of the tubes were compared with it by the balancing tube method. The results are given in Table I. The data of Table I prove that the acid concentration is an important factor influencing the tint of color in the stannous chloride test. No previous investigator has recognized this (3, 6, 7 , 1 0 , l Z ) . Gold cannot be determined by the stannous chloride method unless the acid concentration of the colorimetric medium is taken into account. EFFECT OF ACID CONCENTRATION ON VELOCITY OF COLOR DEVELOPMENT. As the velocity of color development has an important bearing on the precision and reproducibility of the test, the effect of acid concentration was studied from this
