Shorter Method for Determining Copper I odometrically T. H. WHITEHEAD AND H. S. MILLER, University of Georgia, Athens, Ga.
T
H E investigations of Haen (S), Rumpler (7), and Gooch and Heath (2) have established the fact that the determination of small amounts of copper (150 mg.) can be accomplished accurately, precisely, and quickly in the presence of ions which interfere in several other methods. However, there is disagreement among these investigators as to the effect of small quantities of mineral acids on this determination. Gooch and Heath say that not more than 3 per cent by volume of hydrochloric, nitric, or sulfuric acid can be present, but Moser (6) says that the presence of 5 per cent of sulfuric acid actually improved the determination. This question of the amount of mineral acids allowable is important because in the present method of expelling these acids (1) it is necessary to heat to the point of solidification. This procedure is not only time-consuming, but usually causes bumping, with a consequent loss of some material from a beaker even when covered with a watch glass. I n addition to this, there is danger of heating too long and producing basic copper salts which are relatively insoluble in acetic acid (1). Moser checked his results electrolytically, whereas Gooch and Heath did not. It was believed that a repetition of their experiments under uniform conditions might point the way to a shorter method, or to a shortening of their methods.
reduced to about 5 cc. Cool to 25" C., and add 5 cc. of 8 N acetic acid, dilute to a total volume of 50 cc. with distilled water, and titrate with standard thiosulfate solution to within about 2 cc. of the end point, add 3 cc. of 1 per cent starch solution, an$ titrate t o disappearance of color. TABLEI. EFFECTO F ACIDS(8 N', ON IODOMETRIC DETERMINATION OF C'OPPER
N az Sz0s ACID SOLN. ADDED REQUIRED
cc.
Gram
Gram
5 10 25 50
11.40 11.53 11.73 13.15
0.06224 0.06266 0.06402 0.07175
5 10 25 50
11.32 11.40 11.65
0.06181 0.06224 0.06361
0.06158 0.06158 0.06158 0.06158
+0.00066 f0.00107 +0.00242 +0.01017
0.06158 0.06158 0.06158
+O. 00023
NITRIC ACID
?a
.....
.....
$0.00066 + O . 00203
.. .. ...
SULFURIC ACID
5 10 25 50
11.32 11 38 11.54 ?a
0.06181 0.06210 0.06301
0.06158 0.06158 0.06158
.....
.....
$0.00023 +0.00052 +0.00143
... . . . ,
ACETIC ACID
5 10 25 50
11.31 11.31 11.30 11.30
0.06176 0.06176 0.06170 0 06170
5 10 25 50
11.30 11.30 11.30 11.30
0.06170 0.06170 0.06170 0.06170
0.06158 0.06158 0.06158 0.06158
+0.00018 +0.00018 +0.00012 +0.00012
TARTARIC ACID
ACIDS. Sulfuric, hydrochloric, nitric, acetic, and tartaric were of c. P. quality, but were each tested by the methods of Murray (6) prior to use and found t o be satisfactory. SODIUM THIOSULFATE, Nak&O*.5HzO, was of c. P. quality, pure, tested according t o Murray (6) and standardized against electrolytic copper. COPPER. NITRATE,Cu(N0~.)~.3Hz0.These crystals were of c. P. quality, tested according t o Murray. The solution used was made by dissolving 5.6 grams in a liter of distilled water at 20" C. The copper content was determined electrolytically by the method given by Hillebrand and Lundell (4). STARCH SOLUTION.One gram of Baker's soluble starch was heated to boiling in 100 cc. of distilled water and cooled t o 20" C. before use.
0.06158 0.06158 0.06158 0.06158
+o.
00012 +0.00012 +0.00012 +0.00012
End point wa8 so variable that no reliable reading could be obtained, but results would have been very high.
This method approximates a method which would be used in getting copper samples into solution, and so has general application. If other proportions of acids were used, the final solution would be similar to this one, because of the higher boiling point of the sulfuric acid present. TABLE11. COMPARISON OF MODIFIED METHOD WTH STANDARD AND WITH ELECTROLYTIC METHOD
-
Cu FOUNDBY MODIFIED METHOD
METHODUSED
RESULTS The results listed in Table I show that large quantities of mineral acids do cause high results in the iodometric determination of copper, but that 5 cc. of these acids do not cause much higher results than 5 cc. of acetic acid. This suggested that heating all the way to dryness was unnecessary, and a modified method was tried. The modified method was to add to 50 cc. of copper nitrate solution, 5 cc. of sulfuric acid, 5 cc. of nitric acid, and 5 cc. of hydrochloric acid, Boil gently until the total volume is
Gram
DEVN. BETWEDN
METHODS
HYDROCHLORIC ACID
REAGENTS
The general method employed was to take a 50-cc. aliquot portion of the copper nitrate solution, heat till it would just solidify on cooling to room temperature, add the specified amount of mineral acid (see Table I), dilute to 50 cc. total volume with distilled water if necessary, and titrate with standard solution of sodium thiosulfate. All acids were 8 N to compare with the 8 N acetic acid used in the standard methods.
cc.
COPPER FOUND Electrolytic
Iodo-
metric
Gram 0.06181 0.06170 0.06181 Av. 0.06177
Cu FOUND BY Cu FOUNDBY STANDARD METHODELECTROLYTIC MBTHOD Gram 0.06170 0.06181 0.06170
0.06174
Gram 0.06158 0.06158 0.06158 0.08158
The per cent difference between the modified and the standard methods is therefore 0.00003 X 100/0.06174 = 0.048 per cent ( part per 1000). From the foregoing it will be seen that no loss in either precision or accuracy occurs if evaporation of the mineral acids in the standard iodometric method of determining copper is not carried entirely to dryness, provided this evaporation is carried out in the presence of a small quantity of sulfuric acid. This saves time and also does away with the danger of forming insoluble basic copper salts. The authors think that Gooch and Heath were too conservative in their limit of mineral acids allowable for ordinary analytical work. On the other hand, no evidence was found for the statement of Moser that 5 per cent of sulfuric acid improves the determination, and in this they agree with the findings of Gooch and Heath. 15 ~
ANALYTICAL EDITION
16
It is significant that tartaric acid gives more precise reacetic acid, and the senior author intends to follow
SUltS than
this investigation in the hope of explaining the interference of certain acids in this determination. LJTERATURE CITED (1) Fales, “Inorganic Quantitative Analysis,’’ p. 358, Century, 1925. (2) Gooch and Heath, 2. anorg. Chem., 55, 119 (1907).
VoI. 5 , No. I
(3) Haen, Ann., 91, 237 (1854). (4) Hillebrand and Lundell, “Applied Inorganic Analysis,” p. 198, Wiley, 1929. (5) M ~ z, anal. ~ Chem., ~ ~ 43 ,597 (1904), (6) Murray, “Standards and Tests for Reagent Chemicals,” Van ’ Nostrand, 1920. (7) Riimpler, J. prakt. Chem., 105, 193 (1868); Z. anal. C h m . 8 , 465 (1868). R ~ C E I Y EJune D 9, 1932.
Determination of Cadmium A Critical Study of the Evrard Method LORENC. HURDAND RICHARD W. EVANS, University of Wisconsin,
A
LTHOUGH cadmium is a fairly common constituent of many zinc ores and concentrates, there does not exist a rapid and specific method for its determination. In a recent communication Evrard (3) reported a method which appeared to be simple and accurate and which functioned in the presence of large amounts of zinc. Inasmuch as such a method is sorely needed, the writers thought it worth while to repeat the work of Evrard and investigate the possibilities of the method. Evrard’s determinations indicated that the cadmium iodide addition product of allyl iodourotropine was quantitatively insoluble and could be represented by the formula CdIz[ (CH2)6N&Hd]2. Inasmuch as the molecule contains but 11.44 per cent of cadmium and in light of its reputed insolubility in the presence of excess precipitant or in 95 per cent alcohol, the determination appeared to offer excellent possibilities. Zinc, according to the original author, had little effect upon the determination, for in the presence of large amounts of a zinc salt and in moderate concentrations of sulfuric acid a recovery of 99.7 per cent of added cadmium was obtained.
EXPERIMENTAL PROCEDURE PREPARATION OF MATERIALS.Allyl iodide, prepared according to the method of Datta ( I ) , was added to an equimolar solution of urotropine in chloroform. The melting point of the precipitated allyl iodourotropine checked with accepted values. A 10 per cent aqueous solution was used throughout the work, Cadmium sulfate was purified by subjecting c. P. material to the treatment described by Reilly (4). The recrystallized product, freed from iron and zinc, was dehydrated in vacuo, ignited in quartz, cooled, moistened with sulfuric acid, and again ignited. The anhydrous salt was dissolved in water to give solutions containing approximately 0.001 gram of cadmium per cubic centimeter. RESULTSOF ANALYSIS.Evrard reported little detailed information regarding the exact procedure followed in the analysis of his solutions. As nearly as the writers were able to determine, the precipitations were carried out by adding the cadmium solutions to the reagent. This order was followed in a large number of cases, but it was found that the characteristics of the determination were such that the order of addition was relatively unimportant. The precipitated complex was found to be a white flocculent compound which, in the absence of excess precipitant or other electrolyte, exhibited a marked tendency to pass into the colloidal state. In a series of preliminary analyses an attempt was made to simulate as nearly as possible the conditions recommended by Evrard. The precipitations were
Madison, Wis.
made in the cold, and all washings were made in the prescribed manner with a dilute solution of the reagent followed by small portions of 95 per cent ethyl alcohol. An average recovery of 96.4 per cent of the added cadmium was obtained. Although a ninefold excess of precipitant was used in all of the preliminary experiments, it was thought that a further increase might result in a more nearly quantitative recovery. Accordingly a study was made of the effect of excess reagent. It was found that, when an amount of allyl iodourotropine representing a fourteen fold excess was employed, the apparent recovery reached 116.1 per cent. Decreasing the amount of reagent resulted in decreased recoveries. From a study of the data obtained during the execution of some hundred analyses, it became apparent that erratic recoveries of added cadmium resulted from two principal causes. Incomplete removal of reagent or salt adsorbed by the precipitate gave rise to a positive error, whereas the actual solubility of the precipitate contributed to significant negative errors. I n such cases it is only by a fortuitous balance of conditions that results approaching theoretical are obtained. Inasmuch as the precipitate is somewhat less soluble in alcohol than in water, it appeared worth while to attempt to carry out the determination in alcoholic solution. Accordingly numerous analyses were made on solutions, the ethyl alcohol content of which varied from 45 to 67 per cent by volume. Cadmium recoveries ranging from 32.5 to 129.4 per cent were obtained. Prolonged washing with ethyl alcohol dissolved a major portion of the precipitate. Although solutions of zinc salts do not yield precipitates when added to allyl iodourotropine solutions, the presence of zinc has a marked effect upon the cadmium determination. Apparent recoveries of as high as 132 per cent of added cadmium were obtained when the determination was carried out in the presence of small amounts of zinc. The obvious inference is that zinc is co-precipitated or otherwise carried out of solution when the cadmium complex is precipitated. Sulfuric acid in concentrations of 0.01 to 0.05 M had little effect on the solubility of the precipitate. Only when the concentration exceeded 0.06 M , was the relative percentage of cadmium recovered greatly decreased. A series of solubility determinations carried out a t 25” * 0.5” C. indicated that the complex was soluble to the extent of 0.0014 gram per cc. of water. The solubility in ethyl alcohol was somewhat less, an average of 0.00062 gram per cc. being obtained. The use of allyl iodourotropine as a qualitative reagent was also investigated. It was found that a number of metallic ions formed insoluble complex derivatives when treated with the reagent. This is in harmony with the results of
