Determination of Cadmium in Silicate Rocks E. B. SANDELL University of Minnesota, Minneapolis, Minn.
F
chloric acid to effect as complete solution as possible. Add 2 or 3 ml. of sodium citrate and a small crystal of hydroxylamine hydrochloride, neutralize with ammonium hydroxide, and add 2 drops in excess. Extract the main solution (filtrate from any insoluble material.) as follows: Add 5 ml. of 0.02 per cent dithizone and shake in a separatory funnel for 0.5 to 1 minute; allow the carbon tetrachloride to settle and draw it off into another separatory funnel. If the carbon tetrachloride drawn off is not greenish, add 2 t o 3 ml. more of dithizone to the main solution, shake, draw off, and repeat until the carbon tetrachloride layer shows a greenish color after shaking for 1 minute. In like manner extract the solution resulting from the sodium carbonate fusion, using 1-ml. portions of dithizone in this case. Combine all the dithizone extracts and shake with 5 ml. of water; discard the water layer. Shake the carbon tetrachloride extract vigorously with 5 ml. of 0.01 N hydrochloric acid for 2 minutes. Draw off the carbon tetrachloride layer and repeat the shaking with a fresh 5-ml. portion of 0.01 N hydrochloric acid. Combine the acid extracts and discard the carbon tetrachloride. Shake the hydrochloric acid extract with small portions of carbon tetrachloride to remove any colored droplets of carbon tetrachloride in the solution. Finally all droplets of carbon tetrachloride should be separated from the aqueous layer; care should be taken that a film of tetrachloride does not remain on the water surface. The loss of a dro or two of aqueous phase in removing carbon tetrachloride will no harm. Transfer the solution to a flat-bottomed glass-stoppered tube, 1.8 X 15 cm., and rinse the separatory funnel with a milliliter or two of water. Prepare a series of standards in similar tubes containing, for example, 0, 0.05, 0.1, 0.15 . . . . . . microgram of cadmium, dilute with 0.01 N hydrochloric acid to the same volume as the unknown, and mix. Add 2.5 ml. of 25 per cent sodium hydroxide solution to each tube and mix. Then add 1.0 nil. of 0.001 per cent dithizone solution and shake vigorously 10 to 15 times. Compare the colors of the carbon tetrachloride layers by viewing the tubes transversely against a white background. The colors fade on standing, and the hue also changes, so that the color comparison should be made immediately after shaking. The hue of the unknown should be the same as that of the standards. Run a blank on the reagents, taking double the amounts used in the determination itself.
ISCHER and Leopoldi (1) have shown that cadmium can be determined in the presence of zinc, lead, and certain other metals by shaking a solution containing 5 per cent of sodium hydroxide with a solution of dithizone in carbon tetrachloride; cadmium goes into the carbon tetrachloride as the dithizonate, whereas zinc and lead remain in the aqueous phase. Use can be made of this behavior in the determination of the minute amounts of cadmium occurring in silicate rocks. The first steps of the method are practically the same as those in the determination of copper, zinc, and lead in rocks (4). The heavy metals of the sample are isolated by shaking the ammoniacal citrate solution of the decomposed rock with a carbon tetrachloride solution of dithizone. The carbon tetrachloride phase is separated and shaken with 0.01 N hydrochloric acid. Zinc, lead, and cadmium dithizonates are thus decomposed, and these metals go into the aqueous phase as the chlorides; copper and cobalt remain in the carbon tetrachloride. Cadmium can then be determined in the aqueous phase by Fischer and Leopoldi’s method. According to Goldschmidt ( 2 ) the average cadmium content of magmatic rocks is 5 x 10-5 per cent, although he states that this value is very uncertain. According to I. and W. Yoddack (3) the average value is 2 X per cent. The few igneous rocks examined in connection with the development of the method here described showed cadmium contents ranging from about 1 to 2.5 x 10-5 per cent, not far from the limit of the method. When a 0.5-gram sample is taken, 0.02 or 0.03 microgram of cadmium, corresponding to 5 X per cent, can be detected with certainty.
&
Special Reagents Dithizone, 0.02 per cent (weight by volume) in carbon tetrachloride. Dithizone, 0.001 per cent (weight by volume) in carbon tetrachloride. One milliliter of this solution shaken with 10 ml. of redistilled water and 2 to 3 ml. of 25 per cent sodium hydroxide solution should yield a colorless carbon tetrachloride layer. This solution should be prepared shortly before use by diluting the stronger dithizone solution. Sodium citrate, 10 per cent. The solution should be freed from heavy metals by adding a few drops of concentrated ammonium hydroxide to 100 ml. and shaking with successive small portions of 0.01 or 0.02 per cent dithizone in carbon tetrachloride until the latter shows only a very faint pink color. Sodium hydroxide, 25 grams in 100 ml. of solution. Hydrochloric acid, approximately 0.01 N . Dilute 1 volume of concentrated acid with 1000 of redistilled water. The solution should be shaken with a few milliliters of 0.01 er cent dithizone in carbon tetrachloride and decanted from the fatter before use.
TABLEI. DETERMINATION OF CADXIVII Sample Granodiorite5
Cd Present lo-%
Cd Found
Error
io-%
i0-57~
....
4 4
4.5
fO.5
0
,...
5 6 7
5 5.5 6.5 3 4.5
+1 0 -0.5 -0.5 -1 4-0.5
1.0 2 3 3 3 3
0.8 1.5
Addition
,...
.... o ..0o2i b%j ki, 0 NI Extractedb solution of granodiorite
Procedure Weigh 0.5 gram of rock powder into a platinum dish and add a few milliliters of water, 1ml. of 70 per cent perchloric acid, and 5 ml. of hydrofluoric acid. Evaporate to dryness, add 0.5 ml. of perchloric acid and a few milliliters of water t o the residue, and again evaporate to dryness to expel excess perchloric acid. Moisten the residue with 1ml. of concentrated hydrochloric acid, add 5 ml. of water, and heat near the boiling point until all soluble matter has been brought into solution. Add 10 ml. of sodium citrate solution and 0.1 gram of hydroxylamine hydrochloride, neutralize with concentrated ammonium hydroxide using litmus paper, and add 2 drops in excess. If the solution is appreciably turbid at this point, filter through a small filter paper, wash with small portions of water, and ash the paper at a low temperature. Grind the residue with 0.15 to 0.2 gram of sodium carbonate in an agate mortar, transfer the powder to a platinum crucible, and fuse. Leach the melt with water, filter through paper, wash with a few milliliters of water, rinse the insoluble material out of the paper, and heat with dilute hydro-
.... ....
4 4
3 3
-0.2 -0.5 0 0
0.01% Z n 0 01% P b 0 Olb%~C, 33.5 0 . 0 1 5 % Co 3 0 0.04% NI 0 01% N i , 0 . 2 % M n 4 3 -1 0 5 X 10-3% Cd, 0.006% Zn, 0 004% Cu, and 0.002%
+p
Contained 1 * d Solution of granodiorite extracted with dithizone in carbon tetrachloride
a
Pb
t o remove cadmium originally present, and oadmium then added t o the extracted solution.
Discussion Some of the results obtained in applying the procedure as described are shown in Table I. There is a tendency for the results to be low, but it is believed that the method is useful for showing the approximate cadmium content of a rock. Copper, zinc, lead, and cobalt do not interfere, a t least not in the amounts that these elements are likely t o be en364
JULY 15, 1939
365
ANALYTICAL EDITION
”
countered in igneous rocks. A mixture of 2.5 ml. of 25 per cent sodium hydroxide and 10 ml. of 0.005 per cent pure zinc solution (corresponding to 0.1 per cent of zinc in a 0.5-gram sample) shaken with 1ml. of 0.001 per cent dithizone solution gave a trace of pink color in the carbon tetrachloride layer which was distinctly less than that produced by 0.05 microgram of cadmium. Tin, bismuth, silver, and thallium are also without effect in small amounts. Manganese shows a tendency to prevent the complete extraction of cadmium, but this difficulty is overcome by the addition of hydroxylamine hydrochloride. The element most likely to give trouble is nickel. Nickel dithizonate in carbon tetrachloride solution is partially decomposed by shaking with 0.01 N hydrochloric acid, so that more or less nickel will go into the final solution with cadmium. If more than a very small amount of nickel is present in this solution, it will impart a brown color to the carbon tetrachloride layer, the pink color of cadmium dithizonate is then obscured, and the determination becomes impossible. The solution of this difficulty lies in preventing, as far as possible, the extraction of nickel from the ammoniacal citrate solution.
If the solution is made barely ammoniacal most of the nickel will remain in the aqueous phase and all, or nearly all, of the cadmium will be extracted. As much as 0.03 or 0.04 per cent of nickel may be present in the sample without causing difficulty if care is taken to avoid an excess of ammonia. Larger amounts of nickel are likely to cause trouble, so that the method given is not applicable to all rock samples. Moreover, large amounts of any metal extracted by dithizone (copper, zinc, etc.) are undesirable, because the complete removal of cadmium from a large volume of carbon tetrachloride phase by shaking with 10 nil. of 0.01 N hydrochloric acid may be difficult. However, in most silicate rocks the quantity of metals extracted by dithizone is small and the method described is adequate.
Literature Cited (1) Fischer, H., and Leopoldi, G., Mikrochim. Acta, 1, 30 (1937). (2) Goldschmidt, V. M., Skrifter Norske Videnskaps-Akad. Oslo I .
Mat. Natur. Klasse, 1937, No. 4, p. 32. ( 3 ) Noddack, I., and Noddack, W., Svensk Kern. Tid., 46, 173 (1934). (4) Sandell, E. B., IND. ENG.CHEX.,Anal. Ed., 9, 464 (1937).
Colorimetric Determination of Nickel As Nickel-Ammonia Complex Ion GILBERT H. AYRES AND FRANCENE SMITH S m i t h College, Northampton, Mass.
THE
rapid development of photoelectric colorimeters and the wide application of colorimetric methods of analysis suggested to the authors the possibility of determining nickel on the basis of the blue color of nickel-ammonia complex ion, the color being measured photoelectrically. Such a method might find some application in steel and ore analysis, being simple and rapid.
Development of the Method APPARATUS AND REAGENTS. I n the proposed method the concentration of nickel was indicated by the spectral transmission of blue nickel-ammonia complex ions. The instrument employed in measuring the transmission was the Yoe photoelectric colorimeter (4). Special grades of nickel salts low in cobalt and copper were used; all other reagents were of analytical reagent quality. Stock solutions, one each of nickel chloride, nickel nitrate, and nickel sulfate, were made up to contain approximately 5 mg. of nickel per ml., and were standardized gravimetrically with dimethylglyoxime (1). Suitable dilutions of the stock solutions were used in formulating the calibration curve. EFFECTOF AMMONIACONCEXTRATION. A preliminary study of the variations in transmission by nickel-ammonia ions u i t h concentration of ammonia was made in order to determine what concentration of ammonia should be used. From the nickel sulfate solution, two series of standards containing varying amounts of ammonia were prepared; one series had a nickel concentration of 200 mg. per liter and the other 2000 mg. per liter. At an ammonia concentration of 1.5 N the solutions were characterized by a blue color and by the complete absence of insoluble basic salt. With increasing concentration of ammonia, the blue complexes gave way
to the violet complexes, the coloration becoming constant a t about 2.5 N ammonia. Since, in the Yoe instrument as used without color filter, the blue solutions absorb more light than do the violet, they provide the more sensitive means for the determination of nickel. The instrument was therefore calibrated for nickel determination on the basis of the color produced in a solution 1.5 N in ammonia. EFFECTOF ANIONS. From the stock solutions of nickel chloride, nickel nitrate, and nickel sulfate, three series of standards were prepared. The ammonia concentration in all cases was 1.5 N , but vithin each series the nickel concentrations varied by convenient intervals. In Figure 1, the plot of values of log R/50 against nickel concentration shows that the system follows Beer’s law only for nickel concentrations up to about 600 mg. per liter. It is obvious that the anion has no influence on the transmission. EFFECTOF AimfoNIUM SALT. Since application of this method of analysis would, in all probability, involve making the determination in the presence of ammonium salt, the effect of this factor was investigated. The samples tested covered a nickel concentration range from 50 to 2000 mg. per liter and ammonium salt concentration up to 3.0 N . The presence of ammonium salt up to 1.5 N had no appreciable effect on the transmission. EFFECT OF TIME. Solutions read a t definite time intervals over a period of 150 hours showed no significant variations of transmission with time. EFFECTOF COBALT. Because of the frequent occurrence of cobalt with nickel, a short study was made of the effects of the presence of cobalt on the colorimetric estimation of nickel. Using standardized solutions of cobalt chloride and nickel chloride, a series of solutions was prepared which contained 500 mg. of nickel per liter and varying concentrations
,
