Rapid and Accurate Determination of Cadmium T. L. THOMPSON, Chemical
T
8; Pigment
HE separation of zinc and cadmium has long been a
Co., Collinsville, Ill.
analysis and will cause no additional work when this method is used. Precipitation should be carried out in a neutral or slightly acidic solution. However, no free nitric acid or other strong oxidizing agent should be present, as i t would liberate free iodine from the iodide as well as destroy the brucine. Free ammonium hydroxide should not be present, though ammonium salts do not interfere. Unlike many other methods, the brucine-iodide precipitation method covers the entire range of zinc-cadmium ratios. Table I shows the results obtained by analysis of mixtures made u p to represent various samples in the processes of the zinc and cadmium smelters.
stumbling block in rapid and accurate analyses in the nonferrous metallurgical fields, and laboratories having a large number of cadmium determinations have sought a more rapid and accurate method. The standard procedures of the present day call for the separation of zinc and cadmium by repeated precipitations with hydrogen sulfide or by electrolysis under carefully controlled conditions of p H and current density, involving the use of various addition agents. Because of the consanguinity of zinc and cadmium, these methods are not satisfactory. Studies have shown t h a t the precipitation of cadmium cannot give a complete separation at any pH (5, 7). Repeated precipitations can only give a closer approximation, with each handling giving greater chance for error. Other objections to this method are the length of time and amount of manipulation required, the toxic nature of the hydrogen sulfide gas, and its destructive effect on the laboratory equipment. The separation of cadmium and zinc by electrolysis has been extensively studied. The mass of variant literature gives a picture of the confusion in this field. Present methods have been shown to give only partial separation ( 1 ) . As yet no electrolytic method has been found completely satisfactory to the large routine laboratory. Recent developments in the organic-metallurgical field have uncovered a number of substances that quantitatively precipitate cadmium with no effect on zinc (2, 4). Most of these are very expensive. However, in 1937 Korenman (3) suggested the precipitation of cadmium from a cadmiumzinc solution by a brucine-potassium bromide mixture, its subsequent filtration, washing, drying, and weighing. The objectionable features of the method were the long period of precipitation, inability to wash clean, and the tendency of the precipitate, due to its great bulk, to decompose, volatilizing the bromine, before completely drying. I t s advantages were the completeness of separation and the cheapness and cleanliness of the chemical. Experiments have shown that if the cadmium is precipitated from the cadmium-zinc solution with a brucine sulfatepotassium iodide mixture and then washed by special means, the precipitate can be dissolved and the cadmium determined by titrating the iodine present, giving accurate and rapid results if carefully carried out.
TABLE
I.
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ZINC AND
CADMIUM
Zn Present Grams
P b Present Gram
Cd Present Gram
Cd Found Gram
Error
40.005 0.0200 0.100 0.270 0,003 2 IO00 0.100 0.0025
0.500
0.0200 0,0201 0.0250 0.0302 0.0301 0.0400 0.0502 0.0500
0.0201 0,0201 0,0249 0,0302 0.0302 0,0402 0.0503 0.0500
f0.l 0.0 -0.1 0.0 +0.1 +0.2
... ...
0:300 0 : 075 0.022
Ratio of Zn to Cd
.MQ.
+O.l
0.0
1000-1 1-1 4-1 9-1 1-10 50-1 2-1 1-20
Standards consisted of commercial cadmium metal running 99.8 per cent cadmium and 0.1 per cent zinc. They were determined in about half a n hour. Zinc spelter of the ordinary grades may be run in less than 2 hours, compared to a previous time required of approximately 7 hours. Below is given the detailed procedure for the analysis of a raw ore running 60.0 per cent zinc and 0.4 per cent cadmium with many impurities. The time required is one third that of a hydrogen sulfide determination of the same accuracy with the same sample. The procedure includes steps made necessary by the complex nature of the raw ores. Procedures for ordinary cadmium samples are normally simple and rapid.
Procedure for a Sulfide Ore SOLUTION OF SAMPLE.Reigh a 5.00-gram sample into a 400-cc. beaker, add 15 cc. of concentrated hydrochloric acid, and boil until all hydrogen sulfide has passed off. Add 15 cc. of concentrated nitric acid and bake. Cool, add 30 cc. of concentrated hydrochloric acid and 5 grams of ammonium chloride, and boil until the volume is reduced to 5 to 10 cc. REMOVAL OF IRON.Dilute to 200 cc. with water and add ammonium hydroxide until the iron is all precipitated, adding sufficient to redissolve the cadmium and zinc. Boil to granulate the precipitate, then filter, washing the beaker three times, using a drop of ammonia with the wash water each time. Wash the precipitate four times with hot water. (If the iron content is above 5 per cent, dissolve, reprecipitate, filter, and wash as above.) Xeutralize the filtrate with concentrated hydrochloric acid t,o the methyl orange end point, adding 7 cc. in excess. REXOVAL OF COPFER. Add 3 to 5 grams of granulated c. P. lead and boil for 10 minutes to precipitate out the copper. Add another full portion of lead and again boil, then test for completeness by adding a few fresh grains of lead. Remove from the heat and decant the liquor. \Tash the lead and copper three times by decantation with hot water, adding the washings to the liquor. SEPARATION OF THE CADMIUM.Add 5 grams of pure magnesium dust to the solution and boil for 3 minutes. Filter through a No. 4 Whatman paper, washing paper and precipitate until free from acid. Repeat the precipitation by adding another portion of magnesium to the filtrate, again boiling, filtering, and washing. Transfer the precipitate to a 150-cc. beaker using 1 to 1 nitric acid and a fine spray of water. Dissolve the sponge in 5 cc. of concentrated nitric acid and take to dryness. Add 15 cc. of concentrated sulfuric acid and again take to dryness. Dissolve with 25 cc. of water and filter off the lead sulfate, using a fine paper. Wash the beaker twice and the paper four times, using only a small volume of water.
Experimental Work A solution containing cadmium-zinc salts, and freed from lead, copper, and iron, is reduced to a volume of 50 cc., if previous manipulations have resulted in excess volume. The original sample is so taken as to give 20 to 50 mg. of cadmium in the solution. It may be necessary to take aliquot portions of the original solution to get this correct cadmium content. For every milligram of cadmium believed present,, 1.5 cc. of a 1 per cent brucine sulfate solution are added, followed immediately by 1.5 cc. of a 10 per cent potassium iodide solution. The solution is well stirred and allowed to stand for 10 minutes, then filtered rapidly through a Buchner funnel using suction. The precipitate is first washed with a 50-50 mixture of the brucine and iodide solutions, then with an organic solvent such as a 1 to 4 mixture of ethanol and toluene until free from potassium iodide. It is then dissolved by heating in water and the iodine content of the organic iodide is determined in any convenient manner. The cadmium content of the sample may be determined by comparison with the iodine value of standards previously determined. Elements, usually present in nonferrous metallurgy, that interfere with the determination are iron, copper, lead, and antimony. These elements are normally removed during any 164
ANALYTICAL EDITION
March 15, 1941
Add 30 cc. of the brucine sulfate solution followed immediately by 30 cc. of the potassium iodide solution. Stir well and let settle for 10 minutes. Filter through a 5-cm. (2-inch) Buchner
funnel, using moderate suction and a very fine paper. Wash the beaker with a 50-50 mixture of the stock solutions (mixed immediately before use) three times and the paper three times with a-1 to 4 mixture of ethanol and toluene. ESTIMATION.Wash the precipitate and paper back into the beaker with water, making up to 100 cc. Heat until the precipitate has dissolved. Add 5 cc. of a 0.5 per cent solution of eosin Y 9s an indicator and titrate to the absorption end point with 0.03 N silver nitrate (6). The titration value is compared to that of a standard test to give the amount of cadmium in the sample.
Notes I n making u p the brucine sulfate solution, the addition of sulfuric acid u p to 5 per cent is helpful in the solution of the brucine and in the adjustment of the acidity during the precipitation. Care must be taken, however, not to oxidize the brucine. For best results, a fresh solution of brucine sulfate should be made every day. Excessive volume for the precipitation may cause a colloidal precipitate. Difficult filtrations may be aided b y judicious choice of filter papers and filter aids. If there is a large mass of precipitate, it may hold some of the iodide solution, raising the titer and affecting the factor. It may be necessary to dissolve and reprecipitate the cadmium. The work requires only a few minutes and may be well worth the time.
165
As in most analytical procedures, directions must be minutely followed, and a definite technique established. Careless work or minor differences in procedure may vitiate all results. Short cuts or the removal of additional elements will doubtless cause a different titer value. Careful work and a n established technique, always qualities of a good analyst, are fully repaid b y extremely accurate and rapid determinations of cadmium that will effect savings in time and money, especially in the large routine laboratories of smelters and metal-working plants.
Acknowledgment Acknowledgment should be made of the valuable assistance of L. H. Haslip of the American Zinc Company of Illinois for his advice in formulating routine laboratory procedures and of the cooperation of the American Zinc Company for the use of its laboratory in which a portion of the work was done.
Literature Cited (1) Banner, J., 2.anal. C h m . , 75,321-71 (1928). (2) Goto, H., Science Repts. TBhoku Imp. Univ., First Ser., 26, 391413 (1937). (3) Korenman, I. M., Zavodskaya Lab., 6, 1461 (1937). (4) Mahr, C.,Mikrochem. Acta. 3, 300-3 (1938). (5) Reese, J. T., Bur. Mines, Bull. 22, 71-84 (1937). (6) Scott, “Standard Methods of Chemical Analysis”, New York, D. Van Nostrand Co., 1929. (7) Treadwell and Hall, “Analytical Chemistry”, 8th ed., Vol. 11, p. 198, New York, John Wiley & Sons, 1939.
Detection of Sodium Alginate in Dairy Products CHARLES W. SCHROEDER AND PHILEAS A. RACICOT Massachusetts Department of Public Health, Boston, Mass.
A
MONG the thickening agents being used a t the present
time in the dairy industry is the sodium salt of alginic acid, derived from certain seaweeds and put on the market in various forms. The product which is probably most widely used in this country is sold under the trade name of Dariloid, in which sodium alginate is mixed with other substances including dextrin, sucrose, and a phosphate. Other commercial products may consist of the pure alginate. The present investigation was undertaken because of a lack of a specific qualitative test for alginic acid. The test is needed because of the widespread use of alginates in several dairy products, especially ice cream; and because of the possibility of illegal use in milk, cream, or other dairy products. Inasmuah as the commercial product is likely to contain dextrin, a quick preliminary test for dextrin may be used to establish the probable presence of alginic acid in milk or cream. Likewise, milk or cream containing the commercial product is likely to give a positive Rothenfusser test (1) for sucrose.
Tests OPTIONAL PRELIMINARY TESTFOR DEXTRIN. Take 10 cc. of milk or cream, dilute with 10 cc. of distilled water, and add 10 cc. of a 20 per cent solution of trichloroacetic acid. Shake vigorously for 1 minute, let stand for 5 minutes, and a t e r . Make the filtrate alkaline with ammonium hydroxide, evaporate on a steam bath to a volume of about 1 cc., a t e r into a test tube, cool to room temperature, and add 0.05 N iodine solution cautiously to the atrate, a small drop a t a time. A violet to reddish-violet color indicates the possible presence of 0.1 per cent or more of Dariloid. SPECIFICTESTFOR ALGINICACID. To 20 grams of milk, cheese, cream, or ice cream add a volume of concentrated hydro-
chloric acid approximately equal to the water content of the sample taken. Shake thoroughly, add a little sand (acid-washed and ignited), bring to a boil, and boil 30 seconds with frequent shaking. Transfer to a 50-cc. centrifuge tube with the aid of 10 to 20 cc. of alcohol. Centrifuge 10 minutes, or until the solid matter forms a compact cake a t the bottom of the tube, and decant as much of the supernatant liquid as possible from the solid matter, taking care not to lose any of the solids. The centrifuge should be warm enough to keep the fat liquid and should be allowed to slow down without braking, to avoid stirring u the solids. I n some cases, the fat mixed with part of the solids at the surface a dense cake which must be punctured with a stirring rod to allow removal of the liquid. The decantation is best carried out against a white background to facilitate observation of the solids through the dark solution. Wash the solids repeatedly with 75 per cent ethyl alcohol by shaking, centrifuging, and decanting as above, until the wash solution is neutral to litmus paper and is colorless. Then wash twice with ether in the same manner. Evaporate the last traces of ether by warming the tube in a beaker of hot water and directing a current of air into it. Dissolve the residue so far as possible in 10 cc. of 0.1 N sodium hydroxide by shaking a few seconds. Filter, wash the tube and paper with a few cubic centimeters of water, and to the filtrate add an equal volume of 95 per cent ethyl alcohol. Centrifuge 10 minutes, decant the supernatant liquid, and wash the solids with 75 per cent alcohol by centrifuging and decantation until the wash solution is neutral to litmus paper. The solution should be centrifuged and decanted even when it appears clear, as small amounts of gum are invisible beforehand. If the separated gum is now white and free from any yellow or brown color, pass over the next paragraph. Suspend the gum in 10 cc. of distilled water and add 0.1 N sodium hydroxide dropwise, with shaking, until the solution is just alkaline to litmus paper. Add 6 cc. saturated magnesium nitrate solution and shake thoroughly. Centrifuge 10 minutea and decant the supernatant liquid from any precipitate into another centrifuge tube. Discard the precipitate and make the solution acid with a drop of concentrated hydrochloric acid.
perms
