Hydrolytic Precipitation of Cadmium Selenide from Selenosulfate Solutions E. C. PITZER
A-D
N. E. GORDON. The Johns Hopkins University, Baltimore, Md., and Central College, Fayettc, Mo.
C
ADhIIUlM selenide, despite its cost, has found application as a pigment in paints and ceramics. As a source
of hydrogen selenide, it should also be of interest in the production of certain organic chemicals. I n the paint industry, cadmium selenide is usually eo-precipitated with barium sulfate by mixing solutions of cadmium sulfate and barium sulfide in which selenium has been dissolved ( 3 ) . For use in ceramics, pure cadmium selenide may be made by a number of methods, most of which are not feasible commercially (8). As a n intermediate in certain organic syntheses, cadmium selenide offers many advantages, as pure hydrogen selenide is readily obtained upon acidification, and the soluble cadmium salt obtained in the generator is readily reconverted t o selenide, thus effecting a material economy in the process herein described. The proposed method (patent pending, assigned t o the Chemical Foundation) for the preparation of cadmium selenide is based upon a n experiment of Rathke ( 4 ) , who found that upon addition of cadmium sulfate t o potassium selenosulfate a a h i t e precipitate is formed, which changes through various shades of yellow and red to brown. By boiling this precipitate with dilute hydrochloric acid, he obtained cadmium selenide. Unfortunately, Rathke’s article does not disclose his analytical methods. This reaction, rediscovered in the authors’ laboratories, was found to be profoundly influenced by the temperature of precipitation, the acidity of the solutions, the rate of precipitation, and the amount of illumination.
TABLEI. REPRESENTATIVE RESULTS Temperature of Precipitation
(Pure CdSe = 4126% Pe) Remarks
Se in Precipitate
knitated 17 hours Aiitated 8 hours Boiled 1 hour. Stood 17 hours a t room temperature
26 8 36.4 37.2
c. 20
65 100
%
For this experiment, sodium selenosulfate was prepared by agitating amorphous selenium with a solution of sodium sulfite. An equilibrium is established, NazS03
+ Se * NanSSeOl
the extent of the reaction depending upon the temperature. Apparently, the ion SSe03-- is formed, analogous to S z 0 3 - - . The latter slowly decomposes, yielding S--,by a reaction for which several mechanisms have been proposed (1, 5 ) . It is apparent that a number of reactions take place when solutions of cadmium sulfate and sodium selenosulfate are mixed. .4n over-all equation for the main reaction may be written :
Cd’-
+ SSe03-- + H20--+
CdSe
+
2H+
+ SO,--
Experimental When cadEFFECT OF TEMPERATURE UPON PRECIPITATIOX. mium sulfate and sodium selenosulfate solutions are mixed in the stoichiometric ratio, precipitates of low purity and variable composition are obtained. Inasmuch as cadmium sulfite is only sparingly soluble, it precipitates along with the selenide, and the conversion of the former into the latter is very slow if the stoichiometric ratio of cadmium sulfate and sodium selenosulfate are used. Accordingly, the expedient
was adopted of using a n excess of 25 per cent of the latter compound. Table I gives representative results obtained a t various temperatures. Only selenium was determined during the preliminary experiments, and the selenium content was used as a criterion of purity, because of the length of time required for a complete analysis. EFFECT OF ACIDITYUPON PRECIPITATIOK. Accurate pH measurements with ordinary electrodes are not feasible in these solutions because of disturbing “redox” potentials and a tenaciously adherent film of cadmium selenide which separates out upon the surface of the reaction vessel and electrodes. Purely arbitrary limits of acidity and alkalinity were accordingly selected. Assuming that acidification of sodium sulfite proceeds in tivo more or less distinct stages
SO3--
HSOI-
+ H’ + HSO3+ H - +HzS03
it is evident that the limiting acidity compatible with a n appreciable (SOs--) [hence with a n appreciable (SSeOa--) ] is that of a solution of HS03-. Accordingly, a saturated solution of selenium in sodium bisulfite was prepared and treated with cadmium sulfate. X o precipitate formed in the cold even after 2 hours, but upon heating the solution to boiling and allowing it to cool a precipitate formed, which, upon drying a t 100” C., contained 30.6 per cent of selenium. The limiting practicable alkalinity was considered to be that of a n ammoniacal solution of cadmium sulfate containing just sufficient ammonium hydroxide t o prevent the precipitation of cadmium hydroxide. After boiling a solution prepared in this manner, a precipitate containing 37.1 per cent was obtained. Added acid or alkali in even very small amounts evidently produces an inferior product. Optimum results are obtained a t that acidity a t which the reactants sodium selenosulfate and cadmium sulfate come t o equilibrium. EFFECTOF SPEEDOF PRECIPITATION UPON COMPOSITIOS. Since pure cadmium selenide could not be obtained even when %eo3-- was present in 25 per cent excess, the next phase of the problem was to determine whether a pure product could be precipitated by using such a small concentration of one reactant that precipitation would take place very slowly. To use a large excess of cadmium sulfate and a trace of sodium selenosulfate plus sodium sulfite solution would be pointless, as enough C d f f would be present to precipitate SO,--as well as Se--. On the other hand, on adding a trace of cadmium sulfate t o a substantial amount of sodium selenosulfate plus sodium sulfite solution, the product should be pure cadmium selenide as long as the solubility product of cadmium sulfite is not exceeded. I n a n actual test, a cold dilute solution of cadmium sulfate was added to a cold solution of sodium selenosulfate plus sodium sulfite as long as the momentary white turbidity disappeared upon stirring. The solution was boiled gently for a n hour, during which time a brownish crystalline precipitate formed of the composition Cd 5 8 . 7 0 7 Se 41.25d 99.95%
68
FEBRUARY 15, 1938
ANALYTICAL EDITION
69
hexavalent selenium by sulfur dioxide.) The selenium was weighed as usual. c.4DMIUM DETERMINATION. A 0.1000-gram sample was disCd 58.74% 8e 4 1 . 2 6 % solved in nitric acid in an Erlenmeyer flask equipped with a boiling valve to trap the spray. The excess acid was removed 100.00% by evaporat’ion, and the residue was fumed with 1 cc. of sulfuric acid. The resulting cadmium sulfate was then dissolved in This fractional precipitation may be repeated several times. about 5 cc. of water, and saturated sulfurous acid solution wag I n one experiment, three successive precipitations yielded added in successive small portions with intermittent warming, until selenium no longer precipitated. The solution was then precipitates, the average purity of which was 97.29 per cent. EFFECT OF LIGHTUPON THE COLOROF THE PRECIPITATE. filtered and heated gently t o expel most of the sulfur dioxide. The last trace of sulfur dioxide was oxidized with potassium Exposure t o light darkens the color of the precipitate. Thus, permanganate, which also served to oxidize any remaining solutions mixed i n the dark i n the stoichiometric ratio protraces of selenious acid to selenic acid. (The latter compound is less readily reduced at the cathode during the electrolysis which duced a white t,o canary-yellow precipitate of lo^ but varifollows.) able selenium content, whereas the precipitate formed by A slight excess of oxalic acid was now added to reduce the mixing the solutions in ordinary daylight was bright red in excess potassium permanganate, and the solution was neutralcolor. T h e light yellow tints were stable in the dark for ized with sodium hydroxide, acidified with 1 cc. of acetic acid, diluted to nearly 100 cc., and electrolyzed between 50” and 70” C., hours, but a few seconds in direct sunlight sufficed t o redden using a platinum gauze cathode. (By beginning the electrolysis the exposed granules of the precipitate. Qualhative exiyith the smallest voltage that will cause an appreciable current periments with various filters indicated that radiat>ionin the to flow, say 0.05 ampere, and after an hour increasing the curvisible range was most effective. Quantitative studies, rent to 0.10 ampere, a smooth deposit, free from loose crystals, was obt,ained.) however, have not yet been made. After 3 or 4 hours a few cubic centimeters of water were added, and if no fresh deposit formed above the former solution level Analytical on the cathode, the electrolysis was interrupted, taking the utmost, care lest the cadmium deposit be redissolved by the The selenium content of SODIUM SELETOSULFATE SOLUTION. electrolyte after the cessation of the current. The cathode was the sodium selenosulfate solution was determined by adding a thoroughly mashed in distilled water. follon-ed by alcohol and large excess of hydrochloric acid and weighing the precipitated ether, and dried in an oven at 100’ for 5 minutes or until the selenium. odor of ether was no longer perceptible. The gain in weight of Digestion with aqua regia was SELENIUM DETERMINATIOS, the cathode was the weight of cadmium in the sample. unsatisfactory, as low, inconsistent results were obtained. Excellent results JTere obtained by digesting the sample with Literature Cited a small quantity of sodium peroxide in about 5 cc. of water until all dark particles were converted into a white, gelatinous mass. (1) Bassett and Durant, J. Chem. Soc., 1927, 1401. (For a 0.1000-gram sample of cadmium selenide it is convenient 1 2 ) hlellor. J. W.. “Comprehensive Treatise on Inorganic and Theoto use 1 cc. of 30 per cent hydrogen peroxide and 0.5 gram of retical Chemistry,” Vols. IV and X, Sew York, Longmans, sodium hydroxide in a volume of about, 5 cc.) A large excess Green & Co., 1923. of oxidant is to he avoided, because of evolution of chlorine in ( 3 ) O’Brien, W. J., U. S. Patent 1,894,931(assigned to Glidden Co.). subsequent operations. An excess of hydrochloric acid was ‘4) Rathke, J . prakt. Chem., 95, 1 (1865). added, and sulfur dioxide was bubbled through the solution ~ 5 Vortmann, ) Ber., 22, 2307 (1589). until the selenium precipitate was coagulated. (The solution should contain at least, 90 per cent by volume of concentrated RECEIVED November 16, 1937. Contribution from the Chemical Laborahydrochloric acid, in order to facilitate complete reduction of tories of Johns Hopkins Univer-ity and Central College.
The theoretical composition is
Determination of Ammonia and Urea in Milk A. E. PERKINS, Ohio .4gricultural Experiment Station, Wooster, Ohio
I
N CONNECTIOS mith another investigation it was necessary for t h e writer t o make determinations of ammonia in the milk produced by differently fed groups of cows. After unsuccessful experience with several of the methods found in the literature, a n accurate yet simple method was devised which more nearly met t h e requirements. A method has also been devised for the accurate deterinination of urea in milk, using chemicals and equipment found in most laboratories.
Determination of Ammonia The ammonia content of milk has been a matter of interest and study for a t least 80 years. for Bouchardt and Quevenne in 1857 ( 2 ) are reported to have observed an ammonia content of 0.193 per cent in the case of milk made alkaline with sodium hydroxide before distillation. Of course this high result for ammonia was due to a breaking down of other nitrogenous compounds and did not represent a true value for ammonia or ammonium salts in the milk. Modern historv with respect to this determination may be said to begin wiih the work of Berg and Sherman ( 1 ) and of
Sherman anti several collaborators ( 4 , 1.7, IS). hlilk itself was mixed with an equal volume of neutral methyl alcohol, using sodium carbonate as an alkali, and distilled under partial vacuum in a very large flask to overcome the pronounced tendency to foam. Values observed for fresh market milk from S e w York City and vicinity averaged around 0.39 mg. per 100 cc. Milk either untreated or preserved with formalin and stored 8 to 14 days shcwed values for ammonia up t,o 20 mg. p w 100 cc. Tillmans, Splittgerber, and Iiiffart ( I 6) reported a series of results obtained bv their own method of first precipitating the ammonia as ammonium magnesium phosphate from proteinfree milk serum. After filtering and washing, this precipitate was distilled in the presence of alkali. Parallel determinations of ammonia by the Berg and Sherman ( I ) method showed remarkably good agreement. Other groups of workers, notably .Kieferle and Gloetzl ( S ) , Burstein and Frum (3),and Kluge (D), have made different adaptations of the Folin and Hell (;) Permutit procedure originally designed for the determination of ammonia in urine. Recently, Siemrzycki and Gerhardt (1I ) , Polonovsky and Boulanger j I J ) , and other groups of workers hare reported the use of a combination of steam and vacuum distillation carried o u t on an aqueous deproteinized milk filtrate in an apparatus devised originally hg Parnas and Heller (I?), for the determination of ammonia in blood. This method would seem capable of arcurate result$, h i t the apparatus seems too complicated and limited in it.s application to make its general use a t all pruhahle.
