Reducing Action of Mercurous Chloride Separation, Detection, and Estimation of Arsenic, Gold, Platinum, Palladium, Selenium, Tellurium, and Iodine GORDONG. PIERSON, 720 West Main St., Lansdale, Pa.
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changes of color are d e l i c a t e NUMBER of years ago The reducing action of mercurous chloride has (1) the writer observed and must be c o m p a r e d with been adapted specijically to new methods for the standard controls carried along . that mercurous chloride delection and approximate estimation of minute under exactly the same condiwould p r e c i p i t a t e m e t a l l i c quantities of gold, platinum, palladium, teltions as the test. Hydrochloric arsenic from solution in concenlurium, selenium, arsenic, and iodides. These acid is used t o control t h e trated h y d r o c h l o r i c acid and acidity of the solution under serve as a n extremely delicate methods offer a number of advantages ouer older e x a m i n a t i o n a n d should be test for that element. Recently methods, particularly with respect to simplicity, p r e p a r e d b y boiling with a the reducing action of mercurous delicacy, and interfering substances. small q u a n t i t y of mercurous chloride on other easily reduced Under controlled conditions the methods unchloride and standing over the elements such as gold, platinum, doubtedly have value from an analytical as well mercurous c h l o r i d e for about palladium, selenium, and tellu24 hours. I n general, the prorium has been investigated and as commercial standpoint in the recovery of these c e d u r e h a s b e e n t o add 0.1 new methods based on this reacrare elements. The reaction can perhaps be gram of mercurous chloride to tion for separating, detecting, applied to other rare and easily reduced elements. a clean drv 100-cc. beaker. mix and estimating extremely minute in 5 cc. of" d i s t i l l e d watkr or q u a n t i t i e s of these elements have been worked out. Although the reaction appears to be hydrochloric acid to produce the desired degree of acidity, simple and affords the most effective and delicate chemical and add from 1 drop t o 1 cc. of the solution under test. test known to the author, the literature on the subject is Small quantities of the test solutions are preferred to keep apparently confined to an occasional statement in textbooks interfering substances from reaching objectionable concenthat mercurous salts reduce solutions of gold to the metallic trations. With occasional shaking the reaction is complete in a few minutes, after which the mercurous chloride is state. allowed to settle and is collected by tilting the beaker. A general equation for the reaction is With heat the reaction is more rapid, but care must be taken MX HgX = M HgXs to treat the standard controls in the same manner. I n The specific affinities or the potentials of the system which strong acid solutions high temperatures may decompose the cause the reaction to proceed from right to left depend upon mercurous chloride, producing a gray color which interferes the elements involved and such conditions as temperature and and must be avoided. Pretreating the acid with mercurous acidity. Although the reaction takes place with any mercu- chloride greatly retards the development of the gray color. rous halide, the chloride is best suited for detections and It is desirable to extract the test solution with mercurous estimations because it is white. Mercurous chloride is an chloride and use this extracted test solution as a base to preextremely insoluble, very fine powder which not only rapidly pare the standard control. precipitates certain elements from their solutions but also I n addition to the ease and extreme delicacy of these tests, completely adsorbs the precipitated elements which are un- they are also remarkably free from interference. When 1 cc. doubtedly of colloidal fineness. Gold so precipitated may or less of the test solution is diluted to 5 cc., interfering subappear cream, pink, purple, or gray, while arsenic may give a stances are limited to those having very strong oxidizing or buff, pink, brown, or black color depending upon conditions. reducing action under the conditions of the test. Starch, These different colors may indicate allotropic modifications or dextrin, blood, and casein do not interfere. Nitrates, per possibly degrees of fineness of the reduced elements. Mer- salts, iodides, free halides, stannous chloride, and hypophoscurous chloride, being very heavy, settles rapidly, and with phites must be absent, although among this group only the precipitated elements adsorbed can be readily separated nitrates are likely to be encountered. Cupric and ferric salts for observation or recovery. Colored solutions do not inter- may interfere when present in sufficient quantity, apparently fere with the analysis and the tendency of reduced elements to by oxidizing the elements precipitated or about to be precipiform a colloidal suspension too fine to separate or be dis- tated on the mercurous chloride. The amount of C u + + and tinguished is eliminated. The fact that mercurous chloride F e + + + which produces serious interference varies for each sublimes a t a comparatively low temperature offers a simple element. The approximate tolerance limits for 5 cc. of the means for recovering the salt and separating the precipitated solution being precipitated are: gold, 0.003 gram of copper and 0.006 gram of iron; platinum, 0.003 gram of copper in metal in pure form. the cold (no interference in boiling solutions) and 0.003 gram GENERALMETHODOF ANALYSIS of iron; palladium 0.003 gram of iron (copper does not interThis work has been principally concerned with the detec- fere); selenium, 0.0006 gram of copper and 0.010 gram of tion and approximate estimation of minute quantities of iron; tellurium 0.003 gram of copper and 0.002 gram of iron; certain elements. When an approximate estimation of arsenic, 0.0003 gram of copper and 0.06 gram of iron. Boiling quantity is desired, extremely dilute solutions must be used. is necessary to precipitate platinum in the presence of a The estimations are based upon the shades of color produced relatively large amount of the cupric ion, while palladium by the precipitated element adsorbed on the mercurous precipitates readily in a cold or warm solution. The specific chloride powder and beyond certain limits no change in color effect of cupric salts is used as a means of separating platinum is produced by an increased amount of precipitation. These and palladium.
+
+
431
438
ANALYTICAL EDITION
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GOLD. Many methods have been proposed for detecting traces of gold in solution, but have found limited application because they can be applied only to colorless solutions free from many possible interfering elements. The mercurous chloride test is far more simple and delicate, and colored solutions are no hindrance. The ordinary methods usually precipitate gold from very dilute solutions as a colloidal suspension or slime which is very difficult to settle or remove by filtration. Mercurous chloride adsorbs the precipitated metal and provides a mixture which is extremely easy to separate by filtration or sedimentatlion. Because of the extreme insolubility of some forms of powdered mercurous chloride, the loss of this salt i s practically limited to the more soluble mercuric chloride formed by the reduction of an equivalent amount of gold. A number of experiments have been made by the author in order to determine the practicality of recovering gold from sea water. Using synthetic sea water containing one part of ionized gold in one hundred million parts of water, recoveries have been made at an estimated cost which indicates this process to be something decidedly more than a romantic possibility. I n mixtures of gold and mercurous chloride the mercurous chloride can easily be sublimed and recovered by condensation leaving puEe gold. Gold precipitates completely and rapidly from a neutral or acid solution and less than 0.00005 mg. can be detected by a faint coloration with 0.1 gram of mercurous chloride. For quantitive estimations it is best t o use a quantity of gold solution sufficient to give the purplish pink shades. The colors produced on this quantity of mercurous chloride are: GOLD Ma. .s_ 0.20
0.10 0.05
0.02
0.002 0.0002
0.00005
COLOR ON hfERCUROUS CHLORIDE Dark purple Pinkish purple Purplish pink Strong pink Light pink Very light pink Faint coloration
Apparently no method suitable for the detection PALLADIUM. and approximate estimation of very small amounts of palladium in dilute solution has been described in the literature. Mercurous chloride precipitates metallic palladium from a neutral or acid solution, and in solutions comparatively free from interfering substances it is possible to detect 0.00005 mg. by the slight cream coloration on 0.1 gram of mercurous chloride. The approximate colors produced on this quantity of mercurous chloride by different amounts of palladium are: PALLADIUM
COLOROF MERCUROUS CHLORIDE
Mu. 0.2 0.05
0.01 0.002 0.0004 0 . ooO05
Very dark gray Gray Light gray Cream gray Grayish cream Faint cream
PLATINUM. The literature gives no simple or delicate method of detecting and estimating minute quantities of platinum in dilute solution. Mercurous chloride precipitates platinum from neutral or acid solutions. The smallest quantity which gives a slight coloration on 0.1 gram of mercurous chloride is about 0.0002 mg. The approximate colors for this amount of mercurous chloride are: PLATINUM
COLORON M~RCUROUS CHLORIDE
MU. 0.1
0.02 0.01 0.005
0.001 0.0002
Dark gray Gray Creamish gray Grayish cream Cream Slight cream
Platinum and palladium precipitate under similar conditions and the coloring on the mercurous chloride is also similar. However, a suitable concentration of copper sulfate revents the precipitation of platinum except after boiling, whife palladium precipitates readily from a cold or warm solution. The specific effect of copper sulfate offers an effective means of separating mixtures of platinum and palladium in solution. SELENIUM.Selenium is precipitated by mercurous chloride only from solutions containing 6 to 20 per cent of hydrochloric acid, and is not complete with less than 16 per cent. With an o timum of 20 per cent acid it is possible to detect 0.0002 mg. T%e colors produced on 0.1 gram of mercurous chloride me approximately:
SELENIUM MB. 0.2 0.05
0.005 0,002
0.0005 0.0002
Vol. 6, No. 6 COLOR O N MERCUROUS CHLORIDE Salmon red when cold, bright red when warm Salmon pink Strong pinkish cream Pink cream Light cream Faint cream
Mercurous chloride precipitates tellurium under the same conditions as selenium; therefore these elements must be separated to be detected or estimated by mercurous chloride. Sodium bisulfite completely precipitates selenium from a solution containing approximately 20 per cent of hydrochloric acid, while tellurium remains in solution. The selenium can be filtered, redissolved, and estimated by the usual mercurous chloride method. TELLURIUM. This element like selenium is not precipitated from solutions containing less than 6 per cent hydrochloric acid and precipitation is not complete in solutions containing less than 15 per cent acid. About 20 per cent of hydrochloric acid is used in the standard test procedure which permits the detection of about 0.0005 mg. of tellurium. The colorR produced by the precipitation are approximately: TELLURIUM MQ. 0.2 0.05
0.005 0.0005
COLOR O N MERCUROUS CHLORIDB Grayish yellow, turns grayish brown when hot Cream yellow Light cream yellow Faint cream
ARSENIC. A number of methods are available for detecting or estimating minute quantities of arsenic, but are not without objectionable features. The Marsh and Gutzeit methods require special apparatus and an experienced operator to give the best results. The Reinsch test is simple and rapid but does not permit a good quantitive estimation, and a large number of elements interfere. The Bettendorf and Bougaults tests are short and positive under favorable conditions but are not satisfactory with colored solutions or in the presence of many other easily reduced elements. The recently studied Gosio test (8), depending upon the action of certain molds, does not appear to be as practical for general purposes as the strictly chemical methods. Mercurous chloride precipitates arsenic from concentrated hydrochloric acid solutions. Arsenious solutions react rapidly and completely, while arsenates react slowly or incompletely. However, arsenate solutions can be easily reduced by sulfurous acid either before or after the addition of mercurous chloride. Practically all elements, such as gold, platinum, palladium, tellurium, selenium, and iodine, which would interfere with the reaction as a test for arsenic can be easily and quickly removed by making the test solution up to about 20 per cent hydrochloric acid and treating it with a substantial excess of mercurous chloride. Arsenic remains in solution, while the other elements are precipitated. I n a warm solution with occasional shaking the separation is complete in a few minutes after which the solution may be decanted or filtered. The hydrochloric acid concentration should then be increased to 30 per cent or higher, and arsenic precipitated by mercurous chloride. Working with pure solutions, less than 0.00001 mg. of arsenic will produce a visible shade of cream color on mercurous chloride. Using the proportions previously described for the standard test the colors produced are approximately as follows: ARSENIC
MU. 0.1
0.01 0.001 0.0001 0.00002
COLOR ON M ~ R C U R O U S CHLORIDE Bright deep brown Pinkish brown Pink Cream Very slight cream
This method requires no special apparatus, the manipulation is extremely simple and short, colored solutions do not mask the precipitate, interfering substances are very few and easily reduced to a minimum, and its delicacy a parently exceeds all other methods by a substantial margin. As accuracy for approximate estimations greatly exceeds the other wet precipitation methods. Although no comparison has been made, it seems likely that the Gutzeit method might prove more definite and accurate for estimating pur oses. IODINE.Wlen mercurous chloride is added to a solution containing iodides, mercurous iodide forms on the surface of the particles of mercurous chloride, producing shades of color varying
November 15,1934
I N D U S T R I A L A N D E N G I N E E R I N G C H E M I STR Y
439
from greenish black to light yellow depending upon the iodide concentration. In the absence of interfering elements less than 0.003 mg. of iodine produces a faint coloration on 0.1 gram of mercurous chloride. The shades of color produced provide a method for approximately estimating minute quantities of iodine in dilute solution.
dilute with water, keeping the hydrochloric acid concentration below 4 per cent. Add CuS04.5H20to make a 5 per cent cold solution, followed by mercurous chloride which precipitates palladium. The palladium may be estimated by the color on the mercurous chloride, or preferably the precipitate ma be filtered, dissolved, and diluted for estimation by mercurous chcride. The filtrate from the removal of palladium now contains platinum. SEPARATION OF GOLD, PLATINUM, PALLADIUM, SELENIUM, Add a little mercurous chloride to the filtrate and boil gently for about 5 minutes to precipitate platinum. The precipitate TELLURIUM, AND ARSENIC may be filtered, dissolved, and reprecipitated for estimation. The methods of separation are based entirely on reductions 2. SEPARATION OF SELENIUM,TELLURIUM, AND ARSENIC. from solution to the elemental form. It is best to use very Gold, platinum, and palladium are removed from this fresh by mercurous chloride, after which the filtrate is made dilute solutions and to work with a few cubic centimeters of a solution up to a hydrochloric acid concentration of about 20 per cent. solution containing no more than a few milligrams of each Add about 5 per cent of sodium bisulfite and allow to stand for element. Such a solution should first be made acid to the 15 minutes, followed by gentle boiling for a few minutes to preextent of about 1 or 2 per cent with hydrochloric and divided cipitate selenium. The selenium may be filtered, redissolved with chlorinated hydrochloric acid, and estimated with mercurous into two parts, one part being treated for the segregation of chloride. The filtrate from the removal of selenium contains gold, platinum, and palladium and the other for arsenic, tellurium and arsenic. selenium, and tellurium. Tellurium is removed by adding mercurous chloride, warming the solution if necessary t o hasten reaction. The filtrate now 1. SEPARATION OF GOLD,PLATINUM, AND PALLADIUM. Add contains only arsenic. crystals of oxalic acid to make about a 5 per cent solution and In order to remove arsenic the hydrochloric acid concentration boil to precipitate gold. Filter on a fine paper, wash, then dis- must be raised to 30 per cent or higher, after which arsenic can solve precipitate, some of which adheres to the beaker, %it:, dilute be precipitated by mercurous chloride in the usual manner. chlorinated hydrochloric, boil off chlorirlo, and test with mercuroue chioride to detect and estimate gold. The filtrate from the LITERATURE CITED go:d reduction contains the other elements from which platinum cad palladium can be separated. (1) Pierson, G. G., Master's Thesis, University of Wisconsin, 1923. Evaporate filtrate to near dryness with a little sulfuric acid to (2) Smith, H. R., and Cameron, E. J., IND. ENG.CHEM., Anal. Ed., 6, decompose oxalic acid. Any element precipitated during the 400 (1934). evaporation is dissolved with a little hydrochloric acid and potassium chlorate. Drive off free chlorine and excess acid, and R ~ C B I V BMay I D 23, 1934.
Determination of the Common and Rare Alkalies in Mineral Analysis ROGERC. WELLSAND ROLLINE. STEVENS, U. S. Geological Survey, Washington, D. C.
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N MOST rock and mineral On record no mention is made Of rubidium and cesium. Lithium is reported as a trace in many-but this m e a n s l i t tIe quantitativelythough i t has of course been det e r m i n e d occasionally by the
method'
The few re-
Methods are described which afford a determination of each member of the alkali group and are successful in dealing with the quantities of the rare alkalies found in rocks and minerals. The procedures are relatively rapid and based chiefly on the use of chloroplatinic acid, absolute alcohol and ether, and ammonium sulfate, The percentages Of all the alkalies found in a number Of are given*
ported results for rubidium and c e s i u m a r e questionable. A m e t h o d for afi the alkalies is needed, one that will not be too complicated and that will be adapted to the particular purpose of handling the percentages of lithium, rubidium, and cesium met with in rocks and minerals.
Previous attempts to deal with all the alkalies are noted by Hillebrand and Lundell (2) and by Noyes and Bray (4). The method of extracting rubidium and cesium chlorides from potassium chloride by means of hydrochloric acid and alcohol has been used by several analysts. Strecker and Diaz, however, in referring to this step (8) give no details of procedure, and the extraction of rubidium was found to be incomplete by Moser and Ritschel (3); they, however, used relatively large quantities of the two salts in their tests. Experience shows that e ~ o r t should s be directed to dealing with small quantities of lithium, rubidium, and cesium. Attempts to devise a quantitative procedure on the scheme of Noyes and Bray gave unsatisfactory results for small quantities of rubidium and cesium, as the precipitates formed were not sufficient,lvinsoluble. Results were correct only to a milligram or more." A com lex organic compound, sodium 6-chloTo-6nitrotoluene-m-sulgnate, is suggested by Davies (1) as a precipi-
F ~ t ~ ~ quite soluble. The writers found, however, that the rubidium comp o u n d w a s not sufficiently insolublefortheseparation of rubid-
i U $ ~ ~ O,Leary ~ ~ and ~ ~ Papish (6) have reviewed the analytical reactions of rubidium and cesium and proposed some new methods. They precipitate rubidium and cesium with phosphomolybdic acid and obtain excellent separations from potassium although the procedure appears to be rather time-consuming. deparation of most of the potassium, the element generally in excess, however, seems preferable to precipitation of the minor elements first. The methods here described afford a determination of each member of the alkali group and are successful in dealing with the quantities of the rare alkalies found in rocks and minerals. They presuppose that the alkali chlorides have first been obtained free from all other compounds. Should magnesium and calcium be present they will be found mainly with lithium, and sulfate will be found with sodium. The behavior of traces of borate and fluoride has not been determined, but these may easily be removed if known to be present. Spectroscopic confirmation of the presence of mere traces of any of the alkalies should of course be obtained. At the present time the J. Lawrence Smithmethod seems to be preferred for extracting the alkalies from silicate rocks and minerals. It has long been used almost exclusively in the U. S. Geological Survey
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