6 50
T H E J O U R N A L O F I i Y D U S T R I A L A N D ENGI-VEERING C H E M I S T R Y
after experience h a s been gained, t h e a m o u n t of water, a n d in due ratio t h e t o t a l a m o u n t of alcohol, m a y be somewhat reduced. T h e solution, still a t 40 t o 45' C., is r u n through a n u m b e r of fine jets into four liters of g j per cent alcohol, with continuous stirring. T h e whole is left for at least 48 hours with a n occasional thorough stirring after which most of t h e s u p e r n a t a n t alcohol is decanted a n d t h e rest used t o transfer t h e starch t o a two-quart percolator provided with a filter plate which is covered with filter paper or cloth. Here i t is percolated with 9j per cent alcohol, being stirred u p with a stick at intervals t o prevent t h e formation of clumps or fissures, until alcohol comes through t h e percolator of a specific gravity indicating a strength of 90 per cent. T h e starch is t h e n transferred t o a Buchner funnel, well drained with suction a n d spread out t o d r y i n a moderately warm place. T h e resulting product is a fine white voluminous powder, more or less compacted t o friable lumps which completely disintegrate under slight pressure. I n less t h a n a minute, a little of i t €hrown i n t o cold water dissolves sufficiently t o yield a good blue upon t h e addition of potassium iodide a n d iodine. Moistened with water o r dilute alcohol, i t becomes g u m m y a n d subsequently dries o u t t o a horny mass, very slowly soluble in cold water. T h e efficiency of t h e preparation, therefore, resides largely in i t s fine s t a t e of subdivision a n d care must be t a k e n during t h e process of preparation not t o expose i t t o air until after thorough digestion with alcohol of 90 per cent strength. After drying i t should be preserved from moist air, though i t is not injured b y exposure t o ordinary atmospheric conditions. T h e advantages of t h e material for iodine titrations made in t h e field with a portable outfit are sufficiently obvious. It id also a very convenient substance t o have a b o u t a laboratory, although for highly accurate titrations it is somewhat inferior in sensitiveness a n d delicacy t o a properly prepared fresh solution made from r a w starch. Since t h e above described process was worked o u t , Fernbach' has described very similar observations; namely, t h a t a dilute starch paste or solution r u n in a fine stream into a large excess of absolute alcohol or acetone, produces a finely divided precipitate a-hich is soluble in cold water. H e apparently a t t r i b u t e s t h e soluble qualities of t h e resulting product t o t h e dehydrating action of t h e alcohol or acetqne. Inasmuch as t h e horny form of precipitated starch previously mentioned, a n d indeed r a w starch as well, becomes soluble in cold water when finely ground, t h e present writer believes t h a t t h e prime factor which determines t h e readiness with which precipitated starch dissolves in water is simply t h e degree of fineness of i t s mechanical subdivision. It was early found t h a t t h e more dilute t h e original starch solution a n d t h e greater t h e excess of alcohol, t h e finer, more voluminous a n d more readily soluble in water became t h e resulting product, b u t n a t urally, t h e less practical became t h e process. BIOCHEMIC DIVISION, BUREAUO F ANIMALI N D U S T R Y s. DEPARTMENT O F AGRICULTURE, W A S H I N G T O N
u
Ongrnal Communicatrons, Eighth International Congress of A p p l r e d Chemistry. 13 (1912). 131. 1
Vol. 6 , No. 8
DETERMINATION OF SILVER AND BASE METAL IN PRECIOUS METAL BULLION' By FREDERIC P. D E W E Y ~
Received May 15. 1914
T h e determination of silver in gold bullion b y cupellation methods is such a simple m a t t e r a n d so easily executed t h a t i t has long been t h e s t a n d a r d a n d almost universally accepted method, b u t t h e investigations described in this paper show t h a t i t has most serious a n d incurable defects. I n making a n assay of gold bullion for base metal, which is t h e basis for t h e silver determination, t h e base metal is n o t entirely removed f r o m t h e b u t t o n , a n d n o means are a t present available f o r insuring t h e presence of even approximately t h e same a m o u n t of remaining base metals in duplicate buttons. T h e judgment of t h e cupeller is t h e only guide in t h e matter. T h e a m o u n t of gold a n d silver absorbed b y t h e cupel m a y v a r y greatly in duplicates. T h e proofs are subject t o t h e same variations as t h e assays, a n d i t is purely a m a t t e r of chance whether t h e y give t h e proper correction figures or n o t , a n d t h e y m a y fall f a r short of doing so. These difficulties are inherent in t h e work of a n y one assayer, b u t when different assayers work upon t h e same sample, particularly in different laboratories, t h e opportunities for variation are multiplied; when t h e y work upon different samples of t h e same bullion, there is n o telling how widely they m a y differ in their reports, even upon apparently docile bullion. There is urgent need for a better method, b u t a thoroughly better one is not yet available a n d undoubtedly t h e cupellation method will remain in use for m a n y years t o come. This paper further describes a n a t t e m p t t o establish a better method b y means of fusion with cadmium a n d titration with sulfocyanate, which, while not entirely successful, is very promising. Comparative tests b y cadmium fusion a n d Gay-Lussac titrations are also described. During t h e fiscal year ended June 30, 1912,t h e M i n t service of t h e United States Treasury purchased 7 4 , 1 j 9 deposits a n d redeposits of bullion, most of which contained more or less silver. I n t h e vest majority of cases t h e fineness of t h e silver i n t h e bars was not determined directly, b u t b y a method of differences based upon cupellations b y which t h e fineness of gold a n d base metal present were determined, t h e difference between these t w o actual determinations a n d 1000 being called t h e silver fineness. There are two general methods of arriving a t t h e silver result. T h e actual assaying work in both cases is t h e same, b u t t h e methods of calculzting t h e results differ. I n both of t h e m t h e gold fineness is first determined b y a separate assay. T h e n , in one method of calculating, t h e combined gold a n d silver fineness are determined by a second assay a n d t h e gold figure subtracted from t h e combined gold a n d silver figure t o give t h e silver fineness. I n t h e second method of calculating, t h e result of t h e second cupellation is s t a t e d as t h e loss on cupellation, or base metal, a n d t h e s u m of this determination a n d t h e gold determination is subtracted from 1000 t o give t h e silver fineness. 1
4
Published by permission of the Director of the Mint. Assayer, Bureau of the Mint.
Aug., 1914
T H E J O U R N A L OF 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 S T R Y
M a n y of these deposits were purchased a t t h e smaller assay offices of t h e United States, scattered all over t h e country, a n d were subsequently shipped t o t h e larger offices t o be refined.. These larger offices necessarily treated t h e smaller ones just as though t h e y were outsiders, making their own weights a n d assays a n d often remelting t h e bullion before accepting t h e bar. Naturally there are a t times differences between purchasing a n d redeposit offices. Taking t h e figures of t h e purchasing offices as a basis, Table I shows t h e relation between t h e silver determinations of t h e purchasing offices a n d those of t h e redeposit offices upon j j 9 redeposits last year which were not remelted a t the redeposit offices. I t gives t h e number of times they agreed in their silver determinations a n d t h e number of times t h e y differed in fineness b y t h e a m o u n t stated. TABLE1-SILVER
, .
3 21
7
28
F r o m Table I i t is a p p a r e n t t h a t t h e general tendency of t h e redeposit office is t o get higher results on t h e silver t h a n t h e purchasing office. It is barely possible t h a t a very few of t h e results a t one redeposit office were not obtained b y cupellation. Of course, some of t h e differences shown are due t o differences in t h e samples used for t h e assay work, b u t t h e differences are still much t o o large in m a n y cases a n d call for improvement in t h e assaying or for t h e substitution of some other method for determining t h e silver. I t is evident t h a t t h e silver result is naturally open t o considerable variation, because i t s accuracy is dependent upon t h e accuracy of t h e t w o determinations which are actually made. When t h e net errors of t h e t w o actual determinations are opposite it m a y happen t h a t t h e silver figure will be close t o t h e t r u t h , b u t when they are both on t h e same side t h e error in t h e silver determination m a y be quite large. I have published t w o papers1 dealing with t h e accuracy of t h e determination of gold i n bullion, showing t h a t t h e accuracy often claimed for this determination is not generally obtained a n d t h a t in some bullions t h e gold determination cannot be depended upon within one fine. I n t h e presence of certain elements, such as arsenic a n d antimony a n d in t h e case of very impure bullion, such as some cyanide bars a n d some jewelry scrap, t h e go1.d assay is subject ta much wider variations a n d is entirely unsatisfactory of itself, to s a y nothing of its baneful influence upon t h e silver determinations b y cupellation differences. T h e cupellation for base. metal in gold bullion is a ridiculously simple operation a n d m a y be described in a very few words. T h e assay sample of t h e bullion is weighed, wrapped in t h e proper a m o u n t of sheet lead a n d cupelled, accompanied by t h e proper proofs, t o a b u t t o n of gold a n d silver, which is cleaned a n d “The Assay and Valuation of Gold Bullion.” Trans. A . I . M. E.. 40, A n n . Rept., Directov of the M i n t , 1908-9, p. 25. “The Sampling of Gold Bullion,’’ Trans. A . I . .M. E . , 44, 853. Orig. Com., 8th I n l . Cangr. A p p l . Chem., 1, 155. 1
780.
weighed. T h e difference between t h e weight of t h e b u t t o n of gold a n d silver, when corrected b y t h e proof, a n d t h e sample t a k e n , is called base metal t h a t has been driven off or absorbed b y t h e cupel. Aside from carelessness, there appear t o be b u t few chances for variation in t h e operation, b u t I shall show t h a t some of these chances are exceedingly potent. I t is a fair assumption t h a t t h e chances of error in weighing t h e assay sample a n d t h e resulting b u t t o n will be approximately t h e same as i n weighing t h e sample a n d cornet in t h e gold assay. I n view of t h e wide errors t h a t m a y occur further on in t h e work i t is safe enough t o p u t down t h e vitiation of t h e silver determination from this cause as being negligible for the present. I n t h e actual cupellation in t h e furnace for removal of t h e base metal, t h e first point t o a t t r a c t attention
h S A Y S ON
Redeposit office agreed with purchase office... . . . . . . . . . . . . . . . . . . . . . . . . . Redeposit office differed from purchase office. . . . . . . . . . . . . . . . . . . . . . . . . . . Difference in fineness.. . . . . . . . . . . . 112 1 11/2 2 21/2 . . . . . . . 5 7 56 44 28 32 Above.. . . . . . . . . . . . . . . . . . 59 43 41 21 .......................... 44 .......................... 101 115 87 69 53
6j1
5 i 9 REDEPOSITS
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............................................................ 3‘/2 12 6
18
4 4’
