T H E J O L ‘ R N A L OF INDE’S‘TKIAL A N D E N G I N E E R I N G C H E M I S T R Y RESULTS-Twelve samples of oil were used. T h e y were tested f o r water, low fractions a n d a s p h a l t . T h e results of t h e analysis of these oils a s well a s t h e specific h e a t s f o u n d are given below. DISTILLATE PERCENTAGES Sample
Water 1............... 0 2 . . . . . . . . . . . . . . .0.33 3 . . . . . . . . . . . . . . . 0.52 4 . . . . . . . . . . . . . . .7 . 0 5 . . . . . . . . . . . . . . . 1.3 6. . . . . . . . . . . . . . . 0 7. . . . . . . . . . . . . . . 0 8. . . . . . . . . . . . . . 0 9 . . . . . . . . . . . . . . .0 IO . . . . . . . . . . . . . . .0 11. . . . . . . . . . . . . . . 6.2 12 . . . . . . . . . . . . . . . 0 A’o.
0-150° C.
.... .... .... 5.7
0 35.6 8.1 25.8 0
0 0
0
150-300° 300° C. Asphalt Asphalt
....
....
....
23.14 37.4 54.1 27.3 43.3 12.3 22.5 30.9 34.1
.... .... ....
32.1 30.0 4.2 39.7 22.1 48.7 42.3 22.6 30.2
.
.... .... ....
31.7 30.91 6.2 23.5 7.0 36.3 34.7 39.9 35.4
Specific heats at 20° C. 0.3999 0.4143 0.4389 0.5016 0.4788 0.4804 0.4474 0,4832 0.4419 0.4553 0,4559 0.4491
F r o m t h e above results i t can be seen t h a t t h e average specific h e a t of California petroleum is a r o u n d 0.4500. T h e oils I , 2 a n d 3 were oils containing only t h e fractions from 300’ C . u p . T h e results show t h a t a s t h e asphalt content increases t h e specific h e a t decreases while a s t h e water content increases t h e specific heat increases. UNIVERSITYO F CALIFORNIA BERKELEY
DETERMINATION OF SILVER AND BASE METAL IN PRECIOUS METAL BULLION BY FREDERIC P. DEWEY Received May 15, 1914
(Conclarded from the August N z t m b e v ) T H E DIRECT DETERMINATION O F SILVER
T h e determination of silver i n solution b y t i t r a t i o n with a solution of sulfocyanate was described b y Volhard i n J o u r . p v a k t . C h e m . , in 1874, a n d is generally spoken of as t h e “ V o l h a r d m e t h o d , ” b u t priority h a s been claimed for Charpentier, based u p o n publications i n Compt. r e n d . , i n 1871, a n d Bull. S a c . Iitg. Cia. de France, in 1870. I n general, t h e m e t h o d consists i n adding a liberal a m o u n t of a ferric salt t o t h e silver solution a n d t h e n a d d i n g from a b u r e t t e a solution of a sulfocyanate of a convenient s t r e n g t h , finishing d r o p b y d r o p , until t h e characteristic red color of t h e ferric s a l t with sulfoc y a n a t e appears. More or less elaborate descriptions of i t will b e f o u n d i n various text-books. When working upon high-grade silver, 992 fine or over, t h i s m e t h o d probably yields fully as accurate results a s t h e Gay-Lussac m e t h o d (titration with salt solution). N o special a p p a r a t u s is necessary a n d when only a few determinations are required i t is more rapid t h a n t h e Gay-Lussac method. T h e chief a d v a n t a g e of t h e Gay-Lussac method is i t s ability t o t u r n o u t , with proper equipment, a larger volume of work i n a d a y ’ s time. When, however, t h e Volhard m e t h o d is applied t o miscellaneous materials some objections appear. Merc u r y a n d palladium interfere. Moderate a m o u n t s of copper a n d p l a t i n u m are n o t objected t o . It is often said t h a t 7 0 per cent of copper m a y be present, b u t t h i s is certainly a mistake. Nickel a n d cobalt obscure t h e e n d color. I t has been suggested when Ni a n d Co a r e present t o a d d excess of sulfocyanate a n d t i t r a t e back with silver. Chloride of silver m u s t b e absent since i t gradually reacts with sulfocyanate
Vol. 6, No. 9
i n solution. Lower oxides of nitrogen must be a b s e n t . Ferrous iron, i n presence of nitric acid, m u s t n o t be used as t h e indicator since t h i s would mean lower oxides of nitrogen. It should be noted t h a t ferric sulfate as indicator m a y cause precipitation of lead sulfate when t h e bullion contains lead. It h a s been recommended t o use t w o solutions, a s t a n d a r d a n d tenth standard. In dissolving bullion carrying t i n i n nitric acid, purple of Cassius is almost always formed a n d this interferes most seriously i n determining t h e sulfoc y a n a t e e n d point. Stanniferous bullions are quite rare, b u t one office gets placer gold b a r s carrying t i n occasionally, a n d a n o t h e r gets scrap bullion containing t i n a t intervals. When t h e a m o u n t of silver present is known a p proximately a n d t h e e q u i p m e n t is a t h a n d , i t has been recommended t o precipitate nearly all t h e silver with s t a n d a r d salt solution a n d finish t h e t i t r a t i o n with sulfocyanate. T h e necessity of separating t h e AgCl from t h e solution before adding t h e sulfocyanate makes t h i s combination method t o o lengthy. When we t a k e u p t h e question of applying t h i s method t o t h e determination of t h e silver present i n gold bullion, we are a t once confronted with t h e problem of getting t h e silver i n t o solution. Where t h e bullion is less t h a n 300 fine in gold, possibly t h e o t h e r metals m a y be directly dissolved i n nitric acid, b u t above t h a t gold fineness, some preliminary t r e a t m e n t is necessary. I n order t o m a k e a satisfactory determination of silver i n bullion carrying Pt, i t was t h e practice a t t h e Philadelphia M i n t , for m a n y years previous t o 1890, t o alloy with P b on a cupel, removing from t h e muffle a s soon a s thorough alloying was secured. T h e cold b u t t o n was cleaned a n d t h e n t r e a t e d with nitric acid. T h e alloying required considerable care a n d skill a n d t h e r e was often trouble f r o m t h e separation of lead n i t r a t e if s t r o n g acid was used. T h e silver i n t h e solution was determined b y t h e Gay-Lussac method. Primarily for t h e determination of t h e gold, G. Rose’ proposed t o alloy with lead i n a porcelain crucible over a spirit l a m p , J u p t n e r 2 alloyed with zinc a n d Balling3 with cadmium. I n all of these cases t h e solution was available for determining t h e silver; in f a c t , Rose proposed t o precipitate t h e silver b y solution of chloride of lead. I n 1891, Dr. Cabell Whitehead,4 t h e n Assayer of t h e B u r e a u of t h e M i n t , described t h e use of t h e cadmium fusion for t h e determination of silver i n gold bullion, particularly when present i n very small a m o u n t s as i n gold coins, b u t he regards t h e determination of gold b y t h i s method unfavorably. In t h e Annual R e p o r t of t h e Director of t h e M i n t for 1912, page 33, M r . hl. A. M a r t i n , t h e Assayer of t h e San Francisco M i n t , h a s proposed t h e following method of determining silver i n mass melts combining a n u m b e r of deposits: 1 2
3
4
Mitchell’s “ Manual of Practical Assaying,” 1881, P. 688 Z anal. Cham., 1879, p. 105 Crookes’ “Select Methods of Chemical Analysis,” 1886, p 443. Proc Franklin I n s t , 1891, 111, p. 94.
S e p t . . 1914 t
THE
TH.E JOrRlVAL OF IXDI‘STRIAL A N D ENGINEERISG CHEMISTRY
DETERMINATION
OF S I L V E R I>- MASS S I E L T S O F
GOLD B U L L I O N
’
“ F o r several years past we h a v e experienced n o little difficulty i n t h e endeavor t o determine correctly t h e silver content of gold bullion mass melts b y means of cupellation, t h e m a n y varieties of alloy usually present in bullion of t h i s character rendering i t almost impossible t o obtain a close agreement between duplic a t e assays. “ E a r l y in t h e fiscal year we experimented with a different m e t h o d , t h e results of which h a v e been so satisfactory as t o justify i t s p e r m a n e n t adoption. T h e details of t h e process are as follows: “-1scorifier a b o u t two-thirds full of 9s per cent potassium cyanide is placed i n t h e f r o n t of t h e muffle furnace, care being t a k e n t h a t t h e content:, are k e p t beloxT t h e boiling point. T h e assay is weighed up a t 1000 gold weight, wrapped in a small sht.et of lead foil n-eighing about. one g r a m a n d immersed i n t h e molten cyanide. X piece of stick cadmium weighing approximately five g r a m s is immediately added! a n d b y a slight rotation of t h e scorifier is brought i n t o c o n t a c t with t h e klullion sample. T h e metals unite rapidly, a complete alloy usually being formed i n t w o or t h r e e minutes. T h e contents of t h e scorifier are t h e n poured i n t o a round b o t t o m mou183, allowed t o cool for a few moments, a n d t h e c a d m i u m b u t t o n separated from t h e cyanide, which m a y be used a second a n d a t h i r d t i m e before being discarded. T h e b u t t o n is cleansed with h o t water. placed in a n ordin a r y h u m i d assay bottle, a n d boiled in t w o ounces of 3 2 ’ nitric acid. When t h e brownish red fumes have entirely disappeared, t h e solution is diluted with I O O cc. of distilled water a n d j cc. of a s a t u r a t e d solution of iron a l u m a d d e d as a n indicator. T h e contents of t h e bottle are t h e n t i t r a t e d with a solution of a m monium sulfocyanate, I cc. of which is equivalent t o I O gold weight of silver. T h e complete precipitation of t h e silver is, of course, indicated b y t h e formation of a brownish red color which will not disappear on shaking. T h e fineness of t h e sample is obtained b y multiplying t h e direct b u r e t t e reading b y t e n . ” If t h i s method be satisfactory for silver i n mass melts, which are confessedly difficult t o handle b y cupellation, i t o u g h t t o be even b e t t e r for t h e general r u n of deposits i n a v a s t m a j o r i t y of cases. Having already examined t h e cupellation method carefully, we h a v e a good basis for comparing t h e results b y t h e sulfocyanate method viith cupellation results, a n d in making t h i s coinparison we shall get m a n y more comparative determinations b y cupellation i n various laboratories. I n order t o t r y o u t t h e sulfocyanate m e t h o d t h o r oughly, t h e d s s a y e r a t t h e S a n Francisco mint prepared a series of 1 2 samples i n duplicate, representing characteristic bullions which he t h o u g h t would be suitable for t h e process. H e m a d e duplicate determinations of silver in each sample b y b o t h cupellation a n d t h e sulfocyanate method a n d forwarded t h e samples t o t h e Bureau with his results. T h e samples were t h e n distributed t o various o t h e r laboratories i n t h e Service a n d t h e sa:me four determinations were m a d e
729
TABLE XI-COMPARATIVE SILVERDETERMINATIONS (FINESESS) BY CUPELLATION AND SULFOCYANATE METHODS Cupel S-C Cupel S-C Cupel S-C Cupel S-C SAMPLE A h-0. 1 No. 2 No. 3 h-0. 4 2 1 9 . 5 219.5 2 5 4 . 5 250.5 275 277 2 1 9 . 5 2 1 9 . 5 255.5 249.25 275 275.75 223 220 255 239 278 272 220 220 256.5 244 279 2i2 2 2 3 . 5 2 2 3 . 2 5 258 254.25 278.25 277.25 2 2 5 . 5 2 2 3 . 2 5 256.75 254.75 278.25 277.25 226 220 253 243 278 273 224 223 253 242 277 276 225 213 258.5 247.25 279 267 227 226 2 5 7 . 5 247.25 279 275 227 226 2 5 8 . 5 254.75 279 277.25 253 239 275 267 2 1 9 . 5 213 1 . 3 13 5.5 15.75 4 10.25
227 227 2 1 9 . 5 209 7.5 18 754.25 So. 6 3 2 9 . 5 326.25 3 2 8 . 5 326.25 330 324 331 322 3 3 0 . 2 5 330 329.50 329.5 332 327 330 327 332 321 332 326 332 330 3 2 8 . 5 322 3.5 8
2 5 9 . 2 5 255 2 5 2 . 5 239 6 . 7 5 16 654.5 NO. 7 348 344.73 348 345.75 345 343 350 345 349.5 349.5 3 4 9 . 5 349.5 347 342 345 348 342’ 348 343 350 349.5 345 342 5 7.5
279 275 4 717
277.75 266 11,75
so. 377.5 378.5 370 369 376.25 3i6.5 382 375 380 381 382 369 13
8 3iS.25 374.5 371
328.5 328.5 330 331 329.25 330.25 328 329 332 331 332 328 4
348 348 345 348 350.25 350.5 349 345.5 349 349 350.5 345 5.5
344.75 344.75 343 345 350.75 350.75 342 338 345 343 350.75 338 12.75
383.5 379.5 379 368 376.75 373.75 376 372 381 383 383 368 15
350 5 345 5 5 627
350.75 383 338 368 12.i5 15 163.5
325 326. 25 322 324 331 330 327 327 318 328 330 318 12
332 330 328 318 4 12 636.5 h-0. 406 403 399 403 402 402 397.5 397 396 397 406 396 10
37b:75 379.75 379 378 362 372 379.75 362 17.75
37l.5 373.5 371 372 378.75 378.75 376 376 366 377 378.75 366 12.75 379.75 362 I7 75
400 404 399 402 401.5 40 1 396.5 399 397 398 404 39? 406 396 10
169
417.5 393 24.5
5 3 9 . 5 5 4 5 . 7 5 542 545.75 5 3 0 . 5 527 527.25 524 9.5 1 8 . 7 5 1 4 . 7 5 21.75 455.25 1 4 i . 75
on each sample i n each one of four laboratories. E a c h sample was, therefore, assayed t e n times b y each method. I n all, f o r t y determinations of silver were made o n each bullion. N o especial instructions were given t o a n y of t h e
laboratories beyond t h e description of t h e process printed i n t h e a n n u a l report. It was a p a r t of t h e object of t h e investigation t o determine if t h i s description was sufficient, a n d if t h e process was simple a n d a c c u r a t e enough t o yield satisfactory results i n t h e h a n d s of t h e general r u n of assayers without more rigid specifications as t o t h e details of operating t h e m e t h o d . Some of t h e operators, however, h a d h a d more or less previous experience with t h e m e t h o d . All t h e results reported are given in T a b l e XI. T h e l a b o r a t o r y n u m b e r s d o not always mean t h e s a m e l a b o r a t o r y b u t t h e A a n d B samples were always assayed i n t h e s a m e laboratory. T h e figures given are finenesses, or p a r t s per 1000. An inspection of t h i s t a b l e shows t h a t some,of t h e results were evidently vitiated b y unusual accidents which were n o t appreciated a t t h e t i m e of making t h e determination. When t h e reports were received TABLEXII-AMENDED COMPARATIVE DETERMINATION OF SILVER (FIN*NESS) Cupel Sample
Range
CUPELLATION A N D SULFOCYANATE METHODS S-C Cupel S-C Cupel S - C Cupel S - C No. 1 No. 2 No. 3 No. 4 197 174 2 1 9 . 5 2 1 9 . 5 252.5 239 275 271 197 174 219.5 219.5 253 242 275 272 200 I75 220 220 253 243 275 272 200 176 221 220 253 243 276 273 200 177 221.5 220 254.5 244 276 273 255 244 277 274 201 178 2 2 1 . 5 220 201 I78 223 120 255 244 277 274.5 201 178 223.5 220.5 255.5 245 278 274.5 201 179 223.75 220.5 2 5 6 . 5 247.25 278 275 202 180.25 224 222 2 5 6 . 5 247.25 278 275.75 203 185.25 224 2-3 256.5 247.25 278.25 276 256.75 249.25 278.25 27i 203 182.25 224 223 203 191.75 224 223 257 249.25 278.5 277 203 191.75 225 223 257.5 249.50 279 277.25 204 191.75 225 223 257.5 250.50 279 275.25 204 192.75 225.5 223.5 257.5 250.50 279 277.25 204.5 197 226 226 258 254.25 279 277.75 204.75 198 226 227 258.5 254.75 279 .. 205 198 226 . . 258.75 255 279 205.25 198 227 . . 259.25 255 279 25 8.25 24.0 7.5 7.5 6.75 16.0 4:25 6:75 BY
Most of t h e figures given i n various following t a b l e s a r e finenesses or p a r t s per 1000. F o u r of t h e samples were r e t u r n e d t o t h e S a n F r a n cisco M i n t without informing t h e m w h a t t h e y were a n d t h e following t a b l e compares t h e original results with t h e second reports: Sample lstreport.. Zndreport.. Gold ....
No. 1 A B Cupel S-C Cupel S-C 254.5 250.4 257.5 2 5 0 . 4 3.5 49.3 6.5 49.3 256.0 253.5 258.0 254.6 8.0 .5 7.0 .6 654,5
No. 2
A Cupel 201.0 2.0 206.0 6.0
B
S-C Cupel 198.0 204.0 6.9 3.0 195.6 205.0 6.7 5.0 632
S-C 198.0, 8.0 197.8 6.7
Results o n t h e reassays b y t h e S a n Francisco M i n t of t w o other samples i n t h i s set a r e given beyond. A sample of cyanide bullion f r o m another office was assayed twice a t t h e S a n Francisco M i n t a n d once a t a n o t h e r mint. T h e results are given beyond. T w o samples from each one of t h r e e bullions t a k e n a t a purchasing office were assayed i n duplicate b y b o t h methods i n t h e S a n Francisco M i n t a n d at a n o t h e r m i n t with t h e following results: No. 1 Cupel S-C Sample A San Francisco m i n t . 5 3 9 . 5 536.1 41.5 .1 2nd mint . . . . . . . . . . 5 3 7 . 0 532.0 6.0 1.0 Sample B S a n Francisco mint. 5 3 9 . 5 534.0 40.25 5.0 2nd mint . . . . . . . . 5 3 7 . 0 532.0 6.0 Gold. . . . . . . . . . 303.5
No. 2 Cupel S-C 423.75 .25 422.0 1.0
No. 3 Cupel S.C
423.6 .6 421.0 1.0
345.5 3.5 336.0 8.0
337.4 .5 338.a 8.0
344.5 7.25 338.0 6.0
338.5 7.4338.0 4.P 215
423.25 423.6 3.0 2.6 424.0 421.0 3.0 1. 265
T w o samples each from t w o cyanide b a r s were compared i n t h e same way, b u t i n t h e second comparison t h e second mint was n o t t h e same a s i n t h e first. Sample -4 San Francisco m i n t . , . . . . .
No. 1 Cupel S-C
462 57 2nd m i n t . . . . . . . . . . . . . . . . . 459 60 Sample B S a n Francisco.. . . . . . . . . . . . 455 3 2nd m i n t . . . . . . . . . . . . . . 453 0 Gold. . . . . . . . . . . . . . . . . . . .
45:
No. 2 Cupel 438.5 41.5 442.0 2.0
,
454 455
433.5 5.25 438.0 5.0
I
445 5 44 1
S-C 433.1 434.6 5.6
430.8 2.0 434.6 6.6 413
When t h e investigation of t h e S a n Francisco s a m p l e s was inaugurated it developed t h a t one of t h e small offices was already i n t h e habit of using sulfocyanate t i t r a t i o n on some deposits, a n d t h e following t a b l e shows their results on ~j bullions of widely varying. composition arranged b y progressively increasing; silver fineness:
Range No. 9 390 390 391 390 392 390 393 391 394 391 395 391 395 391 395 394 395.75 395 396 395 3 9 6 . 5 397 3 9 6 . 5 399 397 401 399 401.5 399.5 4 0 1 . 5 401 402 401 402 .. 401.5 403 ,.
No. 396 396.5 397 397 397 397.5 398 399 399 399 400 401 401.5 402 402 402 403 403 404 406 10.0
10 393 394 394 396 398 399 399 400 401 402 402 402 402 403 403
..
.. .. ..
ii:o io:o i:o a10 io:75 910 a t t h e Bureau it was t o o late t o investigate these erratic results. If now we discard these -accidental Range
Vol. 6 , NO.9
T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERI>VG C H E M I S T R Y
730
ii:o
results a n d group t h e remaining reports i n regular numerical order we h a v e t h e above table.
Silver
No. 1
2 3 4 5 6 7
8
Cuoel 15.0 13.5 91.0 91.0 120.5 119.25 116.75 7.25 161.5 .5 190.5 3.0 246 0 5.0 261.25 .25
S-C 14.2 15.2 91.2 91.2 112.5 3.5 118.6 7.6 148.0 7.0 189.6 .6 241.3 2.3 260.5 261.6
Silver 7
Gold 897.5
No. 9
897.25
IO
194.0
11
694.5
12
559.5
13
719.0
14
626.75
15
Cuoel 275.0 6.5 319.25 .25 338.5) .J
402.75 402.75 434.0 .25
720.5
440.25 .75 440.5 .5
S-C 276.8 5.8 314.3 5.3 338.6
Gold 652.75,
4oi:i .5 438.0 6.0 434.0 6.0 446.1 5.1
594.5
7 7
608 0 436.75,
548.75 537.75, 549.5
Nos. I a n d 4 u-e1.e cyanide bars, KO. j was a jewelry scrap b a r a n d No. 3 carried lead a s t h e main base metal. T h e original office numbers o n these b a r s were between 8j a n d 1 1 j , mass-melts 3, 4, j a n d 6 , a n d chips b a r Xo. 4.
*
T H E J O C R N A L O F I N D L 7 S T R I A L A N D E N G I N E E RI N G C H E M I S T R Y
Sept., 1914
-4t t h e redeposit (office five of these b a r s were also examined b y t h e sulfocyanate m e t h o d , being original b a r No. 95 a n d mass-melts 3, 3 , j a n d 6 Test 95 . . . . . . . .
KO.
Mass3.........
1st 1 21 IO, 1
2nd 13.2
2Tl.9 .Y
270.2 69.2 232.6 4.7 181.8 7.0 2S8,O 8.0
Mass 4 . . . . . . . . .
235.5
Mass 5 . . . . . . . . .
186 5
..
256. i
Mass 6 . , ,
,,,
,
3rd 12.2 13.2 272.8
....
.o
4th 12.2 13.2 270.8 1.8 238.2 9.2 186.3 4.2 255.5
231 . O 187.3 6.3 258.5
.s
.5
Different samples of t h e s a m e bullion were s u b m i t t e d t o t h r e e laboratories a n d assayed in duplicate b y b o t h m e t h o d s . T e n of t h e 1 2 results fell between 441 a n d 444 fine i n silver, showing a most remarkable agreem e n t . Unfortunately, however, t h e other t w o results were much lower. T h e following a r e t h e detailed reports : Laboratory
Cupel
Sulfocyanate 434.0
a n approximation of t h e a m o u n t of silver i n t h e bullion, such a n approximation for instance a s is supplied b y t h e customary preliminary assay of gold bullion, in order t o have t h e a m o u n t i n a t e s t a trifle over a g r a m . T h e required q u a n t i t y of silver m a y be m a d e u p b y t a k i n g t h e proper a m o u n t of t h e bullion for t h e assay, or sufficient pure silver m a y be added t o give t h e required a m o u n t with t h a t present i n t h e bullion. This extra silver m a y be added before alloying with c a d m i u m or i t m a y be a d d e d t o t h e final solution. If t h e silver is n o t a d j u s t e d a n d i t is a t t e m p t e d t o t i t r a t e a n u n known a m o u n t f r o m a b u r e t t e , t h e operation becomes troublesome a n d less accurate. Our small offices a r e n o t equipped for t h e Gay-Lussac m e t h o d a n d before going t o t h e expense of equipping t h e m i t is essential t o know if t h e m e t h o d is sufficiently b e t t e r t h a n t h e sulfocyanate method. h sample of cyanide bullion, very low i n silver, was t e s t e d in four laboratories with t h e following results: CYANIDEBULLIONV E R Y Low IN SILVER: GOLD,884 Laboratory Cupel Sulfocyanate Gay-Lussac 1st.. . . . . . . . . . . . . . . . . . . . 6 . 0 5.0 ... 6.0 6.0 ... 6.0 5.9 ... 2nd ..................... 6.0 5.0 5.5 6.5 6.0 ...
h very low bullion, I I O fine i n gold. previously mentioned, carrying a very large a m o u n t of lead was cupelled 16 times a n d t h e silver determined b y t h e sulfocyanate m e t h o d 13 times i n various laboratories with t h e following results: Fineness
Assays Cupel S-C
2.0
25 .5
Fineness
..
2 1
..
..
, .
..
3
Assays Cupel S-C
-
..
.. -
16
13
T h r e e samples of bullion f r o m t h e same mill as t h e last sample, a little higher in silver, were examined a t t h e p’urchasing office b y b o t h cupellation a n d t h e sulfoc y a n a t e method a n d t h e n tested a t t h r e e other laboratories b y b o t h these methods, a n d also b y t h e GayLussac m e t h o d , with t h e results shown i n Table X I I I .
TABLE XIII-CYANIDE BULLIONLow Purchasing office. 1 st l a b o r a t o r y .
,
,......................
. . . . .. . . . . . . . . . . . . . . . . . . . .
2nd l a b o r a t o r y . . . . . . . . . . . . . . . . . . . . . . . . . . 3rd l a b o r a t o r y . . . . . . . . . . . . . . . . . . . . . . . . . . . G o l d . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . .
No. 1 GaySulfocyanate Lussac 18.1 .. ‘8.1 .. 11.2 17 16.8 17 19. 2 5 18.6 15 18.25 17.4 15 20.0 16.9 18 21.0 16.9 19 920.5
Cupel 19.0 19.0 16.0 14.0
M a n y o t h e r comparisons between these t w o methods a r e shown i n t h e larger comparisons given beyond. Once having t h e silver i n solution, t h e Gay-Lussac m e t h o d of t i t r a t i o n with chloride of sodium should be a n excellent m e t h o d of determining t h e silver. T o get satisfactory results, however, i n a reasonable length of time, requires especial e q.~ u i p m e n t , a n d i t is essential t o h a v e a suitable shaking a p p a r a t u s t o settle t h e silver chloride precipitate. I h a v e already described t h e
iii
iis
use of t h i s m e t h o d i n our M i n t Service.’ I n order t o follow t h e mint practice closely i t is essential t o have l
“ T h e Gay-Lussac hle1.hod of Silver Determination,” THISJ O U R X A L , M . E . , 45, 256.
6 , 209; Trans. A . I .
IN
SILVER No. 2
Cupel 14.0 14.0 10.0 11.0 12.25 12.5 15.5 14.5 916.5
s-c
14.8 14.8 12.0 12.0 14.2 14.0 11.9 12.9
G.-I,.
..
1s:o 14.5 13.0 13.0 14.0 15.0
Cupel 16.0 16.0 20.0 19.0 16.5 17.0 18.0 18.0
No. 3 S-C 17.0 17.0 15.0 15.2 15.4 .6 15.9 15.9 884.5
G.-L
..
15 16
14 14 16 18
T o p a n d b o t t o m samples of a different cyanide TOP lst laboratory,,, , , ,Cupel ,62 2
2nd laboratory..
..
Gold. . . . . . . . .
S-C 59 9
BOTTOM G.-L. 59 60
Cupel
58 8
S-C 55 6
G.-I.. 61 .O 1.0
6: 2 57
bullion were examined i n t w o laboratories b y t h e three methods with t h e above results.
No. 1 KO. 2 No. 3 Cupel S - C G . - L . C u p e l s - C G.-I,. Cupel S - C G.-L. Cupel 238 . . . . . . 256 . . . . . . 277.0 . . . . . . 350.0 8 ... 6 ... 7.0 . . . . . . 0.0 1st L a b . . . . . . . . . . . 234 228 253 229 2 3 i : o . . . . . . . . . 346.0 29 30 2 30 2.0 . . . . . . . . . 8.0 i 2 1 1.5 . . . . . . . . . . . 36 27 ... Y Y o 3 3.0 . . . . . . . . . 230 . . . . . . . . . 28O:o 182 34910 . . . . . . 29 . . . . . . . 1.0 7 1 50.0 3rdLab. . . . . . . . . . . . . . . . . . . . . . . . . . 275.5 284 . . . 351.75 . . . . . . . . . . . . T.5 4 . . . 3.75 Gold. . . . . . . . . . . . 525 529:is ” ’ 173 Purchasing office..
73 1
No. 4 S-C
G.-L.
. . . . . .
Cupel 409.0 9.0 410.0 9.0
No. 5 S-C
G.-L.
. . . . . . 465:O 4iO:o
,.. ... ... 7.0 0.0 . . . . . . . . . . . . . . .
349:O 8.0 352:l .3 352.1 .1 240
356:s 49.5
... ...
4ii:o 9.0 415.5 6.5
4iO:o 2.5 414.6 3.5 511 5
4iO:o 2.5
... ...
No. 6 S-C
G.-L.
437.0 5.0
433:O 3.0
44O:o
44O:o 37.0
44i:o 4 4 i : j
Cupel 439.0 9.0
..
...
...
39.0
440,iS 4 3 1 . 6 1.75
2.8 258
.. 1.25
..
.O
.. ..
Most of t h e results i n t h e above table were obtained i n settling disputes a n d in some cases different laboratories worked on different samples. T h e p u r chasing office is t h e same i n each case.
T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
73 2
Sample No. 3 contained a great deal of lead. T h r e e of' t h e original S a n Francisco samples were t e s t e d b y t h e Gay-Lussac method i n l a b o r a t o r y No. 3 as follows: No. 2 S - C G.-L. 220 224.0 3 2.5 222 225.5 3 3.5 4 754.25
No. 4 No. 6 Cupel S - C G.-L. Cupel S - C G.-I,. 278 273 277.0 332 327 331 7 6 80.5 0 7 1 277 274 278.0 328 327 331 6 7 7 . 0 9 7 1 717 636.5
Cupel
. . . . . . . . . 226 4 B . . . . . . . . . . 226
A.
Gold..
.
T w o of t h e original S a n Francisco t u r n e d t o t h a t institution without what t h e samples were a n d t h e cupel determinations were repeated a n d terminations a d d e d as follows:
No. 3 Cupel S - C G.-L. 254.5 250.4 5.5 49.3 256.0 253.5 255 8.0 .5 6 257.5 250.4 6.5 49.3 258.0 254.6 i i 6 7.0 .6 5 654.5
...
.. 2 n d . . .................... B 1st ...... ........... 2nd ......................
Alst..
...
........
Gold
samples were reinforming t h e m a n d sulfocyanate Gay-Lussac deNo. 1 Cupel S-C G.-L. 201 198.0 2 6 9 206 195:6 6 6.7 7 204 198.0 3 8.0 . . . 205 197.8 197 5 6.7 7 632
TOP 2nd . . . 2nd m i n t . .
......
Gold. ....................
BOTTOM
Cupel S - C G.-L. Cupel 140.5 136.7 . . . 142.9 4.5 7 ... 3.9 147.0 139:9 139 145.0 39.0 41.0 8 1.0 142.5 139.2 133 1 4 6 . 0 3.5 8.2 4 3.0 645
S - C G.-L. 135.5 .5 141.0 13810 2.0 7.0 136.7 133.0 .2 3.0
...
T o p a n d b o t t o m samples of t w o cyanide bullions f r o m t h e same office a s t h e l a s t were sent t o t h e S a n Francisco M i n t a n d t h e same second m i n t . T h e results reported were as follows: TOP - .~
S a n Francisco . . . . . . . . . 2nd m i n t ,
............
BOTTOM
S a n Francisco . . . . . . . . .
............. Gold, . . . . . . . . . . . . . . . . 2nd mint
Cuuel 417.25 21.25 420.5 19.5
No. 1 S - C G.-L. Cupel 414.7 485.5 7.5 5.9 . . . 417.7 420 4 8 6 . 5 8.7 . . . .5
No. 2 S-C 481.4 4 483:8 6.3
.9
... ...
.7
422 4
480.2 79.1 483.3 4.3 434
...
416.0 21.0 419.5
415.9 418
.5
434
485.75 7.75 487.5 8.5
G.-L
...
489:O 91.0
... 489:O 9.0
T o p a n d b o t t o m samples of t w o more cyanide bullions f r o m t h e s a m e office were sent t o t h e S a n F r a n cisco M i n t a n d t o a n o t h e r second mint a n d were also r e t u r n e d t o t h e purchasing office. T h e results reported were a s follows: Cupel
TOP
.... mint. . . . . . . . . . . . . .
S a n Francisco M i n t , 2nd
Purchasing office.. , . , . .
449.5 2.5 449.0 2.0 9.0 446.5 7.5
No. 1 S - C G.-L.
441.2 0.3 436.0 6.0
...
438.2 9.1
BOTTOM
S a n Francisco ..........
445.0 441.2 ' 38.0 .2 2nd mint . . . . . . . . . . . . . . ., 4 4 8 . 0 445.0 2.0 5.0 9.0 Purchasing office. . . . . . . 449.0 445 6 9.0 8.2 Gold. . . . . . . . . . . . . . . . . . 386
:
...
No. 2 Cupel S-C
...
448.0 7.0 445.0 3.0
437.4 40.2 433.0 3.0
.
452,'O 0.0
444.'8 5.9
... .,.
441.5 0.5 444.0 50.0
437.4 8.3 433.0 3.0
446 6 5 , ,
...
443 4 3
. . . 444: 5
...
5.0
Silver Cadmium fineness assays 45.0 2 46.0 1 4 9 . 0 ...........................
............................. ...........................
Nitric acid assays 1 1 2 1 4
.. .. ..
-1
iik
T o p a n d b o t t o m samples of a cyanide bullion f r o m a small office were s e n t t o t h e S a n Francisco M i n t twice without informing t h e m w h a t t h e samples were. T h e samples were s e n t t o a n o t h e r mint without inf o r m a t i o n a s t o t h e S a n Francisco results. T h e results reported were as follows: S a n Francisco Mint, 1st
acid with t h e retention of only traces of silver b y t h e gold t h e r e does n o t a p p e a r t o be a n y real a d v a n t a g e i n employing t h e cadmium t r e a t m e n t , as is well shown b y t h e following comparisons: T h e very low-grade bullion high in lead previously mentioned was assayed i n various laboratories eleven times b y t h e cadmium sulfocyanate method a n d t e n times by direct solution in nitric acid a n d t i t r a t i o n with sulfocyanate as follows:
... ...
4i6: 2 8.8
380
G.-L.
... 442:O 3.0
... 44i:5 2.0 1.0
... ...
W h e n t h e gold in t h e bullion is sufficiently low t o permit t h e direct solution of t h e other metals in nitric
KO.9
Vol. 6,
1.0
110
T w o other samples also high i n lead b u t very much higher in silver yielded t h e following results: Silver finen e ss
No. 1 Assays Nitric Cadmium acid
No.2 Assays Silver fineness
Cadmium
Nitric acid
T w o ordinary samples of bullion yielded t h e following comparisons : No. 1 Assays Nitric Silver Silver Cadmium acid fineness fin ene ss 352.3 333 . . . . . . . . . . . . . . . . . I .8 4................. 1 3.9 2 6................. 4.4 7................. 1 8. . . . . . . . . . . . . . . . . . . 1 ... 41.. . . . . . . . . . . . . . . . . . 1 ... , . . 4. . . . . . . . . . . . . . . . . . . 1 5. . . . . . . . . . . . . . . . . . . 1 Gold. . . . . . . . . . . . . . . . 215 Gold: 1 '
No. 2 Assays Cadmium
.. ..
1 1
.. .. .. 240'
Nitric acid 1 1
.. .. .. ..
..
..
As t h e a m o u n t of t h e samples, left after making t h e main comparisons, allowed, four of t h e S a n Francisco bullions were assayed b y direct solution i n nitric acid a n d titration with sulfocyanate, yielding t h e following comparisons: h.0. 1 Assays Nitric Silver Cadmium acid fineness 375 . . . . . . . . . . . . . . . . . . . 1 .. 6. . . . . . . . . . . . . . . . . . . 2 7 ..................... '3 8. . . . . . . . . . . . . . . . . . . 2 2 80. . . . . . . . . . . . . . . . . . . . . 3
Gold. . . . . . . . . . . . . . . . . . 394 . . . . . . . . . . . . . . . . . . . 5. . . . . . . . . . . . . . . . . . .
.. 2 2
6.. . . . . . . . . . . . . . . . . . .. ..
................... 8. . . . . . . . . . . . . . . . . . . 1 401 . . . . . . . . . . . . . . . . . . . 3. . . . . . . . . . . . . . . . . . . ' i Gold. . . . . . . . . . . . . . . . . .
163:s No. 2
'i
1 2
i
!I ..
164.5
Silver fineness 40 1 3 4 5 7 10 533 5 7 8 9 40 5
No. 3 Assays
Nitric Cadmium acid 2 1 1 1 1
..4 169' No. 4 1 1 2 1
..
4 1 1
..
..
.. ..
147.75
..
Careful consideration of these comparisons shows t h a t t h e sulfocyanate method is capable of yielding very satisfactory results in m a n y instances, b u t t h e widely varying results so often shown suggest t h e desirability of more explicit instructions for carrying on t h e m e t h o d a n d especially of pointing o u t probable a n d possible sources of error in t h e manipulations. Therefore, each assayer, whose work h a s been used in t h i s p a p e r , was asked t o write a history of his experience with t h e method. Also during t h e course of t h e
Sept., 1914
T H E J O U R N A L O F I N D U S T R I A L A-VD E N G I N E E R I N G C H E M I S T R Y
t e s t s various observations h a v e been reported f r o m t i m e t o t i m e regarding t h e operation of t h e method. A s t u d y of t h i s d a t a h a s b r o u g h t o u t t h e following points: Various assayers prefer t o m a k e t h e c a d m i u m alloy i n a small porcelain crucible or annealing c u p over a Bunsen b u r n e r or small blast l a m p . T h i s allows t h e direct transfer of t h e weighed sample t o t h e crucible or cup before heating a n d t h e omission of t h e lead foil container for t h e sample if desired. D u s t y samples, however, should be enclosed i n lead foil. T h e alloying operation is t h e n easily observed a n d is under b e t t e r control. Where m u c h work is t o be done, a copper s t a n d t o hold several assays m a y be made a n d t h i s m a y be warmed u p o n t h e lip of t h e muffle or even inside t h e muffle after i t h a s cooled down from other work, before heating over t h e l a m p . M u c h less cyanide is required t h a n i n alloying i n a scorifier. One assayer uses only t h r e e g r a m s of cadmium. This, however, raises t h e question of t h e a m o u n t of silver retained b y t h e residual gold. This point calls for f u r t h e r investigation. Personally, I a m also convinced t h a t t h e very foundation of t h e work, t h e question of t h e formation of a proper alloy with t h e c a d m i u m , requires investigation. M u c h serious objection h a s been raised against alloying t h e samples with c a d m i u m i n a scorifier i n t h e muffle, liability t o crack after s h o r t use, liability of particles of sample t o escape alloying with t h e c a d m i u m , difficulty of observing t h e process, liability of overheating with s u d d e n evolution of gas, particularly on adding t h e C d , which m a y project small particles of t h e sample o u t of t h e fusion, especially i n t h e early stages of t h e operation, liability of small beads becoming entangled i n t h e cyanide, particularly o n pouring, are urged against t h i s operation. Its chief a d v a n t a g e i n allowing t h e reuse of t h e cyanide is greatly reduced b y t h e fact, t h a t a much smaller a m o u n t of cyanide will answer i n other containers. T h e r e is very general objection t o pouring t h e fusion a n d i t is preferred t o cool t h e fusion i n t h e crucible or c u p , a n d t o r u n water, w a r m if possible, directly i n t o t h e cool crucible or cup t o dissolve t h e cyanide. Where much work is done a n arrangement of small faucets allows t h e solution of t h e cyanide f r o m several fusions at one t i m e . A wire gauze f r a m e , with gauze cover, t o hold several crucibles a n d a suitable rosesprinkler make a very convenient dissolving arrangement. T w o assayers assert very positively t h a t t h e disappearance of fumes does not give sufficient heating of t h e solution, but t h a t t h e solution must be vigorously boiled after t h a t . Care m u s t , however, be exercised t o avoid mechanical loss of solution i n t h i s boiling. It is, therefore, b e t t e r t o dissolve t h e alloy i n a conical flask r a t h e r t h a n i n a regular silver bottle. T h e t i t r a t i n g m u s t be done absolutely cold, a n d all t h e conditions a s t o t i t r a t i n g must be k e p t a s uniform a s possible. When t h e bullion contains only a little silver some operators prefer t o a d d a known a m o u n t of silver t o t h e solution before t i t r a t i n g . When t i t r a t i n g a solution of pure silver, a s i n s t a n d -
733
ardizing t h e solution, t h e e n d point m a y be sufficiently s h a r p , b u t , a s i t so often happens with volumetric methods i n general when applied t o miscellaneous materials, so t h i s method presents i t s most serious difficulty i n determining t h e e n d point i n practical work. Also, a s usual, t h e personal equation of t h e operator exerts i t s greatest influence a t t h i s point i n t h e method. While i t helps t h e work along t o know approximately t h e a m o u n t of silver present, t h e careful operator is n o t i o b l i g e d t o m a k e a preliminary assay for t h i s purpose.
It h a s been suggested t o determine t h e silver i n t h e base metal assay beads b y sulfocyanate t i t r a t i o n a n d some promising preliminary results were reported, b u t consideration of t h e question of cupel absorption i n t h e base metal assay precludes t h e expectation of satisfactory results b y t h i s proceeding, unless t h e result b e corrected b y carrying t h e assay proof beads t h r o u g h t h e same operations. Manifestly t h i s would consume t o o m u c h labor a n d b e t o o lengthy. However, t h e following interesting set of results on identical samples of a bullion m a y be given: Lab. 1st..
. . . . . . . . . . . . . .. . . . . . . .
2nd.. . . .
3rd... .
. . . . . . . . . . . . .. . . . .
. . . . . . . . . . . .. . . . . . . .
Cupel 309,O 10.0 1.0 2.0 309.0 .5 11.25 2.0 303.0 4.0 4.0 5.5
6.5 7.5
.
R- i
4th . . . . . . . . . . . . . . . . . . . . . . . .
I
9.5 308.9 9.9 .9 .9
Silver s-c G:L. S-C beads Gold 301 . O 2 9 7 . 0 293.0 430.2 3.0 2.0 8.0 .5 2.0 8.0 4.0 .6 8.0 3.0 4.0 1. O 300.0 303.9 294.9 430.0 .9 5.2 2.0 .8 2.5 4.; .5 1.1 3.5 .I 6.9 .6 302.5 304,O 3 0 2 . 5 4 3 0 . 8 .5 4.0 .5 .9 .5 5.5 6.0 1. O .5 9.5 10.0 1 .o ,.. ... ... 1. o ... .2
. . . . . . . . . . . .
367:6 .b
.
..
30i' 3 4 5
300' 1
... . .
4ii:i .I 1 1
During t h e progress of t h e work various abnormal results reported were investigated more or less. N o s t of these m a y be set down a s d u e t o some unusual a n d a t t h e t i m e unobserved accident or t o t h e lack of proper care i n observing obvious necessary precautions. Once or twice I suspect t h e use of water containing chlorides. One case, however, was so peculiar t h a t i t was thoroughly investigated, u p t o t h e practical exhaustion of t h e samples. I t is No. I i n t h e set of I z S a n Francisco samples. Duplicate samples of t h i s bullion were cupelled for silver, i n five laboratories, 1 2 times each, with t h e following results: A 1 1 9 7 . 0 . . . . . . . . . . . . . . . . . . .. . . . . . . 1 200.0.. . . . . . . . . . . . . . . . . . . . . . . . 3 1 . 0. . . . . . . . . . . . . . . . . .... . . . . . 1 2 . 0 . . . . . . . . . . . . . . . . . . .. . . . . . . 2 3 . 0 . . . . . . . . . . . . . . . . . . .. . . . . . . 4.0 . . . . . . . . . . . . . . . . . . . . . . . . . . . ......... 4.5.. 2 j . 0................... . . . . . . 2 6 . 0 . . . . . . . . . . . . . . . . . . .. . . . . . .
.................
B 1
2 1
..
2 2
1 3
..
-
-
12
12
T h e silver was also determined b y t h e sulfocyanate method i n t h e same five laboratories I Z times, a n d on each sample there is a group of corresponding high a n d low results, a s follows:
734
T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y A
B 1 1 1 1 1
.................... ...........
1
.. 1 .. 2 .. ‘i
-
-3
12
12
One of t h e laboratories also determined t h e silver b y the Gay-Lussac m e t h o d at 196 a n d 7 for A, and 197 and 7 for B. The preponderance of t h e evidence is strongly i n f a v o r of t h e high sulfocyanate figures, b u t t h e 1 2 low results o b t a i n e d in t h r e e laboratories c a n n o t be ignored, and i t became m o s t i m p o r t a n t t o ascertain, if possible, the cause of t h e s e low results. T h e balance of t h e samples was, therefore, s e n t t o one of the laboratories with instructions t o examine i t m o s t carefully, and especially t o w a t c h the sulfoc y a n a t e t i t r a t i o n s f o r indications t h a t m i g h t w a r r a n t s t o p p i n g the t i t r a t i o n s within t h e limits of the lower r a n g e of results already reported. Curiously e n o u g h , however, t h i s examination also developed serious difficulty with t h e Gay-Lussac t i t r a t i o n . T h e following is t h e assayer’s r e p o r t u p o n his work. “The samples of bullion transmitted in the letter of the Director of the Mint of the 15th, marked IIA and IIB, have been carefully assayed, partly by myself and partly under my close observation, with the results which are peculiar and surprising, as shown below. “Two assays were made for gold, giving 632.5 and 632.9. By the three methods of determining silver the following results were obtained, proof corrections being made in each method: CuDel
4. . . . . . . . . . . . . . . . .
205;/z 204’/2
Silver
G...I 181 184 184
Sulfocyanate 191 191 190
B
The cupellation results were first obtained and three assays from each sample were then made by the Gay-Lussac and three by the sulfocyanate. “The appearance of the gold in the bottom of the containers, after dissolving two cadmium buttons in nitric acid 32 B., was unusual. The gold remained as a fine dark powder and longcontinued boiling failed t o lighten the color or cause it to become somewhat aggregated, which is quite unlike the usual character of such undissolved remainders. “ I n working by the sulfocyanate method, as the silver was precipitated, the solution became of a dark bluish color, which made the end point difficult t o determine. Quite consistent results were obtained, however, and the accuracy of the operator’s judgment was checked by adding sulfocyanate to a much more decided end point three to five points higher, and then working back with silver solution to the disappearance of the color, closely confirming the original results. “ I n working by the Gay-Lussac method, after addition of IOO cc standard salt solution and agitating, the solution was somewhat dark, but not enough apparently to interfere with a fairly correct determination of the end point.
Vol. 6, KO. 9
“Upon comparing results from the three methods, i t was at once noted that the chloride had given the lowest results. A reassay was, therefore, made upon each sample by that method, filtering to a clear solution t o determine the end point. “These two assays, shown above, varied but little from the original, one being a little lower and the other somewhat higher. “ I t was then thought that the presence of palladium or some of the platinum metals might account for the high cupellation results, but upon trial no indication of such metals was obtained. On the assumption, then, that base metals might have formed compounds interfering with the humid determination, and to eliminate this factor, six new cupellations were then made of one-half gram each, two from the A sample and four from the B sample, there being more of the latter, and the silver content of the button determined. Corrected by corresponding proofs, these gave the following results. A . ....................... A , . . .....................
B ........................ B ........................ B ........................
B ........................
Cupel silver
Humid silver c1
S
c1
S
c1
S
2041/1 203 204 204 205’/2 205
“Each of these buttons, together with each of the proofs used, was fused with cadmium and the actual silver content determined, three by the chloride method and three by the sulfocyanate, giving, after making the proof corrections, the results shown in column headed ‘Humid Silver.’ Those worked by chloride are marked C1, and the others by S. “These results confirmed the high results obtained by cupellation and indicated the presence of silver in some form which failed of solution in the nitric acid. An assay of A and B was, therefore, made, fusing with cadmium and dissolving in nitric acid as usual. Sulfuric acid was then added and a n additional long boiling given. These were then worked by the chloride method and showed silver 182, no better than the original determination by this method. “Two grams of the sample were then fused with cadmium and parted in nitric acid with long-continued boiling. The residue was then filtered off, washed, dried, dissolved in aqua regia, nitric acid removed by evaporation, and, upon redissolving in hydrochloric acid and dilution, silver chloride showed in considerable amount. It was collected on filter, burned and cupelled, giving 40.5 mg. This button was dissolved in nitric acid titrated with sulfocyanate, and showed 39 mg. of silver, the remainder of the button being gold. From previous experience, z1/2 mg. was allowed for the loss in cupellation, making 4 1 ’ / 2 mg. here found in the residue. The gold in the filtrate from the silver chloride was recovered and upon assay was found to contain 3l/2 mg. of silver, making a total in the original residue of 45 mg. “The filtrate from this residue containing the silver soluble in nitric acid was titrated with sulfocyanate and gave 364 mg., which, added to the 45 mg. previously recovered from the residue, gave 409 mg., equivalent to a fineness of 204~/2 silver, which corroborates the higher of the cupellation returns. “Apparently, the sulfocyanate, when titrated directly into the solution containing residue, has some effect upon the undissolved silver which sodium chloride solution has not, as the titration upon the filtered solution above, thoroughly washed, gave returns agreeing with those by the Gay-Lussac method, while those directly titrated had run 7 to 8 thousandths higher. “The filtrate from the precipitated gold by qualitative analysis was found to show traces of mercury, copper, iron, arsenic and tin. “The lack of more of the metal prevents any further investigation of the nature of the base metals contained, but the work above described seems to show that the variation in the results, and the low returns obtained by the humid processes, are due to the imperfect solution of this peculiar material in nitric acid.”
T H E J O r R Y A L O F I , V D I / S T R I A L A N D ENGINEERILVG C H E M I S T R Y
S e p t . , 1914
It is, therefore, certain t h a t t h i s particular bullion is quite unsuited for t h e determination of t h e silver b y either one of t h e volumetric methods. I n elucidating t h e operations of t h e m e t h o d t w o institutions h a v e m a d e m a n y t e s t s upon s y n t h e t i c preparations or proofs. F o r t h e most p a r t these tests were m a d e t o determine t h e effect of copper upon t h e method, b u t various proportions of other metals a n d combinations were also tried. I n one institution 1 2 s y n t h e t i c proofs were weighed u p , fused with c a d m i u m , a n d t i t r a t e d with sulfocyanate with t h e following results: Parts per 1000 weighed uu
Titration fin en es s
F
Silver
iz;:il . . . . . . . . . . . . . . . . . 250 i:::;} . . . . . . . . . . . . . . . . . . . 2.50 z f ; : ; I . . . . . . . . . . . . . . . . . . . 250 ;;;::1 . . . . . . . . . . . . . . . . . . . 250
Gold
250
. . . . . . . . . . . . . . . . . . . '250
. . . . . . . . . . . . . . . . . . . 250
... ... ...
. . . . . . . . . . . . . . . . . . . 250 250
Coppw
Zinc
750
. . . . . . . . .
500
250
. . . . . .
250
500 250
. . . . . . 250 ...
500
250
...
250
500
...
250
250
250
250
...
250
250
250
250
...
250
. . . . . . . . . . . . . . . . . . . 250
. . . . . .
250
500
. . . . . . . . . . . . . . . . . . . 500 . . . . . . . . . . . . . . . . . . . 750
500
:::::/ ................... ;:;::} zit::1 ::?:: 1................... ::::; 1 :;:::1 251.7 ( 251.il
1
Lead
250
. . . . . . . . . . . . . . . . . .
250
I n eight other cases t h e various metals were weighed i n t o flasks, dissolved direct i n " 0 3 , a n d t i t r a t e d with sulfocyanate. T h e following are t h e results obtained: Parts per 1000 weighed up Titration fineness
7 -
z:;:;) ;g;}
7
................
Silver 250
................
250
it; 1; 1 . . . . . . . . . . . . . . . .
Lead
... ... ...
250
Copper
500
Zinc
250
750
...
250
500
;ti.:,1 . . . . . . . . . . . . . . . .
250
...
...
750
................
250
250
250
250
;:;:;/ ................
250
500
250
...
250
500
...
250
249.9 248,8
250
i50
...
...
i:g::l
iz:: 1 . . . . . . . . . . . . . . . . / ................
These results show a general tendency t o get higher results in t h e presence of copper. I n t h e second l a b o r a t o r y I O cadmium fusion t e s t s o n synthetics yielded t h e following results:
--
1 49 49 51 53
2 48 50 50 51
3 48 50 50 52
149 152 152 154
151 152 151 154
45 1 452
449 453
1000 wrigbed up
...
6
Av.
51 51 52
48.4 50.3 50.7 52.0
149.0 152.0 152.0 154.0
149 151 152 154
150.0 152.0 152.0 154.0
449.0 452.0
448
450.0 452.0
4 48.0 50.0 51.5 52.0
5 49.0 53.0 50.5 52.0
152 153 152 153
150.0 152.0 152.0 153.0
450 452
451.0 452.0
...
-Parts per
Titration fineness. 7 -
Silver Gold Copper 50 950
50 50
...
50
850 650 450
100 300 500
150 150 150 150
850 750 550 350
300 500
450 450
550 250
300
...
100
...
Five duplicate t e s t s yielded t h e following results: Titration fineness
7 775 ) 1 ...........................
lo;/
..........................
:i I . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 1 . . . . . . . . . . . . . . . . . . . . . . . . . . ...........................
--Parts per 1000 weighed up Silver 75
Gold 850
C!opper
100
80Q
100
100
600
300
50
650
300
i5
550
375
75
735
These t e s t s still more clearly i n d i c a t e . a tendency t o high results i n t h e pressure of copper. T h e following t e s t s were obtained b y direct solution of t h e metals in nitric acid: Metal taken Silver found
15 199 80 301 51
202
Silver
15 200 80 300 50 200
.
Silver found
100
26 252 102 502
.. .. ..
100
200 200
-Metal taken
Copper
Silver
Copper
25 250
300 300 500 500
100 500
...
... ...
... ...
...
A set of six tests i n duplicate on synthetic proofs alloyed with c a d m i u m yielded most interesting a n d surprising results a s follows: Metals taken
Silver found
.............
Gold
-
Copper
10
900
100
30
800
200
...........................
50
700
300
..........................
100
500
400
200
300
500
400
...
600
1........................... ::;1 . . . . . . . . . . . . . . . . . . . . . . . . . . . :g";
Silver
It will be noted t h a t in every instance one result i n each set is much lower comparatively t h a n t h e other a n d i n five cases t h e lower result is below t h e a m o u n t of silver - t a k e n , notwithstanding t h e pressure of so much copper I t is, therefore, a p p a r e n t t h a t at times some other influence m a y overbalance t h e tendency of copper t o give high results. Manifestly t h i s is t h e visual condition of t h e operator. Some eyes are always b e t t e r t h a n others in distinguishing shades of color, b u t even t h e best of eyes have their off days. T h e net result of this work on synthetics is t h a t if accurate results are desired proofs m u s t be r u n with t h e assays. Proofs will not only correct for t h e composition of t h e metal b u t also for t h e visual variation of t h e operator. Of course, in miscellaneous work t h e bullions m a y be grouped a n d a generalized proof used for several titrations, j u s t as such a general proof is often used i n cupellation work. I n t h e course of some previous work, i n t h e second laboratory, mentioned aboi-e, i n testing t h e GayLussac m e t h o d o n cadmium fusions, i t was t h o u g h t t h a t t h e c a d m i u m alone h a d a slight tendency t o increase t h e silverfigure>estimated a t one one-thousandth. T h e results, however, were n o t entirely conclusive. If there be such a tendency t h e n i t is all t h e more necessary t o r u n proofs in t h e sulfocyanate titrations for accurate work. T h e following is a final general a n d practical illust r a t i o n of t h e whole subject: During t h e time t h a t t h e active investigation of t h e sulfocyanate method was i n progress a small office shipped 1 1 bars weighing 1 1 7 1 8 . 6 0 ozs. t o a mint. On t h i s shipment t h e m i n t reported 2 5.59 02s. of silver in excess of t h e assay office report. Most of t h e difference between t h e t w o instit u t i o n s occurred i n five b a r s weighing 3 1 5 5 . 7 5 ozs. T h e assay office t o p a n d b o t t o m samples of these five b a r s were forwarded to t h e Bureau a n d t h e silver was determined b y cupellation. T h e samples were also sent t o t w o disinterested mints a n d t h e silver
T H E J O U R N A L O F ILVDUSTRIAL A N D E N G I N E E R I N G C H E M I S T R Y
736
Vol. 6, NO. 9
TABLEXIV No. 1 Cupel
TOP
Purchasing office
........ 1st mint . . . . . . . . . 2nd m i n t . . . . . . . . Bureau..
BOTTOM Purchasing oflice
161.75 2.0 160:50
.75 161.75 .75 158.0 160.0 4.0 160.0 .25
S-C
G.-L. Cupel
. . . . . . . ...... ,, ,
, ,
. . . . . .
16i:6 2.7 159.0 161.0
..,
1:: ...
162
,.. ,.,
. . . . . .
. . . . . . . . . . . .
B u r e a u . , . . . . . . . . 16i .'o ... 1 .o 1st m i n t . . . . . . . . . 160.75 iki:6 1.75 2.7 2nd m i n t . . . . . . . . 156.0 159.0 164.: 161.0 Gold. . . . . . . . . . . . i 42.25
... , . .
,..
... ...
...
No. 2 S-C G.-L. Cupel
400.0 . . . . . . .25 ...... .25 ...... 403.0 ...... .25 399.75 404:2 1 : : 401,75 .2 . . . 402.0 401.0 403 4.0 1.0 , . . 6.0 ...... 400.25 .50 1.25 403.50 .75 400.75 1 . 75 402.0 4.0
. . . . . . . . . . . . . . . . . .
...... ... 402:O , . . 3.1 400.0 . . . 2.0 . . . ..I
560.25
467.0 7.0 7.0 471.75 2.0 470.75 1.75 468.0 71 . O 3.0 465.5 .5
.5 470.75 1.0 471.75 2.75 470.0 1
.o
determined b y cupellation a n d b y t h e cadmium sulfoc y a n a t e method. On t w o samples t h e silver was determined b y direct solution i n nitric acid a n d sulfoc y a n a t e titration. Also a few cadmium Gay-Lussac determinations were made. Table X I V summarizes these determinations: G E X E R A L C 0 N C L US1 0 N S
While i n t h e v a s t majority of cases cupellation yields fairly concordant results, y e t t h e determination of silver b y t h i s method i n different laboratories often shows a difference of j i n t h e fineness reported. Occasionally much larger differences a r e shown. I n t h e comparisons, differences of j fine between individual determinations i n t h e group are common. E v e n duplic a t e assays often show a difference of 5 fine. While i n t h e cadmium sulfocyanate determinations wide differences often appear, y e t i n m a n y cases t h e results agree among themselves a n d are often fully a s concordant as t h e corresponding cupel results. M a n y of t h e differences are undoubtedly d u e t o lack of famili a r i t y b y t h e operators with t h e method. T h e method has not been extensively employed a n d is comparatively new, when considered i n connection with t h e m a n y years of wide application of t h e cupellation method. W i t h more extended a n d extensive use m u c h b e t t e r agreement i n t h e results could be expected. On t h e whole, therefore, t h i s paper shows t h a t t h e sulfoc y a n a t e method compares favorably with cupellation a n d s t a n d s a good chance of supplanting t h e older m e t h o d i n large permanent laboratories where accuracy is desired. Notwithstanding t h e unfavorable opinion of m y predecessor, D r . Whitehead, t h i s work o n cadmium fusions has impressed me favorably towards t h i s operat i o n a s a promising m e t h o d for determining gold i n bullion a n d I a m now sending o u t samples t o test t h i s fusion for t h a t purpose. LABORATORY, BUREAUOF THE MIKT WASHINGTON, D. c.
THE EFFECT OF BREAD WRAPPING ON THE CHEMICAL COMPOSITION OF THE LOAF By H. E. BARNARDA N D H.
E. BISHOP
Received J u n e 20, 1914
With t h e passage of s a n i t a r y food legislation a n d t h e appreciation b y b o t h manufacturers a n d consumers t h a t t h e production of clean food is even more imp o r t a n t t h a n t h e prevention of sophisticated food,
No. 3 S-C G.-I,.
Cupel
......
660,O
::: ...
.25 666.0 7.5 665.0 8.0 661.0 5.0 6.0
...... ...... ,., .., 45i:6 .6 469.0
473
9.0 . . . ......
. . . .
. . . . . . . . . . . . . . . . . . . .
470:5
.5 468.0 9.0 486.25
... ... ... ... ...
No. 4
.o
658.:-
./ 3
S-C G.-L.
Nitric acid
... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ...
667 7 662 4
663
665:9 7.0 664
... ... ...
9.25 ... 667,25 ... .50 665 9 667.0 8.0 .9 665.0 661.0 5.0 2.0 245
:
... ... ... .
.
I
... ... ... ... 66;' 7 660
Cupel 668.75 .75 .75 673.5 4.25 673.25 4.25 664.0 9.0 70.0 667 ..5?
.I3
No. 5 Nitric S - C G.-I,. acid
. . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . 65216 3.8 667.0 7.0
:::
...
671
67i;8 .8 670.0
. . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . .
.75 . . . 673.0 . . . ,25 672.25 652:6 .6 3.25 671 . O 666.0 5.0 670.0 242 .75
. . . . . .
. . . . . .
1:: ...
6?2:6 3.8
. . . 671.0 . . . . . .
has arisen t h e d e m a n d for t h e protection of bread a n d bakery products between t h e oven a n d t h e home. Those who are familiar with t h e facts a d m i t , without question, t h a t t h e loaf a s i t leaves t h e oven is practically sterile both in t h e interior of t h e loaf a n d on t h e surface. I n t h e process of baking, t h e interior of a loaf of bread is raised t o nearly 100' C. a n d a t t h e same t i m e is filled with moist s t e a m , while t h e outside is s u b jected t o a t e m p e r a t u r e of about zooo C. Under such conditions yeast c a n n o t live. Most bacteria will n o t resist prolonged s t e a m h e a t a n d all bacteria on t h e outside of t h e loaf are unquestionably' destroyed. Rousse12 has observed t h a t during baking t h e t e m p e r a t u r e of t h e interior of t h e loaf reaches 101-103' C., a n d t h a t of t h e crust 125-140.j' C. This temperat u r e is somewhat higher t h a n t h a t reported b y Mallett,3 who f o u n d t h a t t h e interior of t h e loaf did not get higher t h a n 100' C. a n d usually r a n lower, even as low a s 9 2 O C. Likewise i n a series of determinations of t h e t e m p e r a t u r e of t h e interior of t h e loaf reported i n t h e Joisrnal d e P h a r m a c i e et d e Chimie4 i t is shown t h a t the temperature of t h e interior of t h e loaf a s i t leaves t h e oven is between 97' a n d I O O O even after t h e baking has continued forty minutes. These temperatures are deemed sufficiently high t o kill pathogenic bacteria b u t n o t a s a rule t h e spores: except i n t h e crust. Numerous authors h a v e pointed o u t t h a t unprotected bread acquires a bacterial flora frequently very extensive, both as t o variety a n d number. Sadtler, i n his report on bread wrapping t o t h e National Association of Master Bakers, shows t h a t t h e colon bacillus was present on t h e surface of seventeen per cent of unwrapped bread samples examined a n d t h a t i n addition t o t h i s number twelve a n d a half per cent showed t h e presence of such large n u m b e r s of bacteria as t o be classed as of doubtful quality. On t h e other h a n d , samples of wrapped bread showed relatively low bacterial counts a n d t h e presence of no pathogenic organisms. I n addition t o t h e necessity for keeping bread in a condition suitable for food, i t must be protected 1 2
3 4
Golden, Proc. Ind. Acad. Sca., 1892, 46. J. Roussel. Univ. Paris, Rea. Intend. md.,20, 122-31. Chemical News, Nos. 1616-1516. Journal d e Pharmacie et d e Chimie, Series 5, 27, p . 16.
