Microdetermination of Silver and Copper in Silver-Copper Alloys Using Precision N uII- Point Potentio metry H. V. MALMSTADT, T. P. HADJIIOANNOU, and
H. L.
PARDUE
Deparfmenf of Chemistry and Chemical Engineering, Universify of Illinois, Urbana, 111. Precision null-point potentiometry has been applied to the determination of micro amounts of silver and copper without prior separation. Silver was determined a t concentrations between 20 and 100 p.p.m. with an average error of about 0.1 p.p.m. and relative Copper average deviation of 0.1%. was determined a t concentrations between 0.3 and 4.5 p.p.m. with precision and accuracy better than 0.005 p.p.m.
I
investigation precision nullpoint potentiometry (PNPP) has been applied to the determination of silver and copper in silver-copper alloys without prior separation of the two. The results obtained compare favorably with reported gravimetric (8) and titration ( I ) methods for silver, and gravimetric (s), titration (1-4, and co1oriniet)ric ( 7 ) methods for copper. The method has the advantages of good sensitivity. accuracy, and reproducibility combined rvith a built-in conipensation for determinat’e errors resulting from impurities in reagents, clect’rode solubilities, and temperature variations I n the determination of silver, the unknown silver solution is varied by a known amount until it. matches a standard reference silver solution. In the determination of copper, the unknonn copper sample is reacted with iodide to liberate a n equivalent amount of iodine. The iodine solution produced in this manner is duplicated by elect,rolytic generation of iodine in a blank iodide solution. The poinhs a t rrhich the standard reference silver solution and the unknoxn iodine solution are duplicated, the null points, are determined by the PSPP technique. N THIS
GENERAL CONSIDERATIONS
The general principles presented for the determination of chloride ( 5 ) by P K P P using silver-silver chloride electrodes apply to the determination of silver also. Likewise, the principles presented for the iodometric determination of oxidants (5) b y PNPP apply t o the iodometric determination of copper
by this method, because under proper solution conditions copper(I1) quantitatively oxidizes an equivalent amount of iodide to iodine. Although the tlyo methods are basically the same, differences in procedure result from the types of concentration variation systems, and t h e relative stabilities of the reagents used. I n the silver determination the concentration variation system is capable of both increasing and decreasing the silver concentration of the “variable concentration solution” (VS). Also, stable standard reference silver solutions can be prepared and maintained. Consequently, it is most convenient to work n i t h a standard reference solution as the “null-point concentration solution” (KS) and to vary the unknown solution to match this reference. On the other hand, in the copper determination the concentration variation system is capable only of increasing the iodine concentration in VS. Consequently, the solution containing the lesser iodine content must serve as T’S; in the determination of oxidants this is the blank. Also, because iodine-iodide solutions are unstable, for best results it is desirable t o prepare a separate blank with each unknown. Thus, in the copper determination the unknown is KS and a freshly prepared blank is 1%. I n both cases it is necessary to maintain the ionic strength at some constant high value so that the activity coefficients of the lesser concentrated ionic species will be relatively independent of their concentrations. I n the determination of silver. 1X sulfuric acid \vas used because this reagent was found to be relatively free of chloride. I n the copper determination, 0.5X potassium iodide was used because this served the dual purposes of maintaining the ionic strength high and ensuring t h a t the copper(1) was complexed and not precipitated as cuprous iodide. All iodine-iodide solutions were maintained in darkness or diffuse light t o avoid photochemical oxidation of iodide. APPARATUS
The instrumental components neces-
sary for a null-type instrument have been described (6). The experimental arrangements and instrumental components used for the silver and copper determinations were those described for the chloride and iodonietric (5) determinations. respectively. The concentration variation system for the silver determination n-as a pair of automatic refill burets, one containing addition reagent (0.1000 mg. of silver per ml. in lizl sulfuric acid) and one containing dilution reagent (1-11sulfuric acid). The silver-silver chloride electrodrls used in the silver determination (S-Conc CompEX, E. H. Sargent 8: Co., Chicago 30. Ill.) m-ere stable for months when stored in the reference solution and not permitted t o dry out. The concentration variation system for the copper determination was that described earlier ( 5 ) . REAGENTS
All solutions were prepared in demineralized water from reagent grade materials. Solutions used in the determination of silver were protected against pickup of chloride from the air. Silver Determination. Stock sulfuric acid (10M). Dilution reagent, 1M sulfuric acid. Addition reigent. This contains 0.1000 mg. of silver per ml. in 131 sulfuric acid. Standard reference solution. This contains 0,05000 mg. of silver per nil. in LU sulfuric acid. The latter three reagents are prepared to have identical sulfuric acid concentrations b y precise dilution of the stock solution. Copper Determination. Potassium Iodide (1M). This is stored in darkness Then not in use and discarded a t the first appearance of iodine. Phthalate Buffer. Potassium phthala t e (0.09M-potassium biphthalate (0.2551). SAMPLE PREPARATION
The sample preparation is described for macro samples to ensure that 17eighing errors do not enter into the results reported. hIodification of this procedure to permit handling of micro samples involves scaling down the amount of nitric acid used in dissolving VOL. 32, NO. 8, JULY 1960
1039
Table I.
PNPP Determination of Copper in Synthetic Silver-Copper Alloys
70 c u , 100 -yo Ag
loo& 95 75
s o . of
Samples 11 3 3 2
50
3 3
40
10
Copper Determined, hlg. (in 50-hI1. Sample) Taken Found Error Av. dev. 0.1589 0.2385 0.1891 0 1325 0 0994 0 0197
0.1589 0.2387 0,1891 0 1323 0 0999 0 0193
0.0000 0.0002 0.0000 0 0002 0 0005
0 0004
0,0005 0.0001 0.0001 0 0003 0 0003 0 0002
a Dilution factor used for analytical sample was different than that described under procedure.
served 11-ith no generation current f l o ~ ing (6). The microgranis of copper in the analytical sample are calculated from Equation 2 or 3.
All above symbols have been discussed ( 5 ) . Substituting appropriate experimental quantities and constants the K value for copper is 1.271 p g , per second so that Equation 2 reduces to (3)
the sample and an adjustment of the dilution factor to give concentrations of copper and silver within the optimum ranges. One-half gram of the sample is completely dissolved in a minimum (3 ml.) of 1 to 1 nitric acid, the solution is evaporated to 1 to 2 ml. and then diluted to about 100 ml. Sodium hydroxide, 1Jf (chloride free). is added until the pH is 3 to 5 (pHydrion paper). This partially neutralized solution is transfrrred to a 500-ml. volumetric flask and tliluted to volume. This solution is labeled S. Solutions for Silver Determination. For best accuracy it is desirable to divide the samples into two categories. containing either more or less than 25% silver. For samples containing less, an aliquot of solution S is diluted 2.5fold to give Si and for samples containing more an aliquot of solution S is diluted tenfold to give solution S,. In making these dilutions sufficient stock I O J I sulfuric acid is added to make the final sulfuric acid concentration exactly the same as the standards. Solution for Copper Determination. An aliquot of solution S is diluted 20fold with demineralized water to give solution S,. PROCEDURES
Silver Determination. The sample is prepared and t h e silver is determined by the PKPP technique. T h e null point is established ( 5 ) with KS (a standard reference silver solution, Css = 0.05 mg. per ml.) in both the isolation compartment and the 50-ml. beaker. After the null point has been cstablished the standard reference solu-
Table II.
% 100 - yo 100 90 , % A
50 25
10 5
1040
0
cu
tion in the beaker is replaced by a volume (V, = 20.00 m1.j of Sior S, containing a fraction, A , of the analytical sample. The silver ion concentration is varied by adding T', ml. of the dilution or addition reagent until the null point is reached. The milligrams of silver, W k , .in the analytical sample are calculated from Equation 1,
n-here A is the fraction of the analytical sample used in the procedure, and C Nis~ the concentration of KS. The plus and minus signs correspond to delivery of V, milliliters of addition or dilution reagent, respectively, into the volume Vo of sample solution S Lor S,, as in the chloride procedures (6). Copper Determination. Copper is reacted lvith a n excess of potassium iodide t o liberate a n equivalent amount of iodine which is determined by the PNPP technique. Into each of two 50-ml. volumetric flasks (V' = j 0 ml.) is pipetted 5 ml. of the phthalate buffer. To one flask is added a fraction A of the analytical sample ( 5 ml. of S,) containing 15 to 225 wg. of copper. Then 25 ml. of 111 potassium iodide is added to each flask and both are diluted to volume with demineralized water and mixed thoroughly. The null point is established with the solution containing the sample in both the NS isolation compartment and the 50-ml. beaker, After the null point has been established, the sample solution in the beaker is replaced by an aliquot (V = 25 ml.) of the blank, and iodine is generated in this solution until the null point is reached. The final null point is ob-
PNPP Determination of Silver in Synthetic Silver-Copper Alloys To. of
Samples 10 3
3 3
3 3
ANALYTICAL CHEMISTRY
Silver Determined, M g . (in 20-111 Sample) Taken Found Error Av. dev.' 0 001 2 000 2 000 0 000 0 001 0 001 1 800 1 801 0 002 1 000 0 000 1 000 0 001 0 0003 0 500 0 499 0 0009 0 796 0 004 0 800 0 400 0 396 0 00.2 0 0005
The data presented in Table I have been calculated asouniing that 1 is unity; thus, thew data represent the amounts of sample actuallJ- handled in the analysis procedure. RESULTS A N D DISCUSSION
-4nalysis of solution:: prepared by dissolving ~eighec! amount; of pure silver and copper in various ratios gave the results shown in Table 11. These data represent the amount of silver used in the analysis 5tep mid demonst'ratr that silver can be tletrrniined a t conccntrations from 20 to 100 p.p.m. nith an accuracy and reproducibility within 0.1 p.p.ni. Thp ioiver con(#elitrationlimit for silT-c'r tietermination with this accuracy r ( d t > irom the necessity for adding a -light excess of acid in the solution process for the metal and the pirkup of I.!iloritlr from bhe air. A4nalj-sis of solutions: containing known amounts of copper at different ~ results silver t'o copper ratios g a i thc shown in Table I . These data demonstrate that copper can be t!eterniined a t concentrations b e t w e n 0.3 and 4.5 p.p.m. nith reproducibility m t l accuracy better than 0.005 p.p.m. The lovier conc8entration limit for copper by this method is set by the solution factors characteristic of the iodine-iodide system which h a w been discused ( 5 ) . However! particular attentimi niust be paid to the buffer type. liuffer concentration, and pH. Park ( 6 ) has observed that in tlic iodonietric tletermination of ropper by thiosulfate titration results w r e lo^ when the p H exceeded an upper limit n-liich was dependent upon the buffer type used. This effect was observrd in this \vork a t both high ant1 Ion pH'$ for acetate, phthalate, aiid citrate i d f e r s . The phthalate-bipht'halate buffer investigated aiid the optimum conditions m r e a pH of 4.0 at a final hiphthalate concentration oi 0.025.11. I n the determination of silx-cr, copper(I1) in concentrations: up to 5 X 10-3M dow not interfere. I-nder the conditions of the copper tlcterniination [(I-j = 0 . 5 X ] , silver(1). mercury (11), and manganese(I1) in concentra-
tions up t o 2 X 10-3X do not interfere. Also, no precipitates of copper(1) or silver(1) iodide were observed. Honever, variations of hydrogen ion concentration on the order of 1 X 10-3.11 cause noticeable error in both cases. Because the solutions used in the copper determinations were huffered, this caused no difficulty; hon.ever, to avoid errors in the silver clcterniinationr it \vas necessary to
control the amount of ewes? acid used in dissolving the sample. LITERATURE CITED
(1) Amin, A. M., Chemist Analyst 44, 17
(190s). ( 2 ) Hall, J. L., Gibson, J. A,, Jr., Wilkin-
son, P. R., Phillips, H. O., AXAL.CHEM.
26. 1484 (1951).
(3) Lingane, J. J., Anal. C h i m Acta 21, 227 (1959). (4) llalmstadt,
H. V., Hadjiioannou, T.
P., Zbitl., 21, 31 (1959). ( 5 ) Malmstadt. H. V., Pardue, H. L.,
A N ~ LCHEJI. . 32, 1031 (1960). 16) Park. B.. IND. ENG. CHEJI.. ANAL. ' ED.3,7T (1931). ( 7 ) Pflaum, R. T., Popov, A. I., Goodspeed, N. C., ANAL. CHEJI. 27, 253 (i955). (8) Stathis, E. C., d n a l . f'hzm. . I d a 26, 21 (195i).
RECEIVEU for review January 18, 1960. iiccepteti March 23, 1960.
Colorimetric Microdetermination of Sexivalent Chromium GEOFFREY HALLIWELL Rowett Research Institute, Bucksburn, Aberdeen, Scotland )Chromium(VI) i s usually determined by reaction with diphenylcarbazide reagent. An alternative procedure i s reported here using a stable, colorless cadmium iodide-starch reagent which i s oxidized b y chromium(V1) to the blue starch-iodide complex. The cadmium iodide-starch reagent i s highly sensitive for chromium(V1) and i s free from the difficulties associated with the preparation and storage of the diphenylcarbazide reagent. The method presented here covers the range 0.05 to 0.5 pg. of chromium per ml. in a final volume of 10 ml.
L
( 7 ) has described the use of a colorless and stable cadmium iodide-starch reagent in the colorimetric determination of bromate. The d a r c h iodide reagent, however, does not appear to have been used in the cletermination of chromium(VI), which i i generally determined by the diphenylcwbazide reagent. The conditions i i c c e ~ w yfor the quantitative colorimetric determination of chromium(V1) using the cadmium iodide-starch reagimt have, now h e w examined. ASIBERT
EXPERIMENTAL
Reagents. Cse analyticaal reagent grade chemicals. Potassium Dichromate. Prepare 0.0037, (IT-. ,'v.) in distilled water. Sulfuric Acid, 1.635. Prepare by dilution from the concentrated acid and standardize. Cadmium Iodide-Starch Reagent. 1)issolve 1. I grams of cadmium iodide in 31 ml. of .water and boil gently (to wmove free iodine) for 15 minutes, keeping the volume constant. Meanwhile prepare starch solution by boiling 0.26 gram of soluble starch in 46 ml. of water for about 5 minutes until the liquid clears. Add this solution to the gcntly hoiling cadmium iodide solution
and boil the mixt'ure for about 5 minutes. Then filter through a S o . 3 porosity sintered glass filter (or S o . 50 K h a t m a n filter paper), cool to room temperature, and make up to 100 ml. with water. This reagent' is stable for a t least a month a t room temperature in clear Quickfit Pyrex flasks (Catalog No. Fe100/2) out of direct sunlight. The preparation of the reagent follows that of Lambert ( 7 ) who, however, prefers a purified pobato starch fract.ion. Procedure. Add 0 t o 0.5 ml. of 0.003% (w./v.) aqueous pot'assium dichromate to 2 . 2 ml. of 1 . 6 3 5 sulfuric acid followed b y sufficient water t o make a volume of 9 ml. i i d d 1 nil. of cadmium iodide-starch reagent, mix t'he solutions, and let st'and a t room temperature for about 20 minutes. Read the blue color so produced on a Spekker phot'oelectric absorptiometer (Hilger Ltd., London. England) using orange filters (Iiford S o . 607) in a 1-cm. light path. RESULTS AND DISCUSSION
A straight-line calibration curve is obtained by the present iodometric procedure with p o t a w u m dichromate. Potassium chromate, laboratory reagent, gives a similar curve. The samples analyzed covered the range from 0.05 to 0.5 ml. of 0.003% (n./v.) diclironiate (0 05 to 0.5 pg. of cahroniium per nil in a final volume of 10 ml.). The final concentration of sulfuric acid in the IO-ml. volume was approximately 0.36.1- and gave a p H of 0.76. This value \vas the optimum for development of the blue color in the working range pH 0.T to 0 9. At the same time the color developcd at 25" C., as ne11 as a t 16' + 3" C. Colors were developed out of direct sunlight, but complete darkness was not necessary. Readings were taken about 20 minutes after addition of the cadmium iodideqtarch reagent, and although the valueq
increased slightly during the n r s t 30 minutes (total color devrlopnic,nt t h e n-as 50 minutes) the increase on the maximum reading was only about 3 7 , . Readings then remained condnnt for a further 25 minutes (total color developnient time was 7.5 minutes). with no sign of precipitation. In order to examinc. the method for ita nccurary in estimating dichroniatr, the clsperinient sholvn in Table I was del-i slion-s the recoveries ohtainctl by the standard procedure ~vhen dichromate was added to a medium n.1iic.h had hreri previously fermented hy llyrotheciuni
rrrrucarin. The iodonietric nidiorl described above for chromium(T-I) rtppears to be a t least, as sensitire as the tiiphenylcarbazide reaction (8, 1 1 ) . Possibly the sensitivity of the present iodometric procedure may be increascd by measuring the absorbance a t x defined \vave length-e.g., 615 mp-in place of the filters employed. Furthermore, the
Table I . Recovery of 0.003470 Potassium Dichromate from Fermentation Medium Taken, Recovered, Range of 111. C'a /C Recoveries, 7, 0 1 0.2
101
98-103 100-103
0 3 0 ,3 0 5
07 98
96-99
97
97
101
96-99
Fermentation medium contained initially, per liter, KH2P040.2 gram. ILHPO, 0.15 gram, KaH2P04.2HI0 2 . 3 grams, 0.6 gram, Xa2HP04 1.5 grams, r\",?;O, XaiYO, 3.8 grams, l I g S 0 , 7H20 0.3 gram, and trace amounts-of Zn, Fe, l l n , Cu, and H,BO,, cellulose, and an inoculum of ATfyrotheczunz uprrucurza. The fermented medium \\as clarified by ('entrifugation prior to use. a Mean of 5 determinations for earh set; resriltq given to nearest 102.
VOL. 32, NO. 8, JULY 1960
1041
