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ANALYTICAL CHEMISTRY, VOL. 51, NO. 11, SEPTEMBER 1979
Gravimetric Determination of Silver by Formation of the Hexaammine Cobalt(111)-Dithiosulfatoargentate(1) Complex W. W. White" and P. J. Murphy Industrial Laboratory, Kodak Park Division, Eastman Kodak Company, Rochester. New York 14650
A rapid semimicro method is described for the gravimetric determination of silver in thiosulfate-rich solutions. The stable Ag(S203)2-anionic complex of silver is precipitated when hexaamminecobalt(II1) trichloride is used as the precipitant. The reaction may be represented as follows:
CO("~):+
+ Ag(S20J;-
+
[Co("3),1
[Ag(S,OJ21
The method is attractive for several reasons: (1) The Ag(S203)2-complex need not be destroyed to monitor silver concentrations; (2) no complex laboratory equipment is required; (3) the analysis is selective; and (4) a favorable gravimetric conversion factor exists for silver. The compound weighed has a relative molecular mass of 493.220. Analysis showed that the hexaamminecobalt(II1)dithiosulfatoargentate(I),abbreviated HCDA, contains 32.6 '70 CO(NH&~+ compared to the theoretical value of 32.7%. The Ag(SzO3)?- portion was found to be 67.4'70 compared to the theoretical 67.3%. The compound contains 21.87% silver by weight. The method has been applied to diverse types of photographic systems (e.g. fixer solutions where the silver halide has been dissolved by thiosulfate) and accurate results for silver have been obtained. The method has also been applied to silver solutions containing diverse metal ions, anions, and sequestering agents. Yoshimura et al. ( 1 ) have reported that the elements lead, cadmium, mercury, and silver react with thiosulfate and hexaamminecobalt(II1) chloride to form insoluble precipitates. However, no work was done to develop a selective gravimetric method t o determine silver in diverse matrices. EXPERIMENTAL Reagents. The hexaamminecobalt(II1) chloride (Eastman ~
Table I. Recovery of Diverse Quantities of Silver Silver, mg rel. std. present recovereda dev., 90' 10.0 10.0 2.5 25.0 25.1 3.7 50.6 1.4 50.0 2.0 75.0 74.8 100.0 101.6 0.9 a
Average of 5 determinations.
Organic Chemical reagent 8253) solution was prepared by dissolving 30.0 g in 1000 mL of distilled water. The sodium thiosulfate solution was prepared by dissolving 100.0 g of the pentahydrate in 1000 mL of distilled water. The (ethylenedinitri1o)tetraacetic acid disodium salt (EDTA) (Eastman Organic Chemicals reagent 6354) was prepared by dissolving 50.0 g in 1000 mL of distilled water. The sodium acetate was prepared by dissolving 50.0 g in 1000 mL of distilled water. The silver nitrate (Eastman Organic Chemicals reagent 491) used for evaluation purposes had an assay value of 99.9%. All chemicals used were reagent grade. General Procedure. Transfer by pipet a weakly basic or acidic sample aliquot (containing 10-100 mg of silver) to a 250-mL beaker. Add 100 mL of distilled water, 20 mL of the sodium acetate solution, 20 mL of the EDTA solution, and 25 mL of the thiosulfate solution to the beaker. Adjust the pH of the solution between 6-7 using ammonia hydroxide or acetic acid. Add 30 mL of the cobalt reagent and stir the suspension intermittently until a flocculent precipitate is formed. After the precipitate has settled for approximately 1h, suction-filter it onto a tared Gooch crucible. Wash the precipitate sparingly with distilled water and dry the crucible at 95 "C for 30 min or until a constant weight is obtained. The conversion factor to silver is 0.2187. RESULTS AND DISCUSSION Table I shows the analytical values obtained when known quantities of silver were taken in the absence of potential interfering ions. All the silver was recovered in the 10-100 mg concentration range. At least a 1:4 mole ratio of silver to thiosulfate should be maintained to stabilize the Ag(S203)23complex. Ratios of 1:2 and 1:3 gave approximately 80-9070 recoveries for silver. Quantitive results for silver have been obtained over a pH 4-12 range. It was found that interferences from foreign ions were minimized when a p H range of 6-7 was maintained in sample solutions. Solutions that were very acidic caused decomposition of the thiosulfate, thus resulting in interference from elemental sulfur and silver sulfide formations. Table I1 represents a study of silver recovery in three synthetic matrices to demonstrate the versatility of the method. The relative error for 25.0 mg of silver was 1 3 % . The addition of EDTA aids in making the method more selective by keeping many of the elements from hydrolyzing, and in the cases of lead and cadmium, from reacting with thiosulfate. The method can be applied to the determination of silver in samples composed of AgCl, AgBr, AgI, and AgSCN. Excess thiosulfate is added to dissolve the samples and form
Table 11. Recovery of Silver from Synthetic Samples" diverse compounds o r ions added in quantities of 50 mg each
a
silver recovered, mg
AI(NO3),.9H,O,BaC1;2H20, BeCl,, CdC1;21/, H,O, CO(NO,);GH,O Cr(NO3);9H,O, FeC1;6H,O, Ga(III), In(III), Pb(NO,),, Sr(NO,), H,BO,, La(III), Mg(N0,) ;6H,O, MnSO;H,O, Ni(NO,);GH,O, KBr, ZnSO;7H,O ammonium tartrate, ammonium-thiocyanate, citric acid monohydrate, triethanolamine 2 5 mg of silver taken in each case. Average of 5 determinations.
rel. std. dev., %
mean rel. error, %
24.7
4.6
3.0
25.1
0.4
0.5
25.5
0.7
1.9
0003-2700/79/0351-1864$01.00/0 1979 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 51, NO. 11, SEPTEMBER 1979
how efficiently a silver recovery unit attached to a fixer bath operates. In determining the quantities of silver in fixer solutions, all the steps in the General Procedure need not be followed. Usually there is adequate thiosulfate already present to maintain the integrity of the Ag(S2O3)?-complex. However, the final liquid volume in the beaker should be maintained as well as the pH control.
Table 111. Silver Results Obtained from Analyzing Fixer Solutions Using the HCDA and Atomic Absorption Methods" fixer, type HCDA AA
a
A 10.2 B 0.02 C 1.6 D 1.2 E (bleach-fix) 0.8 Silver data reported in g/L.
11.8
0.03 1.5 1.1 0.8
the ~ ~ ( ~ anionic ~ o ~complex. ) 2 - H ~ negative ~ ~ interference was evidenced when KBr and KI exceeded 500 mg and 25 mg, respectively, in a sample. Table 111 shows the silver data obtained by HCDA and atomic absorption on various types of "seasoned" photographic fixer solutions. There were no significant differences between t,he methods (correlation coefficient, r5 = 0.9995). Some chemicals commonly found in fixer baths are: sodium sulfite, aluminum sulfate, sodium acetate, sodium borate, and ammonium thiosulfate. In the case of a bleach-fix solution, iron(II1) may be present, complexed with an organic sequestering agent. The proposed method has been used to monitor silver in fixer solutions to determine when a bath needs replenishing. Also, the levels of silver found in solution help to determine
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CONCLUSION Hexaamminecobalt(II1) chloride has been shown to be a useful reagent for, the gravimetric determination ~ precipitating ~ ~ of silver in Photographic and nonphotographic systems. EDTA can be added to prevent aluminum and iron from hydrolyzing and elements such as lead and cadmium from interfering. The method has the advantage of not having to destroy the silver thiosulfate complex or wet ashing silverbromide-chloride-thiocyanate matrices for subsequent
LITERATURE CITED (1) Yoshirnura, Jun; Takashirna, Yoshirnasa; Murakami, Yoshio; Kusaba, Toyoaki Bull. Chem. SOC.Jpn. 1962, 35, 1435.
RECEIVED for review February 28,1979. Accepted May 9, 1979.
Sample Cells for Photoacoustic Measurements David Cahen" and Haim Garty The Weizrnann Institute of Science, Rehovot, Israel
Photoacoustic (PA) measurements have been reintroduced in the past few years as an alternative (and at times the only) way to investigate spectroscopic properties of solid, semisolid, and liquid samples in the ultraviolet, visible, and infrared (1-4). The electronic equipment needed for PA measurements does not, in general, pose special problems, except perhaps for the choice of a suitable microphone. On the other hand, sample cells, and especially the combination of these with the microphone, tend to be a limiting factor in determining the success of the experiment. Although there are various descriptions of PA cells available in the literature, most of these need expensive microphones and/or sample cells that require considerable skill for their construction (2,5-7). We have been developing the PA method mainly for the investigation of energy conversion processes (by photocalorimetry), which puts high demands on the sensitivity of the method (8-11). The cells now in use in our laboratory, which are a further development of one described earlier ( 4 ) , were found to be very suitable for normal spectroscopic measurements as well, notwithstanding their simplicity of construction. Figure 1 shows two of these cells, whose main features are: (1)Short effective length, i.e., low gas volume to sample surface area (in contact with gas) ratio (VIA). All other factors being equal, a smaller ratio will give a better signal to background (= cell noise) ratio, SIB. (2) Use of commercially available rectangular glass or quartz tubing. (3) Versatility through the use of different, interchangeable, sample holders improving reproducibility and S I B especially for liquids and flat samples. While the round quartz tubes used in the earlier described cell gave reasonable results, experiments on less ideal samples and difficulties with reproducibility because of ill-defied light 0003-2700/79/0351-1865$01 .OO/O
intensity suggested the use of sample cells with lower V I A . The rectangular tubing (Vitro Dynamics, Rockaway, N.J.) we use, was chosen from tubing with several dimensions as the one giving the best results. Especially a minimal wall thickness (21mm) was found to be necessary. Although initially the sample was placed directly on the cell bottom, this method was found to introduce much noise because of such external factors as small temperature changes, air flow around the cell, and ambient acoustic noise. Closing the cell with a cover to which the sample holder is attached directly decreases the cell volume, while hardly affecting exposed sample surface area and improves reproducibility because the sample can be taken out, treated, and returned to the same position. Moreover the sample is now relatively insulated from outside interference. Silicone vacuum grease is used to seal the sample holders to the cell ( 4 ) . The two kinds of sample holder shown in Figure 1 each have their special advantages. Component 4 is a bath constructed from Perspex with a transparent bottom. Such a holder is particularly useful for liquids and powders. The small V I A of part 4 is important for liquids, which, because of their small surface area among other things, tend to give low PA signals. Figure 2 illustrates this for an aqueous suspension of bacteriorhodopsin-containing purple membranes of Hulobucterium halobium, a t several frequencies ( I O ) . Here light absorption initiates a cyclic photochemical process which drives the translocation of protons from one side of the purple membrane to the other. The occurrence of a photochemical process will cause a decrease in PA signal because less of the absorbed energy is converted into heat. At higher modulation frequencies, earlier intermediates of the photocycle are sensed, 0 1979 American Chemical Societv
