Spectrophotometric Determination of 1 race Amounts of Silver by Dithiol J. P. DUX and W. R. FEAIZHELLER Research and Developmenf Division, American Viscose Corp., Marcus Hook, Pa.
b This method i s based on the abscrption of light at 41 6 mp b y a dispersion of the silver-dithiol complex in an aqueous acid solvent. The absorbance of the dispersion deviated slightly from Beer’s law. The method i s applicable to concentrations of silver from 4 to 40 pg. per milliliter of final solution and precision i s 1 5 to 6% over this concentration range.
C
AVAILABLE methods for deterniining trace quantities of silver in solution are either inadequate or have serious disadvantages. Thus, the romiiion turbidimetric silver chloride method (8) is not very sensitive and requires a ralibration curve to be run with each batch of samples. The dithizone method requires w r y pure solrents (?’), a time-ronsuniing extraction step, and the o d i z e d form of the reagent has a color very similar to that of the silver conipleu. The rhodamine procedure, on the other hand, is very sensitive to acidity and salt concentrations (1, 6) and requires a reagent blank to be run with each batch of samples. The method presented in this paper is simple and direct, it is a “one color” method, and no reagent blank is required, provided the reagents are not contaminated with silver. It is sensitive; quantities as low as 100 pg. may be determined; and the arcuracy is * 5 to 670. The method is based on the formation of a colored complex between silver and toluene-3,4-dithiol (2, S), and is an adaptation of the niethod used by Farnsnorth and Pekola (6) for trace quantities of tin. The silrer-dithiol complex is insoluhle, and is formed in a dispersed state in aqueous sulfuric acid solution, Kith the aid of the surface active agent, dodecyl sodium sulfate. The dispersion appears transparent to the unaided eye, but exhibits the typical Tyndall cone 11hen viewed with a narrow pencil of light. The near perfect transparency of the solution implies that the suspended particles are very small and probably have an inde.; of refraction very clov to that of the qolvcnt. In any ease this tranym-ency niakes it possible to use the absorhaiiw of the sol as an index of the concentration of silver. URREXTLI
The sol has a yellow color which develops immediately. However, an absorption peak does not appear until approximately 24 hours after the reagents are mixed. I n our experiments, this peak appeared a t 416 mp, Although the peak does not appear until 24 hours have elapsed, 65y0 of the final absorbance is reached within 1 hour after mixing of reagents, so this time interval may be used in the practical analyses of samples. Any attempt to hasten color development by heating the solution results in precipitation of the complex. APPARATUS
Absorbance measurements \$-ereniacle with a Beckman hlodel DK-I spectrophotometer Kith I-cm. Corex cells. A Burrell Wrist-Bction shaker was used to mix the reagents gently, to avoid foaming. REAGENTS
Dithiol Reagent, 0.30%. Two solutions were prepared. The first contained 0.15 gram of dithiol (A. D. Mackay, Inc., New York) dissolved in 50 ml. of 2% sodium hydroxide. The second contained 0.15 gram of dithiol and 8 drops of thioglyeolic acid (mercaptoacetic acid) in 50 ml. of 27, sodium hydroxide. The dithiol, received in sealed ampoules, was melted by holding the ampoule under a stream of hot water and the molten liquid r a s then stored in a 5- or 10-ml. glassstoppered flask. Solutions of dithiol were prepared by withdran-ing a portion of the liquefied dithiol. The dithiol solutions, refrigeratcd in 60-nil. glassstoppered bottles biere stable up to 1 week. Used in this way, the dithiol and solutions did not present any stability problems. Dodecyl Sodium Sulfate (Sipon). A 307, solution was prepared and stored in a 60-ml. dropping bottle. Standard Silver Solution. 0.158 gram of reagent grade silver nitrate was dissolved in 1 liter of water. PROCEDURE
Transfer a suitable portion of a, neutral solution containing brtneen 0.12 and 2.5 mg. of silver to a 50-1n1. volumetric flask. Add 4 nil. of 1: 1 sulfuric acid and srirl t o mix. Dilute to about
40 ml. uith nater. Add 2 drops of 307, dodecyl sodium sulfate and place on the shaker adjusted for gentle agitation. If the liquid in the flask does not snirl, add a little more watrr until an actual sn-irling motion is observed. This will allow thorough mixing n ithout excess foaming. Add 1 ml. of the dithiol reagent to the solution and allow time for the solution to mix. Kext add 8 more drops of the 307, dodecyl sodium sulfate solution. Allow the solution to swirl for about 1 minute. Reniove the flask from the shaker, dilute to volume Kith water, stopper, and invert several times to mix. Read the absorbance after 1 hour in I-em. Corex cells a t 416 mp, using distilled water as a reference. Calculate the concciitration from a calibration curve. Sample Preparation. If the sample to be analyzed is of organic nature, a wet ashing technique may be used to destroy the organic matter. The sample is digested with concentrated nitric acid (25 ml. is sufficient for a 1- or 2-gram sample) and 2 ml. of concentrated sulfuric acid in an Erlenineyer flask. Heat is applied to dissolve the sample, after which the excess nitric acid is driven off. Heating is continued until fumes of SO3 are given off and the sulfurlc acid is refluxing on the side of the flask. This solution when cool, is transferred to the 50-ml. volumetric flask and thc color developed as indicated above. Interference
by
Other
Cations.
Although a great number of cations react with dithiol, the common metals most likely to interfere are tliope of Group I-i.e., Pb and Hg, as ne11 as Sn(1V) and -4s (4). The Ag niust he separated froin these ions before rstiniation. Other metals which react with dithiol have complexes of a different color from silver ( 4 ) ,and suitable choice of m-are length for the absorbance measurement choultl reniove an!. interferencc. Thr mcthod n-as developrd primarily for the determination of -4g iii organic materials. Since t h e v s:unpIrs are \vet ashed a.: descrihcd, thc. effect of interfering anions n a. not inr estigatetl. Calibration Curve. Pipet into five 50-nil. volumrtrir flnfks 2- 3-, lo-, 1 5 , xiid 2O-ni1 aliquots of the standard silver solution (0.15C gram .lg?;O1 in 1 liter water). Dilute the solutions to about 40 nil. and add 4 nil. of 1 : I sulfuric acid. Continue a< outlined abovr nith thc color dewlopment. Plot absorbance a t 416 n i p againqt concentrations of 4 v e r (see Figure 3 ) , VOL. 33, NO. 3, MARCH 1961
445
Elapsed itme Afler Colv Dwebpnenl
-4a _____ 24 Hours HOVS
--
---
4Hours Ibur
0
02003
I3
20
30
50
40
HOWS
Figure 1.
using distilled mater as a reference. I n our experiments reagent blanks showed zero absorbance when run against distilled water. Nevertheless, it would be preferable to run a t least one such blank with each new batch of reagent prepared. RESULTS AND DISCUSSION
Role of Thioglycolic Acid. Thioglycolic acid was originally included in the reagent, since this mas used by Farnsworth and Pekola for the determination of tin. I n their work, i t served t o reduce the tin to a lower valence state. However, thioglycolic acid is a known precipitant for silver and so experiments were run with and without this reagent and with thioglycolic acid alone. Thioglycolic acid alone yields an immediate precipitate, even in the presence of the dispersing agent, dodecyl sodium sulfate. However, in the presence of dithiol no precipitate formed, but the absorbance a t 416 mp was enhanced by about 65%. Therefore, the thioglycolic acid was included in the procedure, since the sensitivity is increased. The acid appears to act as a catalyst, decreasing the time required t o reach maximum absorbance, rather than being an actual reactant itself (Figure 1). Silver Dithiol Spectrum. Figure 2 shows the visible spectrum of the solution as a function of time after mixing of the reagents. Initially there is no peak absorbance, only a long trailing "end absorbance" from the ultraviolet. The maximum for this absorbance is below 210 mp and it is possibly due to scattered light. However, as the solution stands, a peak begins to appear in the region of 416 mg, with a corresponding decrease in absorbance in the region of wave lengths greater than 440 mg, Statistical Evaluation of Method. To evaluate the method most efficiently, a statistically designed experiment was run. T h e response
446
Figure 2. Spectra of silver-dithiol complex
Increase of obsorbance with time at
ANALYTICAL CHEMISTRY
416 mp
measured was the mecific absorptivity of the complex, odtained by dividing the absorbance of the solution by the concentration in milligrams of Ag per milliliter and multiplying by 10. The variables studied were concentration, batch of dithiol, and day-to-day variance. Five different concentrations were run, on each of four dags, using
Table
Concentration, /.lg./lVzl.
Specific Absorptivities
Batch I Day 1
4.212 10.53 21.06 31.59 42.12 Day means Batch means Grand mean
100
4.212 10.53 21.06 31.59 42.12 Day means Batch means Grand mean
135 145 179 176 190 165.0
108 157 161 166 138.4
Source concentration Days Batches Total Error
I.
two separate batches of dithiol. The absorbances were read a t 416 mp 1 hour and 24 hours after mixing. Table I gives the results. Table I1 shows the results of the analysis of variance. From the inspection of the data, the analysis of variance shows concentration to be a significant variable. I n other words, the system does not follow
128.8
Batch I1 Day3 Day 3 1 Hour after Mixing 98 81 117 102 146 127 152 140 176 146 119.2 137.8
Day 4 114 125 139
150
137 9
162 138.0
98.2 113.0 142.2 150.8 162 5
I
133.3 24 Hours after Mixing I31 _. 116 157 146 202 175 212 196 227 198 166.5 185.7 ~
165.7
Concentration Means
175.2
184.6
Table II. Analysis of Variance Sum of Squares Degrees of Freedom 1 Hour after Mixing
11,528 921 414 20,810 7,947
4 2 1
19 12
24 Hours after Mixing Concentration 16,672 Days 30 Batches 2,000 Total 19,331 Error 629 F is ratio of variance to that due to error.
135 171 192 206 212 183.5
129.4 154.8 187.2 197.6 206.9
Variance
Fa
2 ,890 460 414
4.36
664
Table Ill.
421 105 21 1 31 6 42 1
Error in Quantitative Estimation of Silver
04002.1 057 040 1 3 1 0 1 4
22
1 9
27
109 5 4 6 2 4 3
5 2
5 3 4 6 6
7 8 9 2 4
Beer’s law. In tlie case of the 1-hour readings, concentration is the only variable which is significant with respect to error. In the cnsc of the 24-hour Samples, dithiol batch is another significant variable. The variance due to error is miich larger in the case of the 1-hour readings than in the case of the 24-hour readings. This is probably due to the fact that in the 24-hour case we are reading absorbance a t a peak, which is not true of the 1-hour readings (Figure 2). On the other hand, the fact that dithiol batch becomes a significant variable in the 24-hour solutions tends to cancel out the apparent increase in precision gained by waiting the longer time. The error variance shown in the analysis of variance table refers to the error in determination of absorptivity. Analytical chemists will be more concerned with tlie error in determination of silver concentration. This was estimated in the following way. Since the dithiol complex does not obcy Beer’s law, a calibration curve must bc constructed to relate absorbance t o concentration. This was done by plotting the mean absorbance of the four determinations a t each concentra-
-
~,k, qc-5 -;
Figure 3.
Calibration curves
tion level against concentration and estimating the best curve through these points. The curves for the two sets of readings are shown in Figure 3. Next, the concentrations corresponding to the individual absorbance readings were read off the calibration curve. This resulted in four estimated concentrations corresponding to each known concentration. The standard deviation was calculated for each set of four estimated concentrations, using the known concentration as the mean and four degrees of freedom, since the mean is known and not calculated from the samples. The relative standard variation was also calculated by dividing the standard deviation by the mean and multiplying by 100. Results are shown in Table 111. The absolute standard deviation increases with increasing concentration, while the relative standard deviation is relatively constant. The mean of the
five coefficients in Table I11 was chosen as the best estimate of the relative standard deviation and hence the precision of the method. This was 6.4% for the I-hour samples and 5.470 for the 24hour readings. LITERATURE CITED
(1) Cave, G. C. B., Hume, D. N., A N A L . CHEW25, 1503 (1952). (2) Clark, R. E. D., Analyst 62, 661 (1937). (3) Ibid., 83, 396 (1958). (4) Clark, R. E. D., Neville, R. G., J. Chem. Edztc. 36, 390 (1959). (5) Farnsworth, M., Pekola, J., ANAL. CHEM.26, 735 (1954); 31, 410 (1959). (6) Feigl, F., 2. anal. Chem. 74, 380 (1929). (7) Sandell, E. B., “Colorimetric,, Determination of Traces of Metals, 2nd ed.. D. 544. Interscience. New York. (8) I b d . , p. 549. RECEIVEDfor review August 31, 1960. Accepted November 23, 1960.
Ebulliometric Apparatus for Studying Number-Average Molecular Weights of Polymers CLYDE A. GLOVER and RUTH R. STANLEY Research laboratories, Tennessee Easfman Co., Division of Easfman Kodak Co., Kingsport, Tenn. An ebulliometric apparatus was needed, which would be simple to operate and would enable rapid, accurate, and reproducible measurement of the boiling point elevation of dilute polymer solutions. The apparatus consists of an ebulliometer of the Menzies-Wright type combined with an 80-junction, copper-constantan thermopile, which serves as the temperature-sensing element. The output of the thermopile is ‘fed through a d.c. microvolt amplifier to a strip chart recorder, where it is continuously
recorded. The sensitivity of the temperature reading is of the order of 0.015 millidegree. Use of the apparatus to obtain data for calculating, b y conventional methods, an ebulliometric constant and the molecular weight of a polymer is illustrated. A limited statistical study is given to indicate the precision of the method.
R
in the field of polymers has resulted in an increased desire for an understanding of the factors APID growth
which influence the physical nature of these materials. One of the most important of these factors is the numberaverage molecular weight. Bonnar, Dimbat, and Stross (4) recently published a thorough review of the methods for determining this property. Four general methods have been used: ebulliometry (11, 12, 15), cryoscopy ( l ) , osmometry (3, 8, 14-16)’ and vapor pressure lowering (1.3). Each method, however, has certain known disadvantages or limitations. After consideration of these methods VOL. 33, NO. 3, MARCH 1961
447
