Colorimetric Determination of Silver with p-Dimethylaminobenzalrhodanine G. C. B. CAVE1 ~ N DAVID D N. HUME Department of Chemistry and Laboratory f o r .Vuclear Science and Engineering, .\Zassachusetts Cambridge 39, M a s s . very sensitive color reaction between silver ion and pT~~ethylaminobenzalrhodanine (p-dimethylaminobenzilidenerhodanine), hereafter called simply “rhodanine,” discovered by Feigl ( 1 ) was shown to be adaptable to the quantitative determination of microgram amounts of silver by Schoonover ( i ) , who noted t h a t the presence of neutral salts or mineral acid affected the color intensity and reproducibility. Sandell ( 2 ) emphasized the need for close control of the acidity and correctly attributed some of the difficulties in the determination to the fact that the color is due to a very finely divided dispersion of the high]?, insoluble silver salt of rhodanine. Since the completion of the experimental work in this paper, Sandell and Seunia!.er ( 3 ) have published a further study of the rhodanine method (using the ethyl analog) and its application t o the determination of trace silver isolated with tellurium as a carrier. The present investigation \\-as undertaken because, in connection with the determination of the solubility of silver thiocyanate in potassium nitrate-potassium thiocyanate mixtures, the authors had need of a method for the determination of silver with a n accuracy of =k2yO in concentrations as low as 6 X lo-’ M. Prcliminary experiments revealed t h a t the rhodanirie method following the procedure given by Sandell had the requisite sensitivity but did not meet the stringent reproducibility requirements stated above, particularly in the presence of high sodium nitrate concentrations. A systematic study was thrn made of the variables affecting the stability, reproducibility, and absorption characteristics of the silver rhodanate suspension. It soon became apparent t h a t the important variables include the acidity of the fmal solution, the concentration of rhodanirie rcagent, the amount of rhodanine used, the time of standing, the kind and amount of agitation used for mixing, thc tcmperature, the amount of exposure to light, t’he presence of trares of anions forming slightly soluble silver salts, the prescncc of organic solvents, and t,he concentration of “inert” salts present. The majority of the tcsts were made with solutions 10-6 JZ in silver nitrate and 1 &Ifin potassium nitrate. X Beckmati DE spectrophotomctcr with 10-cm. Cores c d s was U S C ~for d l absorp t,ion measurements. EFFECT OF AClDITY
The origirial procedure of Sandell involves simply the measurement of the transmit’tancg of a 20-ml. sample, 0.05 .II in nitric acid, t o xhich has been added 0.5 ml. of rhodanine reagent, and water t o bring t h e volume t,o 2.5 ml. T h e effect of acidity \vas examined by following this procedure on silver samples t o which varj-ing amounts of 1 t o 3 nitric acid had been added. The final solution was 50 ml. in volume, coritained 0.0006% rhodanine, and was 1 X 10-6 Ji in silver nitrate. It was found t h a t belox about 0.07 M acid the reagent pi,eri1)itated, and t h a t betlveen 0.08 and 0.10 -11acid, t h e absorbancy changed by 20%, the sensitivity decreasing rapidly \T-ith increasing acidity. Satisfactory control of the acidity was achieved by neutrnlizing the sample solution t o phenolphthalein, if’ acid, then adding a buffer solution consisting of a mivture of sodium bisulfate and sodium sulfate.
determination. Because of the great sensitivity of the color intensity to the amount of rhodanine used, it was found necessary to add the rhodanine from :I microburet and to strive for a precision in addition of the order of 2 parts per thousand. The use of a saturated rhodanine solution as the reagent led to unsatisfactory results, and the b w t practice was found t o involve the use of a solution containing 90 mg. of thepure reagent, (lissolved, filtered, and made up t o 1 liter in absolute ethanol. 0.800
0.600
0400
0.200
400 450 500 550 600 6 3 Figure 1. Extinction Curves for Silver Rhodanate Suspension a n d for Rhodanine
4
In acid aqueous media
A principal obstacle to ovcrrome \vas the gradual preciliitation of the rhodanine from the Iilank solution, and in Icsser degree from the sample solutions containing silver. When the rhodanine reagent \vas added to an aqueous solution, it apparently formrd a supersaturated solution from n.hic*hprecipitation occurred slowly. The rate of this precipitation \vas found to tic markedly retarded l i y any one of the following artifices: (1) increasing the acidity of the aqueous solution, ( 2 ) drvwtsing the amount of rhodariine added, (3) making the aqueous solution 6% in acetonc., and ( 4 ) increasing the temperature of the mixture. By a propor rhoice of values for these four variables, it was found possible to prcyare a blank solution which was stable overnight; thereby the rcliLibility of the procedure was iniproved greatly. STABILITY OF THE SUSPENSION
RHODANINE CONCENTRATION
When the amount of rhodanine was varied, the absorbancy at 480 mp measured against a blank of the same composition, except for the absence of silver, was found t o be very sensitive t o rhodanine concentration. A 1% difference in t h e reagent concentration, in the optimum range, resulted in almost a 1% error in the 1 Present address, Provincial Departinent of Mine.?, Victoria, R. Canada.
I n s t i t u t e of Technology,
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Another difficulty encountered in t’he use of previously published methods was the instability of the silver rhodanate dispersion in 2 M potassium nitrate. Thus, on preparing such a -11silver solution, the absoi.l)dispersion made from a 2 X ancy a t 480 mg was 0.403 initially; then on pouring the saniple from cuvette t o flask, and back into cuvette, the reading \\-as 0.390; on repeating the pouring operation, the reading was 0.375; but on allowing the mixture then t,o remain a t rest in the cuvette for 5 minutes, the reading was 0.375. Added gelatin was found to increase the absorption of light, doubtless due to enhanced dispersion; but a t 20p.p.m. in the final mixture, gelatin had begun to salt out owing to the high concentration of potassium nitrate present in nearly all the samples, and this maximum pract>ical Concentration of gelatin was found insufficient to exert a significant, effect. Sucrose was finally found t o be a remarkably good stabilizer. T h e reason is somewhat uncertain, b u t niay well be due to the minute amounts of surface-active agents kno\\-n
1503
1504
ANALYTICAL CHEMISTRY
to be present even in highly refined cane sugar ( 5 ) . The degree of stability attained by using sugar seemed to depend slightly on the brand. The minimum necessary concentration of sucrose depended on the range of silver concentrations to be used; for silver concentrations of the order 10-8 M,a final solution 6 f 2 % in sucrose vias satisfactory. Because of instability a t high ilalt concentrations it was found necessary to standardize the method of addition and stirring, during the formation of the suspension. It was concluded that gentle manual stirring gave the best results, fairly vigorous mechanical stirring often producing deviations of 5 to 10% in duplicate samples. Time studies were conducted t o determine the minimum period required for a silver rhodanate dispersion to attain a constant absorbancy, and for how long i t remained constant thereafter. This minimum period was found to be about 30 minutes for a mixture of the order 10-6 M in silver, 1 M in potassiurn nitrate, and containing 6yo sucrose and 6% acetone; the absorbancy then remained constant for about 1 hour and decreased about 5 7 , over the next 12 hours. This minimum period varied slightly with t,he concentrations of silver, sucrose, and acetone.
Since the method was to be applied to nitrate-thiocyanate mixtures 2 M in potassium ion, all studies were based on the assumptioil that the samples used would be treated with nitric acid t o destroy thiocyanate ion and thus ultimately contain considerable quantities of potassium nitrate. In the final procedure, suitable amounts of solid potassium nitrate ryere, therefore, added, if necessary, to bring all samples to the same potassium nitrate concentration. 04001
EFFECT O F TEMPERATURE
The abaorbancy of the silver rhodanate dispersion, and of the blank vias found to be very temperature dependent. During the prcparation of the dispersion and of the blank, and subsequently, the temperature had t o be controlled to & l o C. in order to keep the error within flyo. This need for precise temperature control was judged to be the most serious restriction on the general method.
Figure 2. Effect of Varying Amount of Rhodanine on Calibration Curve for Silver A . 0.8 ml. saturated solution of rhodanine in alcohol B . 0.9 ml. saturated solution of rhodaninc in alcohol
FACTORS AFFECTING SEh SITIVITY EFFECT OF CHLORIDE AND THIOCYANATE
Both chloride and thiocyanate ions were found to reduce the sensitivity of the method. A solution 10-2 fil in potassium chloride and 10-6 Af in silver nitrate gave an absorbancy reading 4.8% loa-er than when the chloride was absent, The effect of thiocyanate was much more serious; any concentration greater than 10-5 111 vitiated the results. A pre!iminary evaporation of the sample solution with nitric acid destroyed the thiocyanate completely, all the silver becoming soluble. For 25 ml. of a solution 0.1 M in potassium thiocyanate, 5 nil. of 8 d l nitric acid was ample. An unexplained phenomenon was that for solutions of the order of 10-6 M in silver and from nil to 0.1 111 in potassium thiocyanate, the absorbancy of the silver rhodanate dispersion increased by 30% from the lower t o the higher thiocyanate concentration. Hence it was necessary always to adjust standards and sample solutions to some constant thiocyanate concentration. The evaporation of the silver solutions had to be conducted in S'ycor; it has been shown by Schoonover ( 4 ) that a t elevated temperatures silver is adsorlled onto borosilicate glass. Erratic results were obtained if the samples were taken to dryness on a high-temperature hot plate. It is, therefore, recommended that the evaporation be completed a t a low temperature. OTHER VARIABLES
Other variables considered included the effect of exposure t o light and the order of, and time intervals between, the addition of the several reagents. The absorbancy of a silver rhodanate dispersion was found t o be consistently higher if stored in the dark during the 1-hour period prior to measurement. Variations in the order and speed of mixing of the reagents appeared to have some effect on the absorbancy of the resulting suspension. A statistical study of the factors involved would be necessary to explain all the results, but it seems clear that the standard and the sample t o be analyzed must be treated identically throughout the entire procedure.
T h e sensitivity of the method to silver was found to increase as solution conditions were changed in directions which tended t o promote precipitation of the rhodanine. The maximum practical sensitivity of the method was thus shown to be limited in a fundamental and unavoidable way; this limit occurred when solute concentrations and temperature Fleere such as just to prevent precipitation of the rhodanine reagent. Figure 2 shows how changing one of these parameters, the rhodanine concentration, changed the sensitivity of the method. The solution is 50 ml. of volume, 4% in sucrose, 6% in acetone, 1.4% in sodium bisulfate, 4% in anhydrous sodium sulfate, and 10.1 % in potassium nitrate. This figure illustrates a useful rule deduced from it and from other data-namely, that xhen any oneof thefourparameters listed under Rhodanine Concentration is altered slightly, the form and slope of the calibration curve are unchanged, but there is a horizontal translation of the entire graph. The point a t d i i c h the graph departs from approximate linearity was conTIin silver; this may stant, and corresponded to a solution 10-6 . perhaps denote a critical micelle concentration. On the approximately linear part of the graph the precision of the method is good, but it is much poorer on the curved lower part. RECOMMENDED PROCEDURE
Reagents. RHODANINE.Weigh 0.0450 gram of solid p-dimethylaminobenzalrhodanine (Eastman Kodak Co.) and dissolve in 400 ml. of absolute ethanol, warming to 50" c. if necessary. Let stand overnight in the dark and filter through sintered glass, rinsing the beaker and filter with absolute ethanol. Dilute the filtrate and washings t o exactly 500 ml. with the same solvent and store in the dark in a well-stoppered bottle. BUFFER. Dissolve 90.0 grams of sodium bisulfate (anhydrous) and 200.0 grams of sodium sulfate (anhydrous) in chloride-free water, make up t o 1000 ml., and filter. ACETONE, reagent grade. If unavailable, substitute ethanol or methanol. POTASSIUX NITRATE. Reagent grade, chloride-free, solid. NITRIC ID. Prepare 8 Jf by dilution of concentrated reagent grade nitric acid. CAKESUGAR. Any brand of good quality table sugar, but always the same brand as used in the calibration.
V O L U M E 2 4 , NO. 9, S E P T E M B E R 1 9 5 2 STAXD.4RD SILVER XITRhTE. Prepare a 5 x Jl stork solution 0.01 Jl in nitric acid and store in a borosilicate glass cont>ainerin t h e dark. Renew after 1 week. Prepare dilutions n-it’hchloride-free x-ater immediately before use. Apparatus. Beckman D or DV spectrophotometer with tungsten light source and matched 10-cm. path-lcnTth cells of Corex or silica. Vycor or silica beakers, 125 or 250 nil. lIicroburet, 2 nil. capable of delivering to iO.005, preferably h0.002 Id.
Procedure. Int,o a silica or Vycor beaker, pipet a 25-1111. or smaller sample which should contain between 2 X 10-8 and 2 x lo-’ mole of silver. Adjust the amount of any inert salt present in the samples t o a standard value-e.g., potassium nitrate t o bring the total amount present per sample t o 0.05 mole and potassium thiocyanate t o bring the thiocyanate content up t o t h e standard value of 0.0025 mole. If necessary, add water t o bring the volume t o 25 i 5 ml i n a hood, add 5.0 f 0.2 ml. of 8 JI nitric acid and evaporate carefully t o dryness on an asbestos-covered hot plate a t a low heat. Leave on the hot plate for 30 minutes after the samples appear dry and no fumes of nitric acid can be detected. Cool and add 24 f 1 ml. of chloride-free water, rinsing d o n the ~valls of the beaker. Warm t o dissolve any deposit of salts, then cool 25’ C.-iyhich must be maint o an arbitrary temperature-e.g., tained throughout t h e rest of t h e procedure. h d d a drop of 0.1% phenolphthalein in ethanol and neutralize with 0.2 JP potassium hydroxide. At once add by pipet 10.00 ml. of sulfate-bisulfate buffer. From this point, avoid exposure t o bright light. .4dd 3 rt 0.2 grams of sucrose and then 3.00 i 0.02 nil. of acetone a t a slow, uniform rate. Then, taking about 1 minute and using always t h e same technique and rate of addition, add from a microburet 2.000 f 0.002 ml. of t h e rhodanine solution, stirring continuously and gently during this addition by rotating the beaker. .ifter t h e addition of t h e rhodanine, continue t o rotate t h e beaker for 10 seconds more. Any potassium nitrate which precipitates during these additions may be disregarded, as it will redissolve on dilution. Transfer a t once, quantitatively, t o a 50-ml. volumetric flask, rinsing the beaker with water. Dilute t o the mark and mix the contents of t h e flask thoroughly \\-ith a uniform number of inversions but do not agitate at all violently. Let stand in the dark for 60 rt 15 minutes. Measure the absorbancy of the suspension a t -170 mp, relative to a “blank,” put through the above procedure but containing no silver, set a t 1007, transmittancy.
1505 check t h a t the rhodanine in the “blank” is in solution and not precipitating after the 1-hour standing period. This is best done by directing a bright beam of light t’hrough the flask. A diffuse Tyndall beam is normal and will always be present, but if many granular particles of a pinkish tint arc suspended in the solution, the solution conditions are incorrect and must be altered. This is most expeditiously done by reducing the amount of rhodanine solution used t o the point n-here no precipitate forms in 1 hour. hlthongh this procedure was developed for the determination of silver in nitrate-thiocyanate mixtures, it is evident that with some slight modifications, i t nould be serviceable for determining silver in various other electrolytes-for example, in cyanidenitrate media. Evident advantages of the method are: (1) that the sample solution remains in one beaker throughout the entire procedure, up to the point of dilution of t h e final silver rhodanate suspension; and ( 2 ) that it is applicable in the relatively large amounts of potassium nitrate which n ould be prcsent following the neutralization of any silver solution obtained from a nitric acid digestion. RESULTS
The precision of the method was found t o be adequate for the determination required. Five samples, each 10-6 J-1 in silver and containing thiocyanate, gave a standard deviation of 1 1 % while 10 samples not containing thiocyanate and run a t different times pave a standard deviation of 1 5%. The determination of the solubility of silver thiocyanate in thiocyanate-nitrate media which gave a total silver concentration of about 10-5 ‘11 ]\as performed b y this method and also by a polarographic method. The two methods gave results in agreement to n ithin r t 2 % . ACKNON LEDGMENT
Work was supported in part by the .itomic Energy Comnlission. The Procter and Gamble Corp. likewise lent aid through a fellowhip t o G. C. B. Cave.
DISCUSSION
T h e standard calibration curve is prepared with known samples of silver content between 1.5 X 10-8 and 2 X 10-7 mole of silver (1.5 to 20 micrograms) put through exactly the same procedure and containing exactly the same content of foreign salts. If the tempci~aturecannot be controlled to within & l oC. or if samples and standards are likely to be run several days or more apart, or indeed if the highest accuracy is required, then it is best t o run three or more standards along with the unlinon-n samples. I t is very advisable after preparing a new set of reagents to
LITERATURE CITED
(1) Feigl, F., Z . ancil. Chem., 74, 380 (1929). (2) Sandeli, E. B., “Colorimetric Determination of Traces of Metals,” 2nd ed., p. 540, S e w York. Interscience Publishers, 1950. (3) Sandell, E. B., and Seumaser, J. J., AXAL. CHEW,23, 1503 (1951). ( 4 ) Schoonover. I. C., J . Research S u t l . Bur. Standards. 15. 377 (193s). ( 5 ) T’avruch, I.,- ~ N A L Can!., . 22, 930 (1950).
RECEII-ED for review F e b r u a r y 14, 1952. Accepted J u n e 4. 1%2,
All-Purpose Capillary Viscometer rA. LETOURNEAU, AND ROBERT MATTESON California Research Corp., Richmond, Gal$.
.J. F . JOHSSON, R .
SURVEY of the characteristics of capillary viscometers in
A common use in the petroleum industry led to the idea that
the Zeitfuchs cross-arm type might be the all-purpose capillary viscometer. Originally this viscometer \vas designed for use only on opaque oils over the range of 2 to 37,000 centistokes (15). However, it has actually been continually used for over 3 years in the laboratories of the California Research Corp. as an allpurpose capillary viscometer covering the vi,cosity range from 0.5 t o 100,000 centistokes, and operated a t temperatures irom - 100’ to 500” F. for both transparent and opaque oils. Table I summarizes the important features of seven of the more popular, commercially available viscometers in addition to the Zeitfuchs cross-arm type; Figure 1 shon-s the constructlonal details of the various viscometers compared in Table I. The advantages of the cross-arm type are: (1) It measures both transparent
and opaque oils. (2) It has a wide range. (3) It requires small sample size. (4) I t has convenient over-all dimensions. ( 5 ) It allows easy, rapid cleaning and filling. ( G ) I t attains rapid temperature equilibration. DESCRIPTIOS A S D OPERATIOS
Figure 2 is a line drawing of the c rm viscometer. Two on -4requires a longer positions of the bulb, 12, are shon-n. capillary for a fixed volume of bulb 12, and t’he diameter is increased thereby. This is the type normally w e d for viscometer conPtanti; of 21 or less because an increase in diameter is desirable for the relatively ernall-bore capillaries to avoid the clogging de a free pasra.ge for cleaning. cffert of foreign matter and Pocition B for the larger ts makes powible a shorter in order to limit the volume length capillary, which i,q 11 of liquid in the capillarj- and bulb to the fixed capacity of the horizontal tube.
