are cautiously neutralized with 80 to 85 nil. of 6N sodium hydroxide and checked with p H paper to ensure that the solution is slightly acidic. After cooling again, the contents are titrated potentiometrically with potassium bromide solution of sufficient strength to give a 6- to 10-ml. titer. For convenience, a magnetic stirrer is used during the titration. MICROPROCEDURE
tion flask according to Volhard, using a visual indicator end point. As an alternative, the contents of the flask are transferred to a 50-ml. beaker with repeated 5 m l . water rinses. The silver sulfatecontaining solution is titrated with a standard (0.001 to 0.005N) potassium bromide solution by plotting the millivolts read by potentiometer as the ordinate against milliliters of titrant as abscissa. Calculation
X 2-sq. em. area of photographic film is cut into 5 X 5 mm. squares. They are placed in a 30-ml. Kjeldahl flask and 0.5 ml. of concentrated sulfuric acid and 6 to 8 drops of concentrated nitric acid are added. The digestion hot plate is set on medium. When evolution of brovvn fumes has all but ceased, the flask is cooled and 6 to 8 additional drops of nitric acid are added. This is repeated until all film base materials are decomposed-from 30 to 90 minutes. The heating is continued to white fumes with no evidence of a precipitate remaining in the digestion flask. The clear solution is titrated directly in the diges-
ml. KBr x -1- IiBr X 100 Mg. Ag/sq. dm. = -X 107.88 area of sample TABULATED RESULTS
Typical analytical results of various photographic films and papers are given in Table I. Comparison determinations using the open beaker and Kjeldahl flask are reported in Table 11. The macroprocedure was used in all cases, because there was no need to scale down the sample size. The accuracy of this method is
demonstrated in Table I11 by comparing the acid digestion with the thiosulfate complexation met hod. FUTURE WORKS
K o r k is in progress to utilize t'he highly efficient wet oxidation with perchloric acid, which promises to simplify the procedure and reduce the digestion time appreciably. ACKNOWLEDGMENT
The author appreciates the help of his associates, Delores A. Ellis for the microand Robert Brougham for the niacrodeterminations Special thanks ale clue to E. F. Vashburn, who evaluated the analytical data on a statistical basis. and to Celia Beasor, n 110 assisted in some of the literature work.
R E C E I ~ Efor D revien September 24. 1958 Accepted .4pril 17, 1959. Dinsion of Analytical Chemistry, 134th Ifwting, 4CS Chicngo. Ill., Septembrr 19.5'3
Volumetric Determination of Silver Using Thioacetamide D. G. BUSH, C. W. ZUEHLKE, and A. E. BALLARD Research laboratories, Eastman Kodak
Co.,Rochester 4, N. Y.
,A rapid, simple method is described for the direct titration of silver in the presence of halide ions. The silver, in an alkaline solution as the thiosulfate complex, is titrated potentiometrically with thioacetamide solution using a silver sulfide-calomel electrode pair. A preliminary separation of the silver from the halides, or other anions which form insoluble silver salts, is not necessary, if the silver salts are soluble in an alkaline solution of sodium thiosulfate.
I
1935, Iwanof (6) first reported the use of thioacetamide as a general analytical reagent. Since then, this reagent has become increasingly important as a source of sulfide ions for analytical separations ( I , 2 ) and for the quantitative precipitation of metallic wlfides ( 5 ) . To the authors' knowlrdge, there has been no prior application of thioacetamide as a standard titrant for volunietric analysis. The hydrolysis of thioacetamide is promoted by either hydrogen or hyd r o q l ions in solution and leads ultimately to the formation of hydrogen sulfide or sulfide ion, depending on the pH. Swift and his con-orkers ( 3 , 4,8, K
1368
ANALYTICAL CHEMISTRY
11) have made a 1-aluable study of the mechanism and kinetics of the reactions betn-een thioacetamide and such metal ions as lead, cadmium, and nickel to form sulfides. Their work has included a study of the hydrolysis of thioacetamide in both acid and alkaline solutions. This paper describes a rapid method for the volumetric determination of silver using thioacrtaniide as the titrant, When thioacetamide is added to an alkaline solution containing eilver ions, the precipitation of silver sulfide occurs essentially instantaneously a t room temperature. Because of the exceedingly low solubility of silver sulfide, there is a very large change in the potential of the silver sulfide electrode a t the equivalence point. If the silver is present in the original solution as the slightly dissociated ammonia or thiosulfate complex, the magnitude of the potential break a t the end point is somewhat diminished, but remains satisfactory for accurate volumetric work. Thus, this titration can be carried out in the presence of halide ions or other anions nhich normally form insoluble sill-er salts, ii the silver salts are solublr in sodium thiosulfate solution. The procedure is especially useful for
the deterinination of silver in coated or fluid photographie emulsions and is written specifically for this application. Sebborn (-9) h s e d a somewhat similm method on the titration of silver in hot ammoniacai solution using thiourea. The thioacetamide method described here is preferred because the precipitation reaction proceeds a t room temperature and the use of an ammoniacal medium with its objectional odor is not required. The method specifies the addition of (ethylenedinitrilo) tctraacetic acid to prevent the precipitation of most of the common metals. APPARATUS
A Beckman Model G p H meter was used to obtain the data plotted as potentiometric titration curves. A Beekman Model K automatic titrator was used to obtain the data on precision and accuracy. -4saturated calomel electrode, Beckman No. 1170, was used as the reference electrode. The silver sulfide electrode was prepared by cleaning a Beckman KO. 1261 silver electrode 11-ithcrocus cloth. Sfter being washed in distilled m-ater, the bright electrode was coated with silver sulfide by placing it in a 20% solution of sodium sulfide for 3 minutes.
-6OOC
-300
L
I
4
Figure 1 .
a
I
I2
16
mi. 0.01 N Thioacetamide Effect of alkalinity o n titration curve
Titration of 0.1 meq. of silver with 0.01N thioacetamide in: A. 1.ON N a O H E . 0.1N N a O H C. 1.ON N H 4 0 H Titration of 0.01 meq. of silver with 0.001N thioacetamide in: D. 1.ON N a O H
REAGENTS
Buffer solution, pH 5. Mix 0.1JI potassium acid phthalate and 0.05M trisodium phosphate solutions in the ratio of 50 to 24 parts by volume, respectively. iidd 0.5 gram of pondered thymol per liter. Thioacetamide solution, 0 . 2 s stock solution. Dissolve 7.6 grams of thioacetamide in l liter of pH 5 buffer solution. Thioacetamide solution, 0.OLI- stantlard solution. Dilute 50 ml. of the 0.2&Y stock solution to 1 liter using the p H 5 buffer solution. Standardize as directed in the procedure. Thioacetamide solution, 0.001.Y standard solution. Ililute 5 ml. of the 0 . 2 s stock solution to 1 liter using the pH 5 buffer solution. Standardize as directed. Sodium hydroxide-EDT-1 solution. Dissolve 80 grams of sodium hydroxide in water. - d d 4 grams of lehylenedinitri1o)tetraacetic acid. Dilute to 1 liter. Sodium thiosulfate, 24% solution. Dissolve 240 grams of sodium thiosulfate pentahydrate in water and dilute to 1 liter. Gelatin, 0.4% solution. Dissolve 4.0 grams of gelatin in warm water. Add 0.5 gram of powdered thymol. Dilute to 1 liter. PROCEDURES
Fluid Photographic Emulsions. Weigh a sample containing 5 mg. or more of silver and add t o it 10 ml. of sodium thiosulfate solution. Add 5 ml. of 0.4% gelatin solution and 25 ml. of sodium hydroxide-EDTA solution. Titrate Kith 0.01A' thioacetamide solution t o t h e inflection point of the titration curve obtained using a calomel-silver sulfide electrode system.
, -200 4
8
12
16
mi 001 N Thiwcetamide
Figure 2. Effect of thiosulfate concentration on titration curve Titration of 0.1 meq. of silver with 0.1 N thioacetamide in 50 ml. of solution containing varying amounts of Na&03.5H?O A. 2.40 grams E. 1.20 grams C. 0 . 2 4 gram
For samples coiitaiiiing 0.5 to 5 mg. silver, use 1 ml. of sodium thiosulfate solution and 1 ml. of 0.4% gelatin solution and titrate with 0.001S thioacetamide solution to the inflection point. Coated Photographic Emulsions. Cut an accurately known area of the emulsion coating containing 5 mg. or more of silver and place, emulsion side up, in a beaker. Add 10 ml. of sodium thiosulfate solution and rock gently until the silver halides have dissolved. Add 5 ml. of 0.4Oj, gelatin solution and 25 ml. of sodium hydroxide-EDTA solution, and titrate with 0.01W thioacetamide solution. Photographic Fixing Baths. T o a samplc of the fixing bath containing 5 or more mg. of silver, add 5 ml. of 0.4y0 gelatin solution and 25 nil. of sodium hydroxide-EDTA solution, and titrate with 0.0lN thioacetamide solution. Standardization of Thioacetamide Solution. Transfer 10.0 ml. of 0.01N silver nitrate solution t o a beaker and add a n excess of potassium bromide -e.g., 10 ml. of 0.02hrsolution. Swirl t o mix and add 10 ml. of sodium thiosulfate solution. It is necessary to precipitate the silver as a slightly soluble salt prior t o the addition of thiosulfate to avoid the precipitation of small amounts of silver sulfide which occurs when thiosulfate is added to solutions of high silver ion concentration (7). Add 5 ml. of 0.4% gelatin solution and 25 ml. of sodium hydroxide-EDTA solution, and titrate with 0.01-V thioacetamide solution. For the standardization of 0.0015
solution, or solutions to be used for the titration of silver in fixing baths, adjust the concentration of sodium thiosulfate to be approximately equivalent to that present in the solution for analysis. This is especially important when an automatic titrator is used and the solution is titrated to a predetermined electrode potential. Similarly, when analyzing by automatic titration fluid cmulsion samples which contain a high gelatin content--e.g., substantially more than the equivalent of 5 nil. of 0.4% solution-increase the amount of gelatin added to the solution used for standardization to approximately the amount present in the 5ample. DISCUSSION
Experimental Variables. Figure 1 illustrates the titration curves obtained and s h o w the effect' of variations in alkalinity on the titration curve. T h e higher alkalinity results in the larger potential break a t the equivalence point. Figure 2 shows the effect of varying t'he thiosulfate concentration on the size of the potential change a t the equivalence point'. As expected, a high Concentration of thiosulfate diminishes the concentration of the silver ion in the starting solution and likewise diminishes the potential change a t t'he end point. Accordingly, it is recommended that the thiosulfate concentration be near the practical minimum, especialljwhen 0.001N thioacetamide is used for the titration. VOL. 31, NO. 8, AUGUST 1959
1369
Figure 3. Comparison of titration curve of silver-mercury([[) mixture with curves of the two ions separately A.
4
6.
c.
Mercury(l1) Silver Silver mercury(l1)
+
hydroxide solution. These favorable circumstances permit the use of automatic titrating devices. Stability of Thioacetamide Solutions. Aqueous solutions of thio-
ml 001 N
Thioacetamide
Table I. Stability of 0.01N Thioacetamide Solutions
Decrease in Titer, Experiment No.
70
Buffer Phthalate-phosphate
1 2 3 4 5 6
Tetraborate-phosphate
7
Unbuffered Unbuffered, thymol added
8
Table [I. Comparison of Thioacetamide and Digestion Methods in Analysis of Emulsions for Silver
PH Original Final 4.0 5.0 6.0 7.0 8.0 9.0
4.0 5.0 6.2 7.0 8.0 9.0 6.7 6.7
5.8
6.9
3.410 3.428 3.412 3.426
217.5 217.6 217.3 217.9 218.4 Av . 3.412 3.417 217.7 0 4 Std. dev. 0.008 0,010
217.1 217.4 217.1 218.3 217.7 217.5 0.5
T h e silver sulfide and platinum electrodes were investigated as indicator electrodes. Either may be used, but t h e former is superior because the potential break is more t h a n twice as great. Either a saturated calomel or glass electrode may be used as reference electrode. I n this work, the saturated calomel electrode was used. The silver sulfide electrode comes to equilibrium with the solution very rapidly and no waiting time is required between additions of the reagent. I n the absence of a dispersing agent, the silver sulfide formed by the reaction tends to agglomerate and to form a loose coating on the electrode surface, which, if left undisturbed, Tvould tend Electrode Systems.
1370
ANALYTICAL CHEMISTRY
40 days 3.1 0.5 100.0 1.7 2.8 7.7 87.0 3.0
Table 111. Comparison of Thioacetamide and Digestion Methods in Analysis of Used Fixing Baths for Silver
Digestion, Grams/ Liter 3.419 3.418 3.410 3.401
7 days 0.0 0.0 0.0 0.0 0.9 1.8 1.6 1.6
Thioacet- Diges- Thioacettion, amide, amide, Grams/ Grams/ Grams/ Liter Liter Liter xo. 3
3-0. 1
1 44 1 44 1 45
1 42 1 42 1 43 >To.
3.67 3.61
2 3.63 3.63
4 25 4 24
4 23 4 23 No. 4 0 058 0 0.55 0.055 0.051 0.050 0,054
to make the electrode response sluggish. I n the presence of a small amount of dispersing agent, such as poly(viny1 alcohol) or gelatin, this flocculation is prevented. Gelatin was more effective and was chosen for this lvork. The silver sulfide electrode is unusually free of poisoning effects and requires no special handling. After several months’ use, i t may become sluggish, because of the build-up of a n adherent silver sulfide layer, and require regeneration. The change in potential a t the end point is very large, permitting the use of 0.001N solutions where appropriate. Figure 1 includes a plot of the curves obtained using 0.001N thioacetamide solution (dotted line) in l.OL1’ sodium
acetamide decompose slowly with t h e deposition of elemental sulfur. Because both hydrogen and hydroxyl ions promote t h e hydrolysis of thioacetamide, i t would be expected t h a t maximum stability would be found near p H 7. T h e d a t a in Table I indicate t h a t bacterial action as well as hydrolysis may be responsible for the decomposition of thioacetamide and that bacterial action proceeds most rapidly a t p H 6. I n the case of the solution buffered a t p H 6 and the unbuffered solution which changed in p H from 5.8 to 6.7 during the aging experiment, decomposition of the thioacetamide was essentially complete in 40 days. Solutions buffered a t p H 7 and especially a t p H 5 are much more stable. After the initial work (Table I, experiments 1 through 7) on the relationship betn-een p H and thioacetamide stability had been completed, thymol (added as a bactericide) was found to stabilize thioacetamide solutions, as shown in experiment 8. Other experiments using 0.1N thioacetamide solutions buffered a t p H 5 confirmed the stabilization of the solution by thymol. These results were obtained on thioacetamide solutions which were constantly exposed to air in an automatic buret-filling device. Under these conditions, the reagent specified is a n acceptable titrant for the usual volumetric work. RESULTS
In replicate standardizations of -0,Ol.V thioacetamide with 1.000 meq. of silver, 10.30, 10.22, 10.24, 10.22, and 10.25 ml. of thioacetamide were required. The average is 10.25 ml. with a standard deviation of 0.03 ml. Typical results obtained by analyzing a fluid photographic emulsion and an emulsion coating are listed in Table 11. Similar results obtained by analyzing four photographic fixing baths are given in Table 111. In each case, the sample was also analyzed by an independent method based on the potentiometric titration of the silver with potassium iodide solution after complete digestion of the sample with concentrated sulfuric acid and hydrogen peroxide. The data in Table I1 were obtained uqing samples containing at least 5 mg.
of silver. Under these circumstances, the accuracy and precision of the thioacetamide method are approximately +=0.37,. In the case of the fixing baths (Table 111), the precision of the digestion method was not entirelj- satisfactory, and a comparison between the two methods to establish the accuracy of the thioacetamide method is not useful. The precision of the thioacetaniide method is approximately +1%. The samples analyzed are representative of the diffcrent types of fixing baths in common usage, INTERFERING ELEMENTS
Any metal which forms an insoluble sulfide potentially may interfere in this method. Flaschka (5) has shown that E D T A prevents the precipitation of the sulfides of a number of metals, including copper, cadmium, zinc, cobalt, and nickel. Studies made in these laboratories have shown that, in the presence of EDTA, none of these metals (as well as lead and ferric iron) interferes in the titration of silver with thioacetamide. Among the common metals, only mercury and ferrous iron interfere in the presence of EDTA. Mercury forms a n insoluble sulfide and ferrous iron reduces silver ion to silver metal. The use of E D T A is recommended whenever metal contaminants may be present, and these data were obtained with E D T A present in the solutions being titrated. None of the common anions, including the halides, interferes. Although cyanide does not prevent the formation of silver sulfide, it seriously reduces the
initial silver ion concentration in the solution and, therefore, markedly diminishes the size of the potential break at the end point. Accordingly, only small amounts of cyanide can be tolerated. Oxidizing agents are variable in their behavior and may cause difficulty. Permanganate and chromate are reduced by alkaline thiosulfate to manganese dioxide and chromic hydroxide, respectively. and do not interfere. Free iodine is also reduced, but the oxidation products have a deleterious effect on the end point. Ferricyanide is reduced slowly, and in these experiments, its presence led to low results for silver. APPLICATION OF METHOD TO OTHER METALS
Thioacetamide can be used for the direct titration of mercuric ions in solution. I n this case, the thiosulfate is omitted and E D T A is used to prevent the precipitation of mercuric oxide from the alkaline qolution. The silver sulfide electrode conies to equilibrium rapidly with this solution and no waiting period is necessary between additions of the reagent. Figure 3 shows a typical titration curve for mercury. The curve for the titration of silver in thiosulfateEDTA mixture is included for comparison. I t is apparent that the mercury curve shows a substantially larger potential break a t the end point. Curve C represents the titration of a mixture of silver and mercury in thiosulfate-EDTA mixture and the end point corresponds to the sum of the tn-o titers. There is no indication of an intermediate end point which would differentiate between these ions in a mixture.
It appears probable that the thioacetamide method might be extended to the potentiometric titration of a number of the heavy metals. The problem in this connection \Todd be the discovery of suitable indicator electrodes. Copper, in ammoniacal solutions, has been successfully titrated using a mercuryplated platinum electrode described b y Siggia, Eichlin, and Rheinhart (10). It has been the experience in these Laboratories that this electrode requires frequent regeneration. LITERATURE CITED
(1) Barber, H. H., Grzeskowiak, Edward,
ANAL.CHEM.21,192 (1949). (2) Barber, H. H., Taylor, T. I., “Semimicro Qualitative Analysis,” rev. ed., Harper, New York, 1953. (3) ~, Bowersox. D. F.. Swift. E. H., ANAL. CHEY.30. f289 (1958). ’ (4) Butler, ’E. A.; Peters, D. G , Swift, E. H., Zbid., 30, 1379 (1958). (5) Flaschka, H., 2. anal. Chem. 137, 107 (1952); Chemist Analyst 44, 2 (1955). (6) Iwanof. F. W.. Chem. Zentr. 106, 883 ’(1935 (1935 11): ((7) 7,) Kolthoff. Kolthoff, M., Furman. Furman, T. H.. H., ~~~~~-~~ I. M.. “Potentiometric Titrations,” “Potentiomet,ric Titrations,”’ 2nd ed.; ed., p. 171, Wiley, New York 1949. 1949 (8) Peters, D. G., Swift, h. H .,, Talanta 1,30 1, 30 (1958). 19) Sebborn. W.. Kodak Harrow Research Laboratories, ‘personal communication. (IO) Siggia, Sidney, Eichlin, D. W., Rheinhart, R. C., Ax.4~.CHEM27, 1745 (1955). (11) Swift, E. H., Butler, E. -I., Ibid., 28, 146 (1956). \
’
RECEIVED for review Sovember 3, 1958. Accepted April 20, 1959. Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, March 1958. Communication No. 1986 from Kodak Research Laboratories.
Paper Chromatography of Sugar Phenylosazones as Their Borate Complexes BARBARiN ARREGUjN lnstituto de Quimica de la Universidad Nacional Aut6noma de MGxico, Ciudad Universitario, M6xico 20,
b Few papers have appeared on the chromatography of sugar osazones. In this work a paper chromatographic method was developed which is suitable for the separation of the osazones as complexes with potassium borate. The migration rates of these complexes are different, and mixtures of them separate well on paper.
S
the discovery by Fischer (3) that sugars react with phenylhydrazine \vith the formation of osaINCE
zones, this reaction has been frequently used for the identification of sugars. Recently these derivatives of monosaccharides have been successfully chromatographed using circular chromatography (1). Likejvise, in a n unpublished report ( 7 ) ,the separation by paper chromatography of four osazones was achieved by using filter papcr impregnated with formamide. The reaction of boric acid with glycols and polyhydroxy compounds requires a cis-orientation of the hydroxyl groups. or that they be placed in a semirigid system which prevents the rotation of
D. F.
the hydroxyl groups to a lesb favorable trans-orientation. The behavior of boric acid with polyhydroxy compounds has been reviewed ( 2 ) and the borate complexes of sugars have been separated by paper partition chromatography using boric acid ( 8 ) . I n this connection sugars or sugar derivatives which in themselves have few structural differences, show different complexing abilities towards borate ions, which in turn is reflected in their properties. resulting in different mobilities in chromatographic studies. I n other Tvork with sugars and plienoVOL. 31, NO. 8, AUGUST 1959
1371
