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I N D U S T R I A L A N D ENGINEERING CHEMISTRY
(2) It favors the formation of very finely divided suspensions instead of clots during titration.
The second effect is particularly favorable for satisfactory results by the procedure described by the writer, of titrating to a clear end point, especially while standardizing the standard cyanide solution against pure silver nitrate. Except in rare cases, the usual procedure by this method is to add 0.1 N potassium cyanide solution, containing 5.0 cc. ammonium hydroxide per liter, to the liquid emulsion dispersed in hot water until it becomes clear, which indicates the end of the titration. The trace of ammonia in the cyanide doubtless has a slight solvent action on the silver halides, but allowance is made for this when standardizing the cyanide solution. In order to be successful by this method, there are certain precautions to be taken and several points with which the operator should be familiar. (1) Although pure potassium cyanide solutions are fairly stable, standardization requires only a few minutes and should be repeated a t least every other day in order to be sure t h a t the solution has not decomposed t o any important extent. (2) The end of the titration is indicated by complete solution of the silver halides, but at this point the liquid is often slightly opalescent, depending on the amount and state of the gelatin in the sample. The end point of the titration is taken where two or three drops of the standard potassium cyanide solution cause no further decrease in turbidity. This is not difficult t o see and is important since an attempt t o overcome turbidity due t o gelatin leads t o high results. (3) In the rare cases of coagulated emulsions or other silver halide precipitates, a n excess of potassium cyanide is added as in the first part of Eggert’s method until all the silver halides are dissolved. .4 known amount of silver nitrate is then precipitated in about 50 cc. of water in a separate beaker by a small excess of potassium bromide and the freshly prepared silver bromide suspension poured into t h e clear sample. Titration is then continued as before with standard potassium cyanide solution to a clear end point. Back titrationof the excess potassium cyanide with standard silver nitrate solutions in the presence of gelatin as in Eggert’s method gives low results. (4). The rate of reaction between the silver halides and the entering cyanide varies somewhat with the size of the silver halide particles and the volume of liquid in which the silver halide is dispersed. In nearly every photographic emulsion the crystals are small enough t o react instantly, b u t volume changes of 300 per cent increase the error of a determination by about 1 per cent. Cautious titration and reasonably uniform dilution are desirable to obtain best results.
STANDARDIZATION OF KCN SOLUTION Dissolve a known weight (0.25 to 0.300 gram) of silver nitrate in about 150 cc. of hot distilled water (50”to 90’ C.). Add about 0.4 gram of powdered potassium bromide (too much potassium bromide coagulates the silver bromide formed) stirring the mixture by the aid of a rubber-tipped rod and then run in approximately 0.1 N slightly ammoniacal potassium cyanide solution until the mixture becomes nearly clear; then add the potassium cyanide solution carefully by drops to a clear end point. If coagulated silver bromide or silver cyanide is formed, it is easily seen at the end of the titration as quick-settling white particles, and indicates that the work has not been done properly.
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If coagulated emulsion is encountered, add a known excess of potassium cyanide and stir until the silver halides in these clots are dissolved. Take more than sufficient silver nitrate to correspond to the excess potassium cyanide employed in about 30 cc. of water in another vessel, add a slight excess of potassium bromide solution to precipitate all the silver as bromide, and pour this into the clear sample, and titrate again with the standard potassium cyanide solution to a clear end point. COMPARISON OF RESULTS BY BOTH METHODS Since Eggert’s procedures and the one described here are nearly identical, the only way to compare them is to analyze emulsions by both methods. Both methods give sharp end points in the standardization of the potassium cyanide solution. The following results, however, indicate that the presence of gelatin strongly impairs the accuracy of Eggert’s method, and is responsible for the discrepancy between the true silver conteqt and the results obtained by either method. This method gives results approximately 1 per cent high, while under the same conditions by Eggert’s method the results are 2 to 5 per cent low, according to the amount of gelatin present. STANDARDIZATION OF 0.1 N K C N SOLUTION D t v e c l ltlratton method KCN Ag equiv. per Average AgNOs taken soln. used cc. of K C N s o h . difference Per cent Grams cc Grams 0.2256 27.30 0,005247 -0.10 0.2514 30.43 0.005255 +0.06 0.2732 -n.nz 33.00 0.005253 Av. 0,005252
.
Eggert’s method Rack titrationa KCN AgN03 KCN AgN03 taken s o h . added s o h . used soln. used Grams cc. cc. cc 0.2238 45.00 8.35 27.55 0.2512 45.00 6.70 31.00 0.2735 45.00 5.40 33.72 Av.
.
Ag equiv. per cc. X C N soh. Grams 0.005158 0.005145 0.005153 0.006152
Average difference Per cent +0.12 -0.14 $0.02
ANALYSIS OF SILVER BROMIDE-GELATIN EMULSION ------EGGERT’s METHOD----DIRECTTITRATION METHOD Back titration KCN KCN KCN Ag in Emulsion s o h Emulsion s o h . AgNO3 soln. Ag!n used emulsion taken taken added soln. used used emulslon Cc. Percent Grams Cc. Cc. Grams Cc. Percent 18.55 3.015 3.400 40.02 9.95 19.27 2.92 3.232 26.85 3.017 4.645 40.00 13.63 26.37 4.672 2.92 36.30 3.007 6.135 45.00 10.22 34.78 2.90 6.337 Av. 2.91 Av. 3.013 True per cent Ag = 2.98 True per cent Ag = 2,980 Per cent error = 1.10 Per cent error = -2.35 a Ratio of AgNOa solution to K C N solution was 1: 2.090
+
COMPARATIVE RESULTS SHOWING EFFECT OF ADDEDGELATIN Error by EgError b y digert’s method rect method Per cent Per cent 0.09 0.06 +1.1 -2.2 +1.1 -4.4 +1.6 -3.3 +0.8 -5.2
Gelatin in sample Grams None 0.4 1.5 1.2 2.0
ACKNOWLEDGMENT The writer wishes to express his appreciation to F. F. Renwick for kindly criticisms.
ANALYSIS OF EMULSIONS Completely disperse a known weight or area of emulsion cofitaining about 0.2 gram silver in about 150 cc. hot water, and titrate with standard potassium cyanide solution to a clear end point in the same way as in titrating the silver when standardizing the potassium ,cyanide solution. Remember that the gelatin, especially if present in large amounts, may cause an appreciable turbidity. Unless lumps or clots of emulsion are present, a reading taken before the first addition which causes no decrease in turbidity represents the end point.
Announcement of Chemical Exposition Date Owing t o some confusion which is believed to exist in a few quarters regarding the holding of the next chemical exposition, an announcement has been sent out by the International Exposition Company, under whose management the Exposition of Chemical Industries has been held since 1915. This announcement states t h a t there will be no chemical exposition in 1924. The next Exposition of Chemical Industries will be held September 28 t o October 3, 1925, at the Grand Central Palace, New York, N. Y .
