September, 1924
I N D U S T R I A L A N D ENGINEERING CHEMISFRY
stand overnight. The glue should then be stirred for several minutes or until the albumin is dissolved. Upon heating to 90" C. for 15 minutes the albumin coagulates and brings down with it all suspended matter. The glue may be filtered while hot through either cotton or porous filter paper. The color of the clarified glue may be easily determined by placing a sample in a Lovibond tintometer cell and matching the color with the standard red and yellow tintometer glasses.
PERMANENCE This is another point of first importance in the consideration of the value of any liquid glue. Not only should it retain its adhesive power indefinitely, but it must retain its original appearance-i. e., no coagulation, precipitation, or discoloration should appear on storage for long periods of time. The glue must, therefore, be preserved to prevent bacterial and mold growth, and it must not contain any suspended matter that will settle out on standing. One of the best tests that may be applied to a liquid glue to determine its permanence is its storage a t a slightly elevated temperature-e. g., 37.5" C. If liquid glue remains free from bacterial decomposition and other changes during a month's storage a t this temperature, it is likely that it will keep indefinitely a t ordinary room temperatures, provided it is properly preserved to prevent mold growth.
CHEMICAL COMPOSITION The permanence of a liquid glue is partly dependent upon its chemical composition. If the glue is either strongly acid or alkaline, hydrolysis will take place slowly and the glue will gradually lose its strength. The proper reaction for glues is neutral to litmus and very slightly acid to phenolphthalein. The presence of chlorides has already been considered. Other impurities in liquid glue may be exceedingly objectionable; for instance, the presence of substances corrosive to tin or iron causes many liquid glues to be of little value, although they may be splendid in other respects. A complete chemical analysis of the glue itself is of considerable value in the examination of a liquid glue, provided the amounts of gelatins, gelatoses, gelatones, and amino acids are determined. Only chemists who have had considerable experience in the examination of glues are able to interpret the results of such an analysis correctly. The writer has not done sufficient work along this line to state definitely the maximum and minimum amounts of these substances that good liquid glues should contain, but the gelatin and gelatose content must be high and the gelatin and amino acid content should be low. This is obvious, for gelatones and amino acids have little adhesive power. If a liquid glue is to be used for joining work, the properties of prime importance are permanence, adhesiveness or strength, and hygroscopicity. If the glue is lacking in any one of these three qualities, it is of no value for use in joining wood. Other properties, such as gel point, viscosity, color, and odor, are of minor importance in most cases. Where the glue is to be used for special work, such as joining glass, other properties-for instance, color and elasticity-become of importance. I n buying glue, viscosity, moisture content, and speed of set should be taken into consideration, for these properties determine the relative cost of liquid glues. Obviously, more glue must be used on joints in the case of liquid glues of low viscosity and high moisture content than when glues of high viscosity and low moisture content are used. If the liquid glue sets too quickly after it is applied, it will be impossible to spread uniformly, and therefore more glue must be applied in making the joint than would be the case if a slower setting glue were used.
945
A Titration Method for the Determination of Silver in Photographic Preparations' By Martin Marasco REDPATH LABORATORY, E. I. DU
P O N T DE
NEMOURS % Co.,PARGIN, N. J.
HE possibility of quantitative determination of the
T
reactions between soluble cyanides and silver salts, especially silver nitrate, has been known for a long time. The method was probably first tried by Liebig for the estimation of cyanogen by adding standard silver nitrate solution to cyanide in solution until a permanent turbidity of silver cyanide was formed, or, if some soluble halide was also present, by adding the silver nitrate solution until a precipitate of silver halide was formed which occurred as soon as all the cyanide had been converted to the double salt. I n the photographic industry a reasonably accurate and rapid method of estimating silver halides in emulsions is important, but until recently no satisfactory way of doing this has been published. Such a method has been used successfully in this laboratory for determining the silver content of photographic emulsions (emulsion noodles and dry emulsions on photographic plates and films). It is similar to, but more rapid and accurate than one recently described by Eggert.2 The procedure suggested by Eggert is the same as that given by Rabiere,3 and is probably better known as Denig&s'method which is based on adding an excess of 0.1 N potassium cyanide solution to the neutral silver solution (about 0.2 gram in 200 cc.), then 5 cc. ammonium hydroxide, about 0.05 gram potassium iodide, and titrating back with 0.1N silver nitrate solution until just turbid (due to silver iodide). Comparing the equations representing both methods in their working order, we have for Eggert's method p AgBr m K C N (excess) =
+
+ n K C N + p KBr (1) +p AgK(CN)2 n K C N + n/2 AgNOa = (p + n / 2 ) AgK(CN)2 + n / 2 KNOs (2) K I + AgNOa AgI = turbid solution (31 and n represent the number of chemical equivalents, and
p AgR(CN)z
=
(q, m,
m
- n)
q
=
and for the method described here AgBr
+ 2 K C N = AgK(CN), + KBr = clear solution
(1)
That the second method 1s far simpler and quicker is obvious at once from the equations; it remains to show that it is a t least as accurate. By Eggert's method the emulsion is dispersed in hot water, then a mixture of ammonia and potassium iodide in water is added, followed by an excess of standard potassium cyanide solution. The excess of potassium cyanide is titrated with standard silver nitrate solution until a faint turbidity due to a permanent precipitate of silver iodide appears. Eggert's article might lead one to suppose that the ammonia is added to keep silver cyanide in solution a t the end point, which, of course, is not the case. The real advantages of having ammonia present are: (1) It keeps the solution alkaline and so prevents the liberation of traces of hydrogen cyanide during titration. Without the ammonia, after several hours of continuous work with cyanide solutions in an ordinary laboratory, the eyes become dull and there is a feeling of internal pressure about the temples, with occasional dizzy spells, b u t when a little ammonia is added t o the mixture or the cyanide before titration these effects are no longer noticed. 1
Received M a y 24, 1924. 209 (1924). Bull. 5 0 6 . ckim., 17, 306 (1916)-
* Z. wiss. Phot., 22,
*
946
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 that the solution has not decomposed to 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 to see and is important since an attempt t o overcome turbidity due to gelatin leads to high results. (3) In the rare cases of coagulated emulsions or other silver halide precipitates, an 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 the 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 to react instantly, but 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.
Vol. 16, No. 9
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 to 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 that there will be no chemical exposition in 1924. The next Exposition of Chemical Industries will be held September 28 to October 3, 1925, at the Grand Central Palace, New York, N. Y .
