ACIDIMETRY AND ALICALIMETRY BY THE USE OF SYMMETRICAL DIPHENYL. GUANIDINE H. H. C-MAKJIAN,
TUFTS COLLEGE MEDICAL SCHOOL, BOSTON, MASSACKUSETTS
Sometimes very useful information is lost in the ever-increasing mass of chemical literature. In 1922 C . A. Carlton ( 1 ) published his work on symmetrical diphenyl guanidine as a reliable, easily reproducible standard for a quick titration in acidimetry and alkalimetry. Diphenyl guanidine is an important accelerator in the vulcanization of rubber and may be obtained easily for a reasonable price.* In the standardization of acids and alkalies use is made of chemically pure sodium bicarbonate which is heated to 350-360°C. in a sand-bath for about three hours, and after cooling about 0.5 gram sample is weighed in a weighing bottle and heated again in the sand-bath until constant weight is secured. This process means waiting for a long time before one is able to start the titration, and if any accident happens to one or more of the weighing bottles while heating, the day's work may be seriously impeded. Even if everything goes smoothly some element of doubt is left as to the accuracy of the determination. This doubt may be eliminated by checking the normality of hydrochloric acid, for instance, by a time-consuming gravimetric method such as the silver chloride method, provided the acid is freshly prepared. A three-months old sample of standard hydrochloric acid will have the same chlorine content but lower normality due to the action of the silicatesin the glass container. Checking may also be done by the uses of recrystallized C.P. oxalic acid, or by electrometric titration. The safest and easiest confirmation of the normality of a given acid may be obtained by the use of diphenyl guanidine. Diphenyl guanidine neither gains nor loses weight, and therefore i t may be used to determine the normality of an acid as often as desired and in a length of time just necessary to weigh and titrate two samples, provided diphenyl guanidine has been previously purified by reaystallization from toluene. Symmetrical diphenyl guanidine is a monoacid base, insoluble in water but soluble in alc~holand mineral acids. Evidently two phenyl radicals deprive two secondary amine radicals of their basicity and leave the middle nitrogen with double bond as a strongly basic secondary amine radical. Therefore the reaction with hydrochloric acid may be given. C.H,NH.C/
NH \NH-C~H~
+ HCI +C,H,.NH-C/
N-N /H \CI \NH-C~H%
The weighed sample (about one gram) is transferred to a 250-cc. beaker and dissolved in a volume of alcohol about equal to the volume of acid needed to neutralize it, using 2 drops of methyl red as indicator. The * May be purchased from Eastman Kodak Co. 2060
VOL.8,NO. 10
ELECTRIC METRONOME
2061
molecular weight of diphenyl guanidine being high (211.13), large quantities of it may be used in order to reduce any possible experimental error. One gram of the base will require 47.36 cc. of 0.1 N acid. Another important advantage of the base is the fact that after its purification by three crystallizations the fine white crystalline substance may be kept in glass-stoppered bottles for years without any alteration. In our laboratory the purification of the base was performed in 1923, and we are still using it for our standardization with satisfaction. The purification was carried out according to the method given by Carlton. One hundredgramsof diphenyl guanidine was dissolved in 800 cc. of hot, almost boiling, toluene which was previously dried over calcium chloride and redistilled. The solution was filtered through a bot-water funnel, and the filtrate cooled with vigorous stirring or shaking under running water. The recrystallization was repeated twice more, using approximately 8 cc. of toluene for each gram of the base. The residue of the last crystallization was washed twice on a Biichner funnel with cold toluene, sucked dry, and then dried in an electric oven, first a t 90°C. and finally a t 120' for two hours. The yield is about 80 grams or better. Cold toluene dissolves the impurities in the base but not the base itself. (1)
CARLTON, 3.A m . Chem. Sac., 44, 1469-74 (July, 1922).
