January 15, 1932
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
continuous slope of that portion of the curve. The abrupt rise in temperature in the middle of the laurate fraction is due to an increase in the rate of distillation. The 60-cc. intermediate between the caprate and lauratefractionswas carefully refractionated into 22 cc. at 92" to 93" C., 10 cc. a t 93" to 115" C., and 23 cc. at 115" to 117" C., at a pressure of 5 mm. The apparatus can be used conveniently for amounts of material from 50 to 400 cc. having a boiling point up to about 200" C., a t pressures down to 1 mm. It is most conveniently assembled with the aid of standard ground-glass connections, as indicated in Figure 1. The size of the connection a t E is
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number 9.5, at F number 11, a t D number 15, and at the bottom of the column number 11. For temperatures above 150" C. it is well to insulate the flow divider with "85 per cent magnesia." LITEIL4TURE CITED
(1) Houben-Weyl, "Die Methoden der organischen Chemie," 2nd ed., Vol. I, p. 568, Georg Thieme, Leipzig, 1921. (2) Schwartz, A. M., and Bush, M. T., IXD.ENQ.CHEM.,Anal. Ed., 3, 138 (1931). RECEIVED July 6, 1931.
Wet Microcombustion of Organic Compounds With Particular Reference to Cellulose Derivatives ANDREWCHALMERS, Du Pont Rayon Go., Buffalo, N . Y .
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N A communication to be published in a later issue, the writer discusses the application of a strong oxidizing mixture upon cellulose compounds, and by using a specially designed apparatus, estimates the carbon content by the pressure exerted by the carbon dioxide formed. However, it is sometimes necessary to perform analyses using samples smaller than 0.10 gram, and as the sample size is reduced, the efficiency of the above method decreases. It was considered possible that by using a standard solution of the oxidizing reagent, a much smaller sample could be determined by using volumetric methods. Such a method has apparently been obtained. As an oxidizing reagent, a mixture of a concentrated aqueous solution of potassium dichromate and concentrated sulfuric acid was used. Thip mixture performed exceedingly well with most of the samples used, but some were found which resisted "wetting" to such an extent that it was necessary to boil the mixture for a relatively long time in order to consume the sample completely. Such a procedure did not commend itself to accuracy. A solution of chromic acid anhydride in c. P. phosphoric acid gave the necessary qualifications in that it consumed all the samples used, and since it contained no unnecessary water, steam was not released on subsequent heating. A special form of apparatus, shown in Figure 1, was designed for this method, the design given being the result of experimenting with many other forms of apparatus. The oxidizing reagent is prepared by dissolving one part, by weight of c. P. chromic acid in nine parts by weight of c. P. sirupy phosphoric acid. A more concentrated mixture may be used if desired. Approximately 15 cc. of the oxidizing reagent are run into the reaction flask, A , by means of a standard buret. It is suggested here that where a considerable number of analyses are made, a small buret capable of delivering about 15 cc. be constructed for use in this method. Twenty cubic centimeters of 0.05 N barium hydroxide solution are run into flask B from a constant-delivery buret. The sample (0.0050 gram or less), previously weighed in a piece of platinum foil, is introduced into the reaction flask. A very slow stream of dry air, which has been washed free of carbon dioxide, is conducted through the apparatus and a gentle heat is applied to flask A, using a microburner. When the reaction has ceased, the air should be continued to pass for a reasonable length of time. The apparatus is disconnected and the barium hydroxide solution is washed and precipitated into a beaker, a drop of phenolphthalein indicator
is added, and the excess hydroxide rapidly titrated with 0.05 N hydrochloric acid. The acid mixture in flask A is washed into a 250-cc. volumetric flask and made up to mark with distilled water. A 25-cc. portion is aliquoted and transferred to a beaker. To this 20 cc. of 20 per cent potassium iodide solution are added and the liberated iodine is titrated with 0.1 N sodium thiosulfate solution using starch solution as an internal indicator.
FIGURE1. SPECIAL APPARATUS FOR MICROCOMBUSTION
A blank determination is made upon the equivalent amounts of oxidizing reagent and barium hydroxide used. I n the latter, the blank determination would adjust a n error due to possible contamination by carbon dioxide during transferring of the solution. The actual titration subtracted from the blank titration gives the amount of reagent entering into the reaction. From each of the above results, the per cent carbon present in the sample is obtained, one depending upon the amount of carbon dioxide formed, and the other upon the amount of oxygen consumed. 1 cc. of 0.05 N NC2 = 0.0003 gram of carbon 1 cc. of 0.1 N NazSpOa= 0.0008 gram of oxygen
ANALYTICAL EDITION
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The writer has used this method for a considerable time and found that for all practicable purposes one need only consider the barium hydroxide titration for calculation of results, the other being merely a check. (cc. of blank - cc. of actual titration) X 0.0003X 100 = %carbon weight of sample
Vol. 4, No. 1
It is self-evident that absolute cleanliness is necessary in using this method. Before each series of runs, the flasks should be thoroughly cleaned with cleaning solution, washed well with distilled water, and dried in an oven. RECEIVBD
J d y 14, 1931.
Rapid Method for Fixing End Point of Potentiometric Titration FLORENCE FENWICK, United States Steel Corp., Kearny, N . J .
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time-consuming and boresome. N THE ideal case, the end A S I M P L E M A T H E M A T I C A L formula has It is the object of this paper to point of an electrometric been developed for determining the end point of a point out an analytical solution titration is so sharply depotentiometric titration. I t is applicable only in applicable in many cases which fined by a considerable change in cases in which the electron interchange between p e r m i t s of the determination potential with the appearance of the electromotively active ions and the indicator of the end point directly from a very slight excess of the titratthe titration curve by a simple ing agent that it may readily be electrode is the same on both sides of the end calculation. located either by merely observpoint, and it assumes that a cubic equation fits ing t h e p o i n t of m o s t r a p i d the titration data accurately near the point of DEVELOPMENT OF FORMULA change in potential as the titratequivalence. Its use reduces the necessary plotThe equation ing solution is added, or by deting to a minimum, thereby shortening the time termining the position of the verE = E o - - 0.05915 log 5 tical portion covering the inflecrequired for an analysis. The formula has been N a2 tion of the curve obtained by a applied to a specific titration selected from the may be r e g a r d e d a s defining simple plot of potential versus group to which its use is theoretically least rethe potential of any indicator volume of t i t r a t i n g solution. stricted-i. e., neutralization reactions. The electrode a t 25' C. a1 and az There are, however, many factors results were found to compare very favorably in are the r e s p e c t i v e activities which tend to flatten the titraof $he electromotively a c t i v e accuracy with the exDerimental data. tion curve and thus obscure the substances. Eo is a E o n s t a n t inflection that marlrs the end point, For example, in oxidimetric reactions a relatively small fixed by the theoretical condition of unit values for both difference in oxidation potential between the oxidant and re- al and az, and N the number of Faradays passing. If the ductant has this effect; in precipitation reactions the titration electronic transfer is such that the electrode reactions on curve flattens rapidly with slightly increased solubility of the both sides of the end point for a given titration involve the precipitate; in acidimetric reactions the effect of a high same value for N , it is evident that the curve E versus V concentration of neutral salt is more noticeable under most is symmetrical with respect to the end-point inflection, conditions than in the other types of titration, and the magni- since V , the volume of the titrating solution added, may be tude of the dissociation constants of the acids and bases regarded as a measure of the ion activities concerned in the region of the equivalence point. Many of the most cominvolved determines the clarity of the end point. Figure 1 illustrates the pronounced decrease in the slope monly employed oxidation and precipitation reactions and of a neutralization curve caused by a high salt concentra- all neutralization titrations meet this requirement. It has tion. Curve 1 is the titration curve of*0.004 N sulfuric acid been found in studying the curves for a large number of such with sodium hydroxide. Curve 2 shows the same titration titrations that a cubic equation fits the curve with sufficient carried out in a solution 1.5 M with respect to pure ammo- accuracy over a fairly wide region on either side of the end nium sulfate. Crude ammonium sulfate containing consider- point. The problem, therefore, resolves itself into a quest able amounts of phenolic compounds and salts of pyridine for a rapid method of locating the point of inflection of a was substituted for the pure salt in the titration represented cubic equation. The type equation may be written by the third curve. Hardly a suggestion of an inflection remains, yet it will be shown that even in such unfavorable u V ~ bVa cV d =E cases the end point may be located with a considerable deBy successive differentiation gree of accuracy. I n the event that the titration curve is so flat as to render daE/dV== 6aV 2b uncertain the location of the end point by direct observation, it has been customary to determine from the curve the Since the value of V at the end point must be such that the change in potential, AE, for B given small change in the second derivative vanishes, it is not necessary to set up the volume of the titrating solution, AV; the maximum value complete equation to fix the inflection, as the determinaof the ratio A E / A V fixes the definitive value of V. AE/AV tion of the constants a and b suffices. If four equidistant points, VI, V z , V I ,and V,, are selected is a sensitive function, however, and is considerably affected by even slight irregularities in E. It is therefore advisable from the portion of the titration curve covering the end to plot if the highest accuracy is desired. This process is point, and if k is the constant difference between them, and
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