October 15, 1930
I.C'DCSTRI.4L A S D ELYGINEERING CHEMISTRY
An atteiiipt was made to discover the nature of the substances present in paper which exercise the buffering action. Microchemical examination of a buffered paper extract indicated the presence of a fatty substance. It was suggested that this might have originated in the lubricant present on the rope used in the manufacture of certain papers. Titration curves xere then obtained for a number of papers, some of which were made of rope and others of pure wood or rag. I n all casei: the rope papers shoTved much greater buffering action. I t is concluded that the major part of the buffering action in papers is due to t h e s e f a t t v s u b s t a n c e s . Microscopic examination of a paper extract showed minute liquid droplets in strong Brownian movement. From this observation the writers suggest that the fatty material exercises i t s b u f f e r i n g effect partly chemically and partly by virtue of its colloidal character (3). Attention is called to the first six determinations in Table 111. in which no p o t a s s i u m -- - -2 -__ chloride was used. T h e s e results indicate IU 2M 3M 4M t h a t t h e neutral salt CONCE*jTRATION OF K C I IN CALOMEL M L F CELL effect on the activity of the hydrogen ions is negligible, a t least in the systems used. The results in Table IV indicate that the error caused b y temperature variations in the system may be appreciable. The magnitude of this error is dependent upon the buffered condition of the solution, and in view of the variation of this factor a temperature correction is not practicable. It was found empirically in the case of one typical paper, however, that the room temperature could vary 1 3 " C., provided the extract is cooled to 25" C. immediately before titration, without causing a n error exceeding the required precision. If the room temperature is above 28" C., or below 22" C., the apparatus must be placed in a thermostat in order to obtain satisfactory results.
387
The possible error due to polarization of the electrodes is obviated by keeping in the circuit until neaz the end point a high resistance of 500,000 ohms. Other errors to which the method is subject are within the limits of the desired precision and therefore may be neglected. General Applicability of Method
I n the system X N KC1 xJi' KClllacid solution quinhydronelpt which is that used in the method described, the p H of the titrated solution when the potentials of the two half-cells are equal-i. e., a t the null point-is a function of 5 , the molarity of the potassium chloride. It is evident that by varying 5 v e may vary the pH which is taken as the end point. This increases the usefulness of the method, since it makes it possible to titrate accurately weak acids and bases whose neutralization points lie between p H 6.13 and 7.67. Table 'i' and Figure 3 show the relation between the concentration of potassium chloride and the pH a t the null point. HglHgCl
+
+
Table V-Relation of pH of T i t r a t e d S o l u t i o n a t Null P o i n t to C o n c e n t r a t i o n of P o t a s s i u m C h l o r i d e i n C a l o m e l Half-Cella MOLARCONCN. OF KCI PH 7.67 4 . 1 2 (satd.) 3.00 7.48 2.00 7 30
i.on
0
0.50 0.10 From d a t a of Fales and hludge ( 4 )
7 03 6 75 6 13
At potassium chloride concentrations below 0.1 molar the conductivity of the solution becomes prohibitively small. From Figure 3 it is possible to select the potassium chloride concentration corresponding to the p H to which it is desired to titrate. It is evident from the curve that the cell is more sensitive to accidental alterations in potassiuni chloride concentration as this concentration diminishes. Literature Cited T. 31. Tentative Standards, 1927, D-202-37-T. (2) Clark, "Determination of Hydrogen Ions," p. 306, Williams & Wilkins, (1) A. S.
192s.
Clark, I b i d . , p. 53. Fales and &fudge, J . A m ,C'hem. Soc., 42, 2434 (1920). Leeds Pr Northrup, Bull. 768, p. 13. Pinkhof, "Over de Toepassing der elektrometrische Titraties," Dissertation, Amsterdam, 1919. (7) Sharp and MacDougald, J . . I m . Ciiem. Soc., 44, 1193 (1922). (8) Treadwell and Weiss, H e l ; ~ .Chim. Acta, 2, 680 :1910). (3) (4) (5) (6)
Determination of Wax in Shellac' A. G. Stillwell T H E STILLWELLL X B O R A T O R IIKC., ES, 41
IVE grams of the finely powdered shellac are dissolved by boiling in 150 cc. of water containing 3 grams of sodium carbonate. The solution is cooled and filtered by suction through a specially prepared Soxhlet siphon, thoroughly washed with water until colorless, and finally i-dh about 50 cc. of 70 per cent alcohol. I t is dried in an oven until most of the alcohol has been driven off and then attached to a condenser and extracted with carbon tetrachloride into a weighed flask. The solvent is distilled off and the wax, when dried, weighed to constant weight. The siphon is prepared by placing a perforated porcelain plate in the bottom and on this a wad of absorbent cotton is placed, wet and well packed down. Enough asbestos is 1
Received July 30, 1930.
WATER
ST., S E W
X'ORK.
S. Y .
placed on the cotton to form a layer, when well sucked do~vn, of about 5 mm. in thickness. Another wad of cotton is placed on top of the asbestos, forming a layer, when sucked down, of about 12 mm. After drying to constant weight the tube 1s ready for use. The insoluble matter and most of the wax are caught on the top layer of cotton and, as it clogs this layer, it can be stirred with a wire to hasten the filtration. The object of thc asbestos layer is to catch any very fine wax that goes through the cotton. This procedure can be completed in 3 hours and checks with the standard method of the Shellac Association. I t has the advantage that no allowance need be made for was dissolved in the alcohol and no special attention paid t o temperatures except that the first cooling should be carried down to at least 15" C.
