January 15, 1935
ANALYTICAL EDITION
seal and render leaks the more easily detected. This swivel seems to be simpler and more positive than the one described by Palkin and Nelson (5). Control of regulation by this valve was so good as to cause but a slight rimle on the surface of the sulfuric acid in the manostat. The effe^ci upon the mercury level would certainly not be greater than *0.015 mm. When dispensing with relays, no difficulty due to the increased current flowing through the manostat was experienced in many hours of operation. All work was done with alternating current, but with direct current, inasmuch as both electrodes are in the same arm, all electrolytic gases will doubtless be reunited a t the interrupting electrode as they Seem to be by the alternating current. (Even if all electrolytic gases were to enter the system a t L, Figure 1, the quantity formed as calculated for 1 mm. pressure by Faraday’s law and Boyle’s law would be less than 1 cc. per minute, a value negligible compared to the capacity of the pump.) An apparatus of this type has been in use a t the laboratory at Rutgers University since July, 1933. It has proved
71
capable of performing complete distillations a t predetermined pressures* It is quiet in ‘peration and simple.
ACKNOWLEDGMENT Many thanks are due to the following for technical suggestions: P. A. Hoover, D. L. Cottle, P. V. McIiinney, all of Rutgers University; E. H. Huntress of the Massachusetts Institute of Technology; and J. A. Doremus of Lafayette College. (1)
LITERATURE CITED Hershberg, E. B., and Huntress, E. H., IND.E m . CHEM.,Anal.
Ed., 5, 344 (1933). (2) Huntress, E. H., and Hershberg, E. B., Ibid., 5, 144 (1933). (3) Jacobs, G. W., master’sthesis, Rutgers University, 1934. (4) Miller and McKinney, IND. ENQ.CHEM.,20,522 (1928). (5) Palkin, S., and Nelson, 0. A,, Ibid., Anal. Ed., 6, 386 (1934). RECEIVEDMay 7, 1934.
Application of the Glass Electrode to Dairy Products L. R. PARKSAND C. R. BARNES,Pond Chemical Laboratories, Pennsylvania State College, State College, Pa.
T
HE common methods of measuring the pH of milk and its products are the hydrogen electrode, the quin-
hydrone electrode, and the colorimetric. The glass electrode and the stick antimony electrode have not been accepted as standard methods for dairy products. The purpose of this investigation was t o determine the pH of whole milk, buttermilk, cream, commercial ice cream mix, and butter; by glass, quinhydrone, hydrogen, and stick antimony electrodes.
PROCEDURE The electrical potentials were measured by a vacuum-tube potentiometer of design similar to that of Rosebury (8),who used a General Electric FP. 54 vacuum tube and a Leeds & Northrup type 7651 potentiometer. The circuit used for this investigation was modified somewhat to permit the use of a type K Leeds & Northrup potentiometer. A four-pole doublethrow switch was used for &, connected so as to short-circuit the galvanometer terminals of the type K when the potential readings were being made. The glass electrode was of the bulb type, made of 015 Corning glass according to the method of Robertson (7). The bulb, TABLEI. PH DETERMINATIONS (At 25’ C.)
GLASS
QUINHYDRONE
(GOLD)
HILDEBRAND ANTIMONY
PABTEURIZED WHOLE MILE
6.585 6.587 6.587 6.585 AV.
6.586
6.589 6.586 6.588 6.586
6.585 6.583 6.583
...
6.94 6.95 6.96 6.95
6.587
6.584
6.95
PASTEURIZED BUTTERMILK
Av.
Hg
I
HgCl KC1 (sat.)
4.302 4.304 4.305 4.304
4.304 4.301 4.304
4.84 4.84 4.87
4.306
4.304
4.303
4.85
..
I
Unknown solution O.1NHCI Glass membrane
111
1
%?\sat.)
1
TARLE 11. PH DETERMINATIONS ( A t 260 C.) QUINHYDRONE
(GOLD)
HILDBBRANDBAILEY
ANTIMONY
COMMERCIAL ICB CREAM MIX
6.614 6.614 6.615
6.617 6.617 6.615 6.617
6.615 6.615 6.615 6.617
6.598 6.609 6.613
.,.
7.09 7.05 7.08 7.07
Av. 6.614
6.616
6.615
6.607
7.07
...
PASTEURIZED STNQLB CRBAM
Av.
Hg
The bulb was washed thoroughly mTith distilled water before each determination. The e. m. f. usually increased during the first two or three readings, after which constant values were obtained. The electrode was then checked with a phthalate buffer of pH 5.697 as determined by hydrogen and quinhydrone electrodes. After checking the calibration of the glass electrode with the buffer, another determination was made to make sure that the asymmetry potential of the glass membrane was still constant. The change of asymmetry potential which sometimes occurred during the first two or three readings was probably due to the formation of a thin fatty film over the glass membrane. Quinhydrone electrodes were prepared from platinum foil and plated with gold as recommended by Popoff, ICunz, and Snow (6). To each 50-ml. sample of dairy product 0.2 gram of Eastman’s quinhydrone was added and the mixture was stirred thoroughly before readings were taken. The Hildebrand electrodes consisted of platinum foil 1 em. square, coated with platinum black ap recommended by the Leeds & Northrup Company (3). These electrodes were used with electrolytic hydrogen purified by passing through solutions of mercuric chloride, alkaline permanganate, alkaline pyrogallol, dilute sulfuric acid, and water. The rate of flow was controlled as suggested by Duncombe (8). The Bailey electrode was used as directed by Bailey (1). Antimony electrodes were cast from Mallinckrodt’s c. P. anti-
GLASS
4,305 4.305 4.307 4.307
...
after being blown, was filled with 0.1 N hydrochloric acid and was aged in distilled water for about a month. The cell used was
BUTTER BERUM
6.603 6.601 6.603 6.603
6.600 6.598 6.600 6.600
6.605 6.605 6.603 6.605
6.90 6.88 6.98 6.88
5.995 5.993 5.993
6.003 6.003 6.005
6.001 5.990 5.994
5.994 5.993 5.994
6.64 6.64 6.64
6.603
6.600
6.605
6.91
Av. 5.994
6.004
5.995
5.994
6.64
INDUSTRIAL AND ENGINEERING
72
mony. The potential readings were taken at the end of five minutes as specified by Parks and Beard (5). The butter serum was prepared according to the method of Nissen (4). The results for the different electrometric methods are given in Tables I and 11. DISCUSSION The hydrogen-ion concentration of dairy products may be determined by the glass electrode, the quinhydrone electrode, or the hydrogen electrode with an accuracy which is within experimental error. A decided drift occurs when platinum electrodes are used with quinhydrone, and is especially noticeable with ice cream mix and butter serum. This drift, however, may be eliminated by the use of gold electrodes. The stick antimony electrode determinations were from 0,307 to 0.646 pH units higher than those of the glass electrode. The errors appear to increase with high concentration of serum solids and with the lactic acid content. The effects upon the stick antimony electrode of solutions containing citric acid, lactic acid, and lactose are emphasized in the following experiments. A solution containing 0.235 gram of citric acid per 150 ml. was neutralized with sodium hydroxide to a pH of 3.72 as meas-
CHEMISTRY
Vol. 7, No. 1
ured by the glass electrode. This solution showed a pH of 4.26 by the stick antimony electrode. Another solution containing 2 ml. of c. P. lactic acid per 150 ml. was neutralized with sodium hydroxide to a pH of 3.68 as measured by the glass electrode. This solution by the antimony electrode gave a pH of 4.45. A solution of 5 grams of lactose in 150 ml. showed a pH of 5.86 by both the glass and antimony electrodes. These results indicate that the errors of the antimony electrode in dairy products are due t o citrates and lactates. Since citrate and lactate ions form complexes with antimony in solution, it seems probable that a similar complex formation takes place a t the surface of the antimony electrode. As a result the electrode does not measure the true hydrogen-ion concentration of the solution. LITERATUR~ CITED (1) Bailey, C. H., J.A m . Chena. Soc., 42, 45 (1920). (2) Duncombe, J. Dairy Sci., 7, 86 (1924). (3) Leeds & Northrup Co., Notebook 3.18 (1930). (4) Nissen, B. H., IND. ENQ.CHEM.,Anal. Ed., 3,374 (1931). ( 5 ) Parks and Beard, J. Am. Chem. Soc., 54, 856 (1932). (6) Popoff, Kuna, and Snow, J. Phys. Chem., 32, 1059 (1928). (7) Robertson, G. R., IND. ENQ.CHEW,Anal. Ed., 3, 5 (1931). (8) Rosebury, F., Ibid., 4, 398 (1932). RECBIVED May 28, 1934.
Routine High-Vacuum Distillation of Oils An Apparatus and Conversion Chart K. M. WATSONAND CHARLESWIRTH, 111, Universal Oil Products Company, Riverside, Ill.
A
GENERALLY standardized method for evaluating apparatus are the adequate size of the vapor delivery tube the boiling points of heavy oils is badly needed for the and the method of connecting the McLeod gage direct to the extension of both the practical and scientific aspects flask to avoid errors due to pressure drop in the delivery tube. of petroleum technology. I n addition to the importance of However, in using an apparatus similar to that of Davis and the boiling range as an indication of volatility, it has been Hornberg, it was found impossible to avoid errors due to found (6) that most of the difficultly measurable physical diffusion of small amounts of condensable vapors into the properties of petroleum fractions are better correlated on the McLeod gage. As a result the pressures read were appreciably basis of boiling point than any other easily determined prop- too low, even though the amount of condensate formed was so small as to be invisible. Errors as great as 26.7' C. (80' F.) erty. Various types of equipment have been proposed for opera- were encountered in the boiling points of pure compounds as tion a t pressures ranging from a few hundredths of a milli- determined in this way. Errors due to condensation in the McLeod gage can be meter up to 10 or 20 mm. of mercury. The low pressures have the advantage of permitting the distillation of heavier minimized by careful manipulation, although this procedure stocks without decomposition. On the other hand, operation is both difficult and uncertain. A trap cooled with liquid air a t low pressures increases the difficulty of manipulation and or solid carbon dioxide and placed in the line connecting the uncertainty of converting the observed temperatures to the flask to the McLeod gage was found to eliminate all condensation in the gage. The amount of condensate collected normal boiling points a t atmospheric pressure. The apparatus here described was developed to permit in the trap never exceeds a few tenths of a cubic centimeter operation a t pressures of approximately 0.1 mm. of mercury. and introduces a negligible error in the distillation if suitable As compared to previously proposed methods, it has the precautions are taken. The most important of these is flushadvantage of satisfactory accuracy of pressure measurements, ing the entire McLeod gage and connecting lines with air due to the elimination of condensable vapors in the McLeod before evacuating. The distilling flask is evacuated while the gage, It is also unique in general design through the exten- gage is filled with air a t atmospheric pressure. The air in the sive use of ground-glass connections, which allows a minimum gage is then slowly released into the flask before heat is of difficulty in manipulation and cleaning. A new nomo- applied, sweeping out all vapor and filling the connecting lines graph has also been devised for accurately converting ob- with air. Care is also taken to maintain a constant or diserved temperatures to normal boiling points. As a result of minishing pressure throughout the distillation. the improved convenience of both the chart and apparatus, APPARATUS the method is satisfactory as a routine inspection and has The apparatus finally adopted is shown in Figure 1. been used as such for the past several months with gratifying Special attention was given to obtaining an arrangement easy consistency. Davis and Hornberg (4) recently developed a vacuum to clean and reassemble. For this reason ground-glass joints distillation apparatus using a McLeod gage for accurate were extensively used to minimize the difficulty of keeping the measurement of pressure. Outstanding features of this system vacuum-tight.
