January 15, 1932
INDUSTRIAL AND ENGINEERING CHEMISTRY
109
If 5 cc. of phosphate solution (25 P. P. m. the organic matter oxidized by digesting with sulfuric acid were the Of the standard was 2o m m . ~ and perhydrol according to Youngburg and Youngburg (4). and that of the water 24 mm., then 20/34 X 25 = p. p. m. of POc in the water. LITERATURE CITED IkmPLE.
Of
The procedure is the Same for any of the water analyses, except that for natural waters the more dilute standard phosphate is used. For the determination of total phosphorus in plankton water, the sample must be evaporated to near dryness and
(1) Kuttner, T.,and Cohen, H. R., J. Bid. Chem., 75, 517 (1927). (2) Scarritt, E. W., IND. ENG.CHEM.,Anal. Ed., 3, 23 (1931). (3) Truog, E., and Meyer, A. H., Ibid., 1, 136 (19ag). (4) Youngburg, G . E., and Youngburg, M. V., J. Lab CLin. Med.. 16, 158 (19301. R ~ C ~ I V B DJ
~ ! YR, 1931.
Potentiometric Titration of Acidity in Oils ROBERTR. RALSTON,LTniversily of Michigan, Ann Arbor, Mich., C . H. FELLOWS AND K. S. WYATT,T h e , Detroit Edison Company, Detroit, Mich.
T
HE A. S. T. M. reconiTHE PLATINUM-CARBON electrode Blank determinations on the solvent were made f r e q u e n t l y I n e n d a t i o n (1) for the couple is shown to possess distinct advantages and required only 0.10 to 0.15 cc. determination of acidity over others previously suggested for the acidimetric of alkali. of oils, t h o u g h e x t e n s i v e l y titration Of Oils. used, is subject to several limiAPPARATUS An explanation of the second inflection point t a t i o n s . The use of phenolphthalein and other indicators in the difSerential plot of results on oil titrations S e v e r a l d i f f i c u l t i e s encountered in the use of the agar($) for detecting the end point is posfulaied as being due to the presence two of an oil titration is obviously agar bridge of Seltz and Mcdifferent classes of organic acids. Kinney (6) have been experiu n s a t i s f a c t o r y in the case of A system is described which permits acidity dark-colored oils b e c a u s e of e n c e d in preliminary work in determinations on oil samples as small as 0.5 the difficulty of observing the this laboratorv. chief a m o n g indicator change. Another disgram. which was a t e n d e n c y of t h i agar-agar to dehydrate slowly a d v a n t a g e is the f a c t that most oils are not completely soluble in the hot alcohol solu- and permit leakage. Isolation of the reference electrode in a tion. A two-phase system results, and the success of the separate vessel through use of a bridge was found to be imdetermination of the acidity of the oil is dependent upon practical because of the relatively high resistance of the the distribution of the acidic ingredient between the two solution. The silver-silver chloride electrode, when used phases. Seltz and McKinney (6) have shown the possi- as described by Seltz and Silverman (7), proved quite satisbility of the detection of the potentiometric end point for factory if freshly prepared, but after some days ceased to oil titration in isoamyl or n-butyl alcohol solutions. Later function as a standard. Such a deterioration was shown Seltz and Silverman ( 7 ) suggested improvements in the by the failure of the electrodes to give any potential break apparatus by substitution of a silver-silver chloride elec- a t the end point. In both of these systems (6, 7) a quinhytrode for the quinhydrone reference electrode and its agaragar bridge. BURET The work to be described was undertaken in connection with the research on the deterioration of high-tension cable now in progress in this laboratory. It represents an attempt to develop the potentiometric method as a practical laboratory means of determining acidity in small samples of oil obtained a t different points in the cable insulation.
-i-l
SOLVENTS For use in titration work and especially in potentiometric titrations, a solvent should be capable of effectively dissolving the sample, the titrating agent, and the “conducting salt,” and should possess a dielectric constant sufficiently high to permit the desired ionic reaction to take place rapidly, as well as to permit ionization of the conducting salt in order to increase the conductivity of the solution. Both isoamyl and n-butyl alcohol are excellent oil solvents, are satisfactory solvents for potassium hydroxide and lithium chloride (used as conducting salt), and have dielectric constants of about 15 to 18 (5). I n the work to be described the solution used as a solvent for the oil was isoamyl alcohol saturated with lithium chloride in order to give higher conductivity. The neutralizing solution consisted of a 0.025 to 0.050 N solution of potassium hydroxide in isoamyl alcohol standardized against Bureau of Standards benzoic acid.
CARBONROD
FIGURE 1. CELLFOR POTENTIOMETRIC TITRATIONS drone electrode served as the indicator electrode. This quinhydrone electrode itself was found to be slow in coming to equilibrium potential in addition to its potential being notably unreliable in alkaline solutions.
ANALYTICAL EDITION
110
Holt and KahIenberg (4) have shown that certain metals and non-metals coupled together are very satisfactory endpoint indicators in acidimetric titrations. Some of these recommended couples failed entirely when tried in nonaqueous solutions, but the platinum-carbon pair proved to be quite excellent. Inasmuch as carbon and natural graphite gave equally good results, small arc-light carbons were used.
Vol. 4, No. 1
The series of measurements was extended over a period of about six weeks. In the titration of turbine oils a double inflection could be found with any electrode system that functioned a t all satisfactorily in isoamyl alcohol solutions. This has suggested the possibility of the presence in the oils of acidic ingredients possessing widely different ionization constants. To check up on this point, a number of acid mixtures have been titratid using the platinum-carbon electrode pair and some of the results are represented graphically in Figures 2 and 3. Since the end points of the component acids may be detected in this manner, it should be possible to distinguish between acids having widely different ionization tendencies when present in oils. For this reason the writers believe the two inflections to be,caused by the different types of acids present in oils. ADAPTATIONS FOR SMALLSAMPLES Previous work reported (2, 6, 7) on the potentiometric determination of acidity of oils has specified oil samples of the order of 10 to 20 grams per determination. The necessity arose for determining the acidity of oil samples of less than 1 gram. Accordingly, a cell of Pyrex (Figure 1) was designed to take the place of the 250-cc. flask previously used. This smaller cell had an inside diameter of 3 cm. and a height of 12 to 15 cm. I n this cell, 25 cc. of solvent were used with an oil sample of 0.5 to 2.0 grams (or 0.020 gram of ben-
AE AV
0.50
CC KOH
FIGURE 2. TITRATION CURVEFOR MIXTURE OF PICRIC AND BENZOIC ACIDSUSINGPLATINUM AND CARBON ELECTRODES The advantages of this electrode pair are (1) simplicity, (2) compactness, (3) high electrical conductivity, (4) low resistance contact with the solution, and (5) large potential change at the end point (100 to 250 millivolts). Figure 1 shows the apparatus that was used in titrations wherever possible,’ including cell, electrodes, buret, and nitrogen-stirring arrangement. The buret was of 10 cc. capacity, graduated in 0.05-cc. divisions, and carried an extended tip for convenience in passing through the cell stopper. A Leeds and Northrup hydrogen-ion type potentiometer I 1 was used in conjunction with a Weston model 440 tableCC KOH type galvanometer. FIGURE 3. TITRATION CURVEFOR MIXTURE OF STEARIC, BENTABLEI. ACIDITYOF A TURBINEOIL USINGA PLATINUM- ZOIC, AND PICRIC ACIDSUSINGPLATINUM AND CARBON ELECCARBON COUPLE TRODES DETERMINATION ACIDNUMBER Mg. KOH/grams oil 1 4.74 2 3 4 5 6 7 8 9 10 11 12
4.62 4.44 4.76 4.65 4.73 4.54 4.48 4.72 4.61 4.48 4.61 Mean 4 . 6 2 Max devn from mean 0 . 1 8 ( 3 . 9 % ) . Av. devn. irom mean, 6-10 ( 2 . 6 % ) .
DEVN.FROM M l A N Mg. KOH/grams oil +0.12 0.00 -0.18 +0.14 +0.03 +O.ll -0.08 -0.14 +o. 10 -0.01 -0.14 -0.01
zoic acid when standardizing alkali). This cell and equipment have been used quite extensively with satisfactory results. LITERATURE CITED
Table I shows some typical results of titrations of unfiltered turbine oil using the platinum-carbon electrodes. Weighed samples of from 1.00 to 2.00 grams were used.
(1) Am. SOC. Testing Materials, “Tentative Standards.” p. 378 (1930). (2) Clarke, B. L., Wooten, L. A,, and Compton, K. G., IND.ENQ. CHEM.,Anal. Ed.,3, 321-3 (1931). (3) Evans, R. N.,and Davenport, J. E., Ibid., 3, 82-5 (1931). (4) M. L.. and Xahlenberg, - L., Trans. Am. Electrochem. Soc., -, Holt. 57, 361-79 (1930). (5) International Critical Tables, Vol. VI, pp. 87-9 (1929). (6) Seltr, H.,and McKinney, D. S., IND.ENQ. CHEM.,20, 542-4 (1928). (7) Seltz, H., and Silverman, L., Ibid., Anal. Ed., 2,1-2 (1930).
1 In titrations where the agar-agar bridge and differential electrode systems were used a somewhat larger cell waa neoessary.
RECEIVED August 11, 1931.
