Determination of Acidity in Insulating Oil ./
U
Use of the Glass Electrode in n-Butanol R. N. EVANS AND J. E. DAVENPORT Research Bureau, Brooklyn Edison Co., Inc., Brooklyn, N. Y.
This paper outlines a potentiometric method for the titration of acids in oil i n which the glass electrode in n-butanol is employed. The need for a small amount of water in the solvent is pointed out. Fac-
tors relating to the titration procedure are dealt with experimentally. The effect of easily hydrolyzable substances on the shape of the titration curve and on the estimation of acidity of oils is discussed.
T
glass electrode. Thus, since small amounts of water are involved, the entire operation must be carried out in the absence determined if one attempts to estimate the deleterious of air. This phenomenon is the subject of further study. substances in the oil bv more refined chemical procedures. Thus, in the past, a great deal of work has been done in atTO CURRENT SUPPLY tempting to correlate electrical and chemical test results, with little or no success because of lack of refinement in chemical test procedures together with an incomplete interpretation of the significance of the test data a c t u a l l y o b t a i n e d . For example, it is rarely emphasized that such substances as metal soaps and peroxides may be included in the simple neutralizaFLOW METER tion number test, although the effect of the latter substances on the electrical and c h e m i c a l properties of the oil far outweighs the effect of the elemenEDUCED Cu 0 WIRE tary acid group carboxyl. Inasmuoh as the work herein reported is a result of a study of the deterioration of high-voltDEHYDRI TE ASCARI T age cable oil in particular, the procedure for acidity determination must of necessity be capable of revealing smallamounts of acids-i. e., a neutralization number of 0.01 or less. The value of the glass elecATMOSPHERE trode (3,B)in aqueous solutions is well known under conditions which militate against the use of hydrogen, quinhydrone, or A : REFERENCE ELECTRODE, A y - A y C l analogous substances to stabilize the potential. Similar obB: GLASS ELECTRODE jections may be raised to the C: CAPILLARY TIP use of these substances in nonaqueous solvents. Although D : N-BUTANOL the authors have found that the presence of water in the E: QROUND JOINT titration medium is essential F: FOR ADDITION OF SAMPLE for the precise determination of acidity in oil with the glass 0’ BURET electrode, it will be necessary to modify the apparatus considerably to connect the activity of the water in the solvent OF TITRATION APPARATUS FIGURE 1. DIAGRAM definitely to the behavior of the
HE usefulness of an electrical oil may be more accurately
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FIGURE 2. WIRINGDIAGRAM OF VACUUM-TUBE POTENTIOMETER
Apparatus TITRATIONCELL. The titration assembly is sketched in Figure 1. The beaker with a ground-glass cover was supported by an aluminum plate immediately above two electromagnets. The electromagnets,by means of a collector ring, were energized from a 7.5-volt storage battery which also served as a source of power for a small toy alternating or direct current motor, 8 to 12 volts. A rubber band was used as a belt to connect the motor shaft with the frame supporting the electromagnets. A glass ball filled with iron filings moved in its circular path bounded by the wall of the beaker and the gas bubbling tube. The groundglass cover was fitted with holes for the two electrodes, buret, addition of the sample, and the gas entrance and exit. The gas was directed to the perforated glass tube beneath the surface of the liquid or to the space above the liquid by means of a two-way stopcock. Thus, it was possible to agitate the liquid by means of the gas stream or by means of the glass-covered revolving ball. The exiting tube was connected t o an alkali trap which indicated a slight pressure above atmospheric in the titration vessel. The buret (graduation 0.02 cc.) and alkali storage reservoir were permanently connected to the purified nitrogen system. The capillary tip of the buret extended beneath the surface of the liquid in the titration vessel.
ELECTRODES. In the authors' work, the cell platinumquinhydrone, picric acid 0.003 M in n-butanol-glasslz-butanol plus oil sample-saturated potassium chloride in n-butanol-silver chloride-silver was employed. The silversilver chloride reference electrode was prepared as described by Brown ( I ) . The experimental results substantiate Dole's (8) conclusions in that water was apparently necessary to bring about reproducible potentials. Thus, experiments with %-butanolas a solvent dehydrated by means of the azeotropic mixture ( I S ) with benzene (water content too small to be measured by the subsequently mentioned method, less than 0.1 per cent) and with potassium hydroxide either in n-butanol or methanol, show that, in titrating acids, before the neutral point, the change of potential upon addition of alkali was extremely erratic. However, when sufficient water of neutralization had been formed the potential change was uniform and points of inflection were obtained in agreement with theory. Furthermore when sodium butylate was substituted for potassium hydroxide, haphazard potential readings were
obtained and no weak acid inflection point resulted. Therefore, for routine work, the solvent (n-butanol) should contain approximately 1 per cent of water by weight-an amount insufficient to interfere with the oil sample solubility. VACUUM-TUBE POTENTIOMETER. The potentiometer circuit (Figure 2) employs two tubes ( F. P. 54) in a modification of an arrangement suggested by the General Electric Company (5, 7). The changes are directed toward ease of operation and making the circuit suitable for potential measurements. An external galvanometer (sensitivity 4 X 10-10 ampere) was used as a null point detector and resulted in a precision of 1 per cent of the measured voltage with a current in the cell of less than 10-16 ampere. SOLVENT. The following routine was adopted as effective in purifying the solvent suitable for electrometric titrations: Separate distillations of commercial n-butanol from potassium hydroxide, alpha-naphthol, m-phenylenediamine hydrochloride, and freshly heated lime were made, in the last case taking the fraction boiling at 117" * 0.5" C., 760 mm. The resulting solvent was aldehyde- and peroxide-free but contained 0.5 t o 1.0 per cent of water and possibly a small amount of weak acid (of. paragraph on acidity). The water content was measured synthetically employing the data of Perrakis (9). The distillate was stored in a darkened container in a nitrogen atmosphere. The nitrogen was purified b passage over reduced copper oxide, dehydrite, and ascarite. 111 connections in the still and receiver were of ground glass. ACIDITYIN SOLVENT.It was considered impractical to attempt to remove the last traces of acid in the solvent. Furthermore, carbon dioxide and other acidic gases in the atmosphere quickly dissolved in the alcohol during the interval between the removal from the storage container and the actual titrations. To reduce this variable amount of titer to a minimum, the solvent was purged with purified nitrogen for a 30-minute period (Table I). The residual acidity of different solvent batches varied from 0.05 to 0.15 cc. of 0.01 M alkali. A measured amount of stearic acid (0.005 M in toluene) was added as a blank after the gas stream had been diverted in such a manner that it did not bubble through the liquid. The titer representing the acid removed
JULY 15. 1936
ANALYTICAL EDITION
from the solvent by the nitrogen was surprisingly large1.00 to 1.2 cc. of 0.01 M alkali per 100 p. of n-butanol which had been distilled without protection from air contamination. The magnetic stirrer was operated during the entire gassing period. ALKALI. The most stable solvent for the alkali was anhydrous methanol prepared by distillation from sodium. Potassium hydroxide was dissolved in the alcohol, forming a concentrated solution. Separation of the insoluble carbonate was effected by centrifuging and an aliquot part was pipetted and added to the alcohol which had been distilled into the alkali storage reservoir. The resulting alcoholic potassium hydroxide was 0.01 M , as measured by titration of strong (hydrochloric, picric) or weak (benzoic, stearic) acids. I n other words, the weak acid salt content ( I C ) of the alkali was too small to be detected and remained so for several months (Figure 3). %-Butanolcontaining potassium hydroxide was found to be more susceptible to oxidation and light action, although it was satisfactory for use in oil acidity titrations.
Experimental Results The general applicability of the present experimental setup in its use for oil acidity determinations is discussed and illustrated below. VOLATILE ACIDITYAND STANDARDIZATION. It is possible to measure volatile as well as nonvolatile acidity. The problem of volatility of acids becomes particularly important where small amounts of acids are being determined. The use of an inert gas for agitation in conjunction with any electrode pair is open to serious criticism unless some precaution is taken to eliminate the loss of volatile acid. Furthermore, the solvent becomes quickly contaminated with volatile acid upon exposure to the atmosphere either before or during the titration period. The removal of the volatile acid is illustrated in Table I, where 100 cc. of n-butanol were employed as a solvent for 1 cc. of 0.005 stearic acid in toluene. TABLEI. REMOVAL O F VOLATILE ACIDITYFROM n-BnTAxoL BY MEANSOF NITROGEN (Gas flow approximately 1 0.01 M KOH Equivalent t o Residual Acid in Solvent
cc .
0.14 0.10 0.06
00.
per second) Time of Gassing
Min. 20 30
60
As the volatile acid was more completely removed by the longer gassing period the point of inflection of the stearic acid became more distinct. The oil sample should be obtained under conditions which prevent atmospheric contamination whenever an inert atmosphere is employed to protect the oil during its life as a dielectric. The alkali standardization curves are given in Figure 3.
289
FIGURE 3. STANDARDIZATION OF 0.0097 M ALKALI A. Titration of benzoic acid 5.4 m g . Titer, 4.55 cc. E . 1 cc. of 0.0052 M stearic Acid and 2 cc. of 0.0053 M hydrochloric acid. Titers, 1.11 and 1.74 cc. C. 1 cc. of 0.0052 M stearic acid. Titer, 0.63 co. D . Addition of 1 cc. of 0.0052 M stearic acid to excess alkali resulting from C. Titer, 1.18 cc.
ALKALITO ACID TITRATIONS. The electrode system enables one to proceed from acid to alkali or in the reverse direction. It is thus suitable for saponification experiments. In Figure 4 curve A shows the titration of 0.9 cc. of 0.01 M stearic acid in 100 GC. of n-butanol, free of volatile acids but containing the equivalent of 0.15 cc. of 0.01 M alkali residual acidity. When 2.2 cc. of alkali had been added, the solution was back-titrated with 0.01 M stearic acid in methanol, which is shown in curve B. Curve C illustrates the titration of excess acidity resulting from curve B. Curve C differs from curve A in that the rapid change in potential upon the first addition of alkali is absent. This phenomenon is normally observed in the titration of weak acids in aqueous solutions. The agreement between acid and alkali is satisfactory considering that 0.10-cc. increments were added. It is obvious that for the saponification test an excess of stearic acid may be added to the residual alkali and titrated in the usual manner. Apparently, the drift in potential a t the equivalent point is small as indicated by the relation between acid and alkali independent of the direction of neutralization. PEROXIDES. The use of quinhydrone with oils that contain peroxides should be questioned. When using the glass electrode no quinhydrone and conducting salt are necessary 10,11, l a ) . However, peroxides in the titration medium present an unavoidable difficulty, in that they are attacked
D:
Curve A shows the titration of Bureau of Standards benzoic acid, alkali 0.0097 M. The molarity of the alkali employing recrystallizedpicric acid was 0.0096. Curve B represents the titration of a mixture of 2 cc. of 0.0053 M hydrochloric in methanol and 1 cc. of 0.0052 M stearic acid in toluene. The difference in titers is 0.63 cc. which is the titer obtained with 1 cc. of 0.0052 M stearic acid alone (curve C). The addition of 1 cc. of 0.0052 M stearic acid to the excess alkali from curve C followed by titration gave 0.55 cc. as the titer of the 1-cc. portion of stearic acid. It follows that the residual acidity of the solvent was 0.08 1 cc. of 0.0 M alkali. The stearic acid, which probably contained some palmitic acid, was not used as a standard, but merely to prove the absence of weak acid salt in the alkali and to determine the residual acidity in the solvent.
50-
z 4
Or 50
T
"
t 0.5
1.0 CC.
OF ALKALI
1.6
,
2.0
FIGURE4. ALKALITO ACIDTITRATION A . Titration of 0.9 cr. of 0.01 stearic acid. Blank, 0.15 cc. 0.95 cc of alkali = 0.90 cc. of arid B. Back-titration of excess alkali f r o m , d with 0.01 M stearic acid. 1.10 cc. of alkali = 1.15 cc. of acid C. Titration of excess stearic acid. 1.15 cc. of alkali = 1.10 cc. of acid
INDUSTRIAL AND ENGINEERING CHEMISTRY
290
50
0
so -
6
OO
lence in curve A, Figure 5, immediately above the peak in curve B-a point on curve A which would never have been taken as the end point. Obviously, the alkali was combining with substances which were not present as acids in the original oil sample. Until the exact relation between alkali and the easily hydrolyzable substances in the oil sample is known, the difficulty will remain unsolved ACIDSTRENGTH.One can distinguish between strong and weak acids in the oil sample-a result which in general can be obtained with any electrode pair. Curve C, Figure 5, represents the formation of strong acid in cable oil in the presence of oxygen and finely divided copper. The second point of inflection may represent a moderately strong acid but most probably the hydrolysis of a copper salt of a strong acid. Curve D, Figure 5, shows the formation of acids in a duplicate sample of cable oil in the absence of copper, but under the same oxygen pressure. COPPERSOAPS. The use of quinhydrone in the presence of aqueous copper sulfate solutions has been investigated by O’Sullivan (8). He reported the slow formation of a precipitate when quinhydrone was used in a neutral copper sulfate solution, bringing about a gradual drift in potential. The use of the glass electrode eliminated this possible interference when titrating insulating oils, which frequently contained copper colloidally dispersed in the elementary state or combined as soaps. However, the estimation of acids in electrical oils was unavoidably complicated by the fact that certain soaps hydrolyze so easily that they behave as acids when titrated potentiometrically. Thus, a mixture of copper stearate recrystallized from n-butanol and stearic acid (curve A, Figure 6) gave but one point of inflection and a titer equivalent to the sum of the two constituents was obtained. Copper cyclohexane carboxylate recrystallized from n-butanol (curve B, Figure 6) behaved in a similar manner. The potentiometric determination of soaps is the subject of a paper which will be submitted in the near future.
d-1, 30
20
10 I
I
BOOC
0.5
10 CC.
O F ALKALI
1.5
ZO
I
FIGURE 5. EFFECTOF PEROXIDES ON TITRATION CURVE A . 15 grams of cable oil, peroxide No. 45, after removal of volatile compounds by means of nitrogen
B. Titration of excess alkali from A after addition of 1 CC. of 0.01 M C.
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stearic acid
9 grams of cable, oil, peroxide No. 0.
Soluble copper, 7 p. p. m. Volatile constituents not removed D. 18 prams of cable oil, peroxide No. 1.5. Volatile constituents absent
by the alkali in the region of the weak acid point of inflection. This effectbrings about a flattening in the region of inflection and thus may render the end point obscure. Figure 5, curve A, represents the titration of a commonly used highvoltage cable oil with a peroxide number of 45 after exposure for 48 hours to ultraviolet light. Addition of stearic acid at the end of the titration, followed by a second titration, gave a somewhat more distinct point of inflection (Figure 5, curve B ) . The oil sample in Figure 5, curve D, had a peroxide number of 1.5. The peroxide number of the oil was obtained by the method of Yule and Wilson (15) and is defined in the authors’ work as the grams of active oxygen per thousand kilograms of oil. In general an oil with a high peroxide number presented an obscure end point upon titration] independent of its volatile acid content. There appeared to be no stoichiometrical relation between the peroxide number of the oil and the magnitude of the flattened portion of the titration curve. When benzoyl peroxide was added to the titration medium containing the stearic acid, it was impossible to obtain a point of inflection. I n the region of the end point, after addition of alkali, the potential drift would reverse after its normal change due to the alkali and there would follow a slow drift in potential in the acid direction. It must not be assumed that peroxides were the sole contributing cause to the difficulty discussed in the previous paragraph. Any functional group which reacts with alkali would bring about the same effect. In curve C, Figure 5, the oil sample contained 7 p. p. m. of soluble copper but possessed a peroxide number of approximately zero. The manner in which certain copper soaps interfere with oil acidity determinations is discussed below. The question naturally arises as to just where the point of equivalence between acid and alkali should be taken when interfering substances are present. The cell potential is of some aid, but even if an arbitrary potential were chosen the dficulty could not be overcome. Consideration of the relation between acid and alkali would call for a point of equiva-
0 50
u
0 0 0 L w
> so I
a I
IO
20 CC. OF A L K A L I
30
FIGURE 6. EFFECTOF COPPERSOAPSON TITRATION CURVE A . Titration of. mixture of 3.2 mg. of copper stearate and 3 mg. of stearlc acid. Theory, 2.14 cc.; found, 2.25 cc.; blank, 0.15 cc. Titration of mixture of 3.35 mg. of co per cyclohexane carboxylate and 1 cc. of 0.005 Mstearic acid: Theory, 2.73 cc.; found, 2.75 cc.; blank, 0.05 cc.
B.
Conclusions
It is difficult to state definitely the precision with which acidity in oils may be determined with the procedure outlined in this paper. Oils from different sources obviously contain variable amounts of peroxides, metal soaps, and weak acids of different strengths-factors which tend to render the anticipated point of inflection for the oil sample indefinite. When interfering substances are absent-a case which practically reduces to the determination of two solvent blanksthe precision of the method is +0.02 cc. of 0.01 M alkali. For a 20-gram sample of oil, this amount of alkali is equivalent to a neutralization number of *0.001. Acknowledgment The authors wish to express their appreciation of the assistance of Ward F. Davidson, director of research, particu-
JULY 15, 1936
ANALYTICAL EDITION
lady for the design and construction of the vacuum-tube potentiometer.
Literature Cited Brown, A. S., J.Am. Chem. SOC.,53,645 (1931). Clarke. B. L.. Wooten. L. A.. and ComDton. _. - . K. C.. IND.ENG. CHEM.,Anal. Ed., 3,321 (1931). Dole, M., J . Am. Chem. Soc., 54, 3095 (1932). Evans, R. N., and Davenport, J. S., IND.ENG.C H m f . , Anal. Ed., 3, 82 (1931). Gen. Elec. Co., Pub. CET-249A, “Circuits for Amplification of Direct Currents Using the FP-54 Pictron,” February, 1932. MacInnes, D. A,, and Belcher, D., J . Am. Chem. SOC.,53, 3315 (1931).
Moles, F. J., Gen. Elec. Rev., 36, 156 (1933).
291
(8) O’Sullivan, J. B., Trans. Faraday SOC.,23, 52 (1927). (9) Perrakia, N., Compt. rend., 177, 879 (1923). (10) Ralston, R. R., Fellows, C. H., and Wyatt, K. S., IND.ENO. CHEM.,Anal. Ed., 4, 109 (1932). (11) Seltz, H., and McKinney, IND, ENG.CHEM.,20, 542 (1928). (12) Seltz, H., and Silverman, L., Ibid., Anal. Ed., 2, 1 (1930). (13) Timmermans, J., and Martin, F. S., J. chim. phys., 25, 411 (1928).
(14)
Wooten, L. A,, and Ruehle, A. E., IND.ENG.CHEM.,Anal. Ed.,
6, 449 (1934). (15) Yule, J. A. G., and (1931).
Wilson, C. P., IND.ENG.CHEM.,23,
1254
RI~~CDIIWD April 17, 1936. Presented in part at the Pittsfield meeting of the Committee on Electrical Insulation, Division of Engineering and Industrial Research, National Researoh Council, October, 1935.
Energy Equivalents of Vitamin D Units ROBERT W. HAMAN AND HARRY STEENBOCK, University of Wisconsin, Madison, Wis.
T
HE widespread acceptance of irradiated food products and
irradiated ergosterol, as well as the general use of cod liver oil and its concentrates] has demanded the formulation of unitary expressions of vitamin D activity. I n this country the Steenbock unit, prior to the acceptance of the International unit by the U. S. Pharmacopceia, had received widespread use. The Steenbock unit was originally defined in terms of the amount of calcium deposited in a standard rachitic rat under standard feeding conditions in 10 days. A reference preparation of irradiated ergosterol of known potency expressed in such units was made available for use in other laboratories. At the time of the adoption of the International unit by the Permanent Commission on Biological Standardization of the League of Nations in 1931, the authors immediately initiated experiments comparing the Steenbock unit with the International unit. Their initial experiments were of the therapeutic type. Preliminary experiments necessitating certain aomparisons revealed provisionally that 1 Steenbock unit was equivalent approximately t o 2.7 International units. This factor was sufficiently accurate for the purpose at that time (7). Unfortunately, it was accepted by others as the final conversion factor for purposes requiring a far greater degree of accuracy than demanded by the authors’ particular experiments. A limited number of quantitative studies relevant to the amount of energy required t o synthesize vitamin D from ergosterol have been reported (2, 4, 5 ) , and their results are generally concordant in showing that within certain limits there exists a definite relation between the amount of radiant energy absorbed and the amount of vitamin D synthesized. The authors have now used a similar technic to determine the amount of radiant energy required to synthesize one International unit of vitamin D in comparison with the Steenbock unit, using ergosterol as the substrate.
abled the authors to move either compartment into the path of the monochromatic radiations incident to the thermopile slit, thereby eliminating reflection and absorption by the quartz and the solvent as factors in the quantitative evaluations. As a source of light the authors used a capillary quartz mercury arc similar to one described by Daniels and Heidt (1). The degree of resolution of the spectrum from this arc as well as the energy values of different lines is shown in Figure 1. It is evident that the dis ersion was sufficient for the purpose. their first quantitative evaluations of the Steenbock unit the authors irradiated ergosterol in absolute alcohol solution with the 303 p mercury line. (The ergosterol was obtained from Chaa. Pfizer and Co. It had a melting point of 158” C., uncorrected, a rotation of CY)^ = -134.3‘ in chloroform, and an extinction coefficient of 11,000 at 282 mm.) Five cubic centimeters of a 0.1 per cent solution were placed in one compartment of the cell and 5 cc. of absolute alcohol in the other. The cell was sealed with a glass plate and placed on the rack in front of the thermopile slit. Readings of the galvanometer deflections for the determination of the energy transmitted by the absolute alcohol were made before and after the irradiation period for these short exposures. Since the radiant energy incident to the ergosterol solution waa all absorbed, this measurement represented the radiant energy absorbed. The am erage and voltage of the arc were always checked. Each soktion after proper exposure to measured amounts of radiant energy, ranging from 500 t o 5000 ergs, was fed in oil to rats using the 10-day line test technic with ration 2965 as the rachitogenic diet.
81
Earlier results indicated that healing comparable t o that produced by one Steenbock unit was obtained when 3000
Experimental Known amounts of radiant energy of known wave lengths were obtained with the use of a Bausch and Lomb quartz monochromator equipped with a Coblena linear thermopile (12 copper-Constantan junctions) in conjunction with a Leeds & Northrup galvanometer (sensitivity, 10.4 mm. per mv.). The thermopile-galvanometer system was standardized with a carbon filament lamp obtained from the U. S. Bureau of Standards. The instrument was adapted for the purpose by the insertion of a quartz lens with a focal length of 5 cm. between the exit slit and the thermopile. This made it possible to place in front of the thermopile slit a cell, 2.5 cm. wide and 1.5 cm. thick, consisting of two compartments constructed of ground and polished plates of quartz. A rack provided for this cell en-
d
FIGURE 1
