Determination of Water in Insulating Oil Manometric Procedure R. N. EVANS' AND J. E. DAVENPORT Research Bureau, The Consolida'ted Edison Co. of New York, Inc., Brooklyn, N. Y.
I
was readily found by approaching the final state from either direction. However, in region OABC, the same pressure reading could not be obtained in approaching the equilibrium value from either direction. When at the start all the water was condensed on the lithium chloride, a low pressure was found and the reverse was true if the water diffused from the vapor space to the lithium chloride. For these reasons, the vapor pressure of the saturated hydrate (region C D ) was adopted as the reference pressure. In preparation for an analysis the entire apparatus, of which a diagrammatic sketch IS shown in Figure 2, was evacuated for 5 minutes with all stopcocks open except B and G, while heating lithium chloride, L, to approximately 300" C. with heater W . Stopcocks D and K were closed, the heater was removed, and the dry ice bath, X , was raised about trap J. Stopcock B was opened to admit air dried by desiccant in A through capillary Y , and 20 ml. of insulating oil were introduced by hypodermic syringe through serum rubber stopper H and stopcock G into degassing chamber F . Pumping was continued for 5 minutes while the dry air entering the degassing chamber through the porous disk, E, carried the water into the cold trap, J . By actual experiment] the volume of air passing through the disk was found to be 3 ml. at one atmosphere. This volume, however, corresponds t o approximately 3 liters at the reduced pressure. Stopcock I was
T WAS shown in studies on the electric hygrometer (1) that because of an indeterminate hysteresis effect the hygrometer unit could not be used to determine the water content of insulating oil. However, during the course of these studies, a constant-volume or manometric procedure was devised which shows great promise as a rapid and accurate procedure for the determination of small quantities of water in oil. The method consists of two pressure readings-namely, the initial pressure of the gases removed by evacuation from the oil sample and a final pressure reading after exposure of the evolved gases t o a film of lithium chloride monohydrate. Adsorption of the organic oil vapors is reduced to a minimum because of the small quantity of lithium chloride monohydrate employed (approximately 1 mg. in an almost invisible film) and equilibrium between the hydrate and water vapor is attained very rapidly. The manner in which the unknown amount of water vapor is accommodated b y a &xed weight of hydrate will be clear from the description of the apparatus. The theoretical relation between the vapor pressure of water in contact with 0.5 mg. of anhydrous lithium chloride (3) is shown in Figure 1. In region OA, water vapor can exist in the presence of anhydrous lithium chloride and the vapor will obey Boyle's law. At A , at a pressure of 0.38 mm. and a temperature of 30" C., lithium chloride monohydrate will be formed and further addition of water will result in no increase in pressure, providing the two solid phases, lithium chloride anhydrous and lithium chloride monohydrate, are present. At B, all the anhydrous lithium chloride has disappeared and the pressure will now increase t o that of a saturated solution of lithium chloride hydrate, during which addition of water to the system none will be taken up by the lithium chloride hydrate. At C the saturated solution appears and a constant pressure results on still further addition of water until the solid phase disappears at D. In region DE the solution becomes more dilute with corresponding increase in vapor pressure. Experimentally, the pressure along CD could be demonstrated, but the length of C D was found to be shorter than theoretically demanded by the solubility data. The explanation of this effect could not be found unless the Solubility of the hydrate is greater in the film than in bulk. For this reason, region C D must be obtained experimentally for any selected quantity of lithium chloride. The equilibrium value of the pressure listed in (5)
500 1000 1500 MICROGRAMS OF WATER ABSORBED ON L l C L .
FIGURE 1. THEORETICAL RELATION BETWEEK WATERVAPOR PRESSURE AKD WATERCOMBINED WITH 500 MICROGRAMS OF LITHIUJf CHLORIDE
Present address, Westinghouse Electric & Manufacturing Co., E a s t Pittsburgh, Penna. 1
TABLEI. CONP-4RIsOX O F RESULTSBETWEEN MAXOMETRIC A S D COMBUSTION PROCEDURE Sample No. 471 471
Sample Manometric Procedure Grams Micrograms P. p . m . 15 340 23 15 360 23
Combustion Procedure P. p . m. 30
Sample No. 478
615
102
1.5 15
5QO
39 40
43
595
30 30
695 705
23 23
22
6 6
800 785
133 132
146
475 475
18 15
640 490
35 33
33
476 476
481 481
12 6
775 415
62 62
62
472 472
480
9
515
57
56
Std. Ytd.
132
l l a n o m e t r i c Procedure Micrograms P. p . m.
Combustion Procedure P. p. m. 100
Sample Grams 6
ANALYTICAL EDITION
September 15, 1942
733
A . Dehydrite drying tube B , D, G , I, K ,.V. Two-way stopcocks C . Waste oil flask E . Porous disk F . Degassing chamber H . Serum rubber stopper J. D r y ice t r a p L. Lithium chloride chamber M. Xlanometer 0, Q, S , C . Three-way stopcocks P , R , T, r'. Auxiliary flasks W . Heater X . D r y ice b a t h Y. Capillary Z. Nichrome heater
FIGURE2.
DIAGRAMMATIC SKETCH
then closed and pumping continued until the manometer reading was zero-about one minute. Stopcock N was closed, also the three-way cocks 0, Q, S , and U by turning to the position shown a t S , the cold bath, X,was lowered and trap J was warmed to 50" C. with a water bath for one minute, If the vapor pressure indicated on the manometer, M , was greater than 12 mm. of mercury it was reduced by allowing the water vapor to expand into the added volume of auxiliary flask P by turning stopcock 0 to the position shown a t Q. If this volume was insufficient, flask R could be included in the system by turning stopcocks 0 and Q to the positions shown. When necessary, flasks T and 6' may be added to the system. The water bath was removed and after allowing 5 minutes for the trap to reach room temperature the manometer reading and the gas volume were recorded. Stopcock 0 was closed by turning to the position shown a t S and stopcock K was opened. After allowing 5 minutes for the water vapor to reach equilibrium with the lithium chloride in L, the manometer reading was again recorded together with the temperature. I n preparation for a succeeding analysis the oil in F was forced into the evacuated flask, C, through stopcock D. The water content of the oil was then simply calculated from the gas law, employing the water pressure P H ~ obtained O by combining the initial, final, and lithium chloride monohydrate saturated solution pressure readings. In equation form, PH,O may be derived as shown below, where Pail signifies gases other than water vapor. =
Pinitid
Poi1
=
P initial
In Table I, experimental results are given on samples of synthetic transformer oils of the chlorinated aromatic type, which had been withdrawn from service. The corresponding water content as obtained b y the combustion procedure (8) is also included. Duplication of results was readily obtained with the manometric procedure with the same or different weight of sample. It is gratifying t o note the good agreement of the results from the two independent methods. The manometric procedure appears t o be capable of the greater precision. The apparatus has been so designed that water may be determined in concentrations ranging from 8 to 170 parts per million, using a 30-gram sample. Few samples of oil will be found which exceed the maximum concentration and b y increasing the mTeight, samples in the lower range may be readily accommodated. I n the field, b y replacing the mercury b y a high-grade vacuum pump oil as the manometer fluid, a factor of approximately 16 is obtained in the displacement of the fluid. Thus, the pressure readings may be made with a metric rule with sufficient accuracy for most purposes. A complete analysis may easily be carried out in 30 minutes.
PLiC1.HzO
(2)
- PLiCLHzO
(3)
Acknowledgment
(4)
The authors wish to express their appreciation t o W. F. Davidson, director of research, for his interest and advice in this work.
Substituting 3 in 1 PH20
mately 1 mg. and it would accommodate between 250 and 950 micrograms of water-i. e., the length of CD in Figure 1. It was introduced as 2 drops of a 1 per cent solution of the salt in 95 per cent ethanol from a pipet which delivered 20 drops per ml.
(1)
+
= Pfinal
APPARATUS
+
P H ~ O Pail
Pfmd = PoiI
OF
- Pfinal
+
PLiCl.HZ0
All pressure readings were taken with a cathetometer capable of reading to 0.1 mm. The choice of the magnitude of the fixed volumes V , T,R, and P as well as the quantity of lithium chloride will depend on the nature of the liauid to be analyzed. Insulating oil ranges in water Concentration from o to 150p. p. m. except in rare-cases where emulsions are encountered. The volumes selected in Figure 2 were V = 250ml.; T = 125ml.: R = 70ml.; P = 30ml.; manifold = 10 ml. The weight of lithium chloride was approxi-
Literature Cited (1) Evans, R. N., and Davenport, J. E., IND.ENG.CHEM.,ANAL. ED., 14, 507 (1942). (2) Evans, R. N., Davenport, J. E., and Revukas, A. J., Ibid., 13, 589 (1941). (3) International Critical Tables, Vol. 3, p. 368; Vol. 7, p. 301.
