June. 1925
INDUSTRIAL A N D ENGINEERING CHEMISTRY
637
New Calomel Half Cells for Industrial Hydrogen-Ion Measurements’ By Henry C. Parker and Carl A. Dannerth THELEEDS& NORTHRUP Co., PHILADRLPHIA, PA.
URING the application of hydrogen-ion apparatus to industrial problems several new types of calomel half cells have been developed which appear to offer desirable features even in laboratory hydrogen-ion measurements.
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Improved Laboratory Type Calomel Half Cell
Figure 1 shows a cell designed primarily for laboratory work. A potassium chloride reservoir is fastened to the cell by a rubber tube a t the point A . -4pinch clamp is used on this rubber tube at some distance from the cell. The groundglass stopper, E , has an enlargement or reservoir which may be used, if desired, to presaturate the potassium chloride solution with calomel. The calomel may be introduced into the reservoir and prevented from entering the lower vessel by a small plug of glass wool. The stopper has an inner seal supporting the tube H and has a hole, G, bored in the ground glass to make liquid connection to the side1tube which dips into a beaker a t K . Electrical connection to the cell is made through the mercury in the side tube, D. This tube is supported by a brace, C, which is an importantyfactor in reducing breakage. Breaks in this tube usually occur just a b o v e C, where they m a y be r e a d i l y reFigure 1-Improved Laboratory Type Calomel Half Cell paired, rather than at the platinum seal where the cell would be incapacitated. The most important new feature, however, consists in the mercury seal a t B. This seal, by removing the necessity for stopcock grease on the ground-glass joint, prevents a possible source of contamination. It also prevents creeping or evaporation of the potassium chloride solution and is the principal feature which prevents contamination from the outside. The ground-glass joint cannot dry out and stick with the mercury seal above and this removes a frequent source of breakage. With the top sealed free from contamination, the only other source is through the tube K . The cell is designed so that this contamination occurs a t the top instead of close to the mercury surface, and fresh potassium chloride solution will easily flush out the upper part of the cell where this contamination is supposed to enter. The enlargement, J , likewise prevents $he mercury from accidentally coming in contact with the upper part of the cell even though the cell were to be laid on its side. The tube from G to K is designed to reduce the internal resistance of the cell to a minimum. The total length of this Presented before the Division of Indus1 Received November 28, 1924. trial and Engineering Chemistry at the 69th Meeting of the American Cbemical Society, Baltimore, Md., April 6 to IO, 1925
side tube is reduced by having only two bends present and it is considerably shortened by coming from the top of the cell rather than from the middle section. Coming from the top it likewise reduces the danger of accidentally draining the potassium chloride out of the cell when the latter is standing idle. It is a common practice to have the constriction a t K continue around the bend a t this point, whereas the only constricted portion necessary is a t the extreme end and a considerable saving of resistance is effected by having the tube enlarged just beyond the constriction. The construction shown has reduced the internal resistance to about half that in cells of a similar type. Measurements may be easily taken with the stopper E twisted so as to shut 08the opening, G. Measurements made in this manner prevent both contamination and diffusion. When in use, the pinch clamp on the rubber tube connecting to the potassium chloride reservoir is closed and the stopper, E, is turned so that the drop of red glass, F , is pointed towards the side tube, K . When out of use the stopper E may be turned off and the pinch clamp also closed with about 2 feet of rubber tubing between the clamp and the cell. When left in this condition the rubber tube takes care of the thermal expansion and the cell may be left indefinitely without contamination. A desirable feature is that air may be completely expelled from the cell and the possibility of oxidizing the calomel is thus reduced. The cell can also be used in a water bath since it can be immersed somewhat above the brace, C. Industrial Flow Type or Laboratory Type Calomel Half Cell
Figure 2 shows an industrial type cell suitable for recording the pH of a flowing liquid. The liquid to be measured flows in a t M and out a t K . The electrodes a t L are a special type of tungstenmanganese sesquioxide electrode which will be described in a later article. They require a supply of neither hydrogen nor air and a p pear to be especially free from the so-called poisons which affect the c o r r e s p o n d i n g electrodes. The calomel cell, C, was designed esFigure 2-Industrial Flow Type pecially for removing Calomel Cell the necessity of a potassium chloride reservoir and the bother of a continuously flowing potassium chloride solution. The design is especially suited for a “saturated” calomel
638
INDCSTRIAL A N D ENGINEERING CHEMISTRY
cell, having the solution saturated with both calomel and potassium chloride. Electrical connection to the cell is made through B. Liquid connection to the tungsten electrode is made around the ground-glass plug, G, and through the dense porous cup, F. The glass rod, A , is in such positiofi as to just touch G when the rubber stopper a t the top of the cell is fitted tightly. This prevents the ground-glass plug from being raised accidentally, when the porous cup is being fastened on'the rubber stopper, E , or by striking against the crystals of potassium chloride. Around the calomel cell is placed a section of large rubber tubing, D. The cell is rested on this rubber tubing in the top of the larger vessel. The calomel cell can thus be readily removed for adding potassium chloride crystals to the cup or flushing with fresh solution. The latter operation is most easily accomplished by removing the porous cup, loosening the rubber stopper, pushing up the ground-glass plug until about 10 cc. of solution has been run out, and then refilling with fresh solution. When in service the cell requires flushing about once a week and the addition of extra crystals of potassium chloride to the cup may be made a t the same time. The calomel cell as shown in the figure is thus capable of running for a full week or longer without any attention whatever. This removes one of the greatest difficulties in applying electrometric methods to the frein d u s t r y-namely, quent necessity of expert attention. Figure 3-Combination Calo m e l Cell and Hydrogen ElecThe calomel half cell, when trode removed from the flow tvDe vessel, makes an extremely convenient electrode for 21around use in the laboratory. It is essentially a calomel cell with a salt bridge attached-the porous cup being the salt bridge. When being transferred from one solution to another the porous cup may be rinsed off with distilled water, and there is but slight danger of contaminating one solution with the other, since the cup is so dense that diffusion through it is extremely slow. It possesses all the advantages of a metallic electrode as far as convenience and mobility are concerned. When used in this manner a rubber tube may be fitted between the upper part of the porous cup and the rubber stopper a t E , in order to prevent accidentally knocking off the cup. The adherence between the cup and stopper is sufficiently great for ordinary purposes. The internal resistance of the calomel cell is comparatively low in spite of the ground-glass stopper and the porous cup. A resistance of 5700 D was obtained between the electrode and the calomel cell when potable water was flowing through the vessel. W i t b a buffer solution in place of the potable water a resistance of 5000D was obtained. A convenient method of lowering the resistance was found when a very fine thread or hair was slipped between the ground-glass surfaces and the plug was pulled down tightly upon the obstruction. It was found that the resistance could be reduced to about
VoI. 17, No. 6
2000 D by this method. It would be necessary to flush out the cell somewhat more frequently if used in this manner. The use of the porous cup, containing a saturated solution with extra crystals added, appears to offer the only method of obtaining a cell which will require attention only a t infrequent intervals. Several different types of cells were tried before use was made of the porous cup, but none of them proved successful. Among the most promising was a cell similar to that of Figure 2, without the porous cup. When potable water was being measured in this cell it was found that the potassium chloride solution would gradually diffuse out of the ground-glass section causing an enormous increase in the internal resistance of the cell, while if the rubber stopper at the top was loose the potassium chloride solution would soon flow out. The use of the porous cup, on the other hand, insures the presence of a solution saturated with potassium chloride on both sides of the ground-glass section. The large area of the porous cup reduces the resistance offered to the passage of current while its density insures very slight diffusion. The ground-glass plug effectually prevents diffusion of the solution in the porous cup up into the cell, while the tendency for this diffusion is reduced by both solutions being saturated with potassium chloride. The chance of any contamination reaching the mercury in the calomel cell is slight indeed. On account of using saturated potassium chloride solution a t the liquid junction, it was expected that any liquid junction potential would be small. However, it appeared possible that a capillary potential might be present. In an investigation which diil be reported later it was found, not only that such an effect was not preFent, but also that m o r e uniform and c o n s i s t e n t results were obtained by using the porous cup than with the usual type of salt bridge. When used in a flowing solution the porous cup gives an effect very similar to that of a flowing junction. The saturated solution gradually diffuses through the cup, but its supply is maintained by the excess crystals. When the cell is to be placed out in the plant, in industrial use, it is protected by a wooden c a s i n g which surrounds the whole. The solution to be measured is Ty e Calomel Cell usually fed from a Figure 4-Industrial a n d Tungsten Efictrode bleeder into the bottom of the cell, although it may be pumped from a tank or sump or drawn through the cell by means of an aspirator. Combination Hydrogen Electrode and Calomel Half Cell
In Figure 3 is shown a combined calomel cell and hydrogen electrode which has been found very convenient for routine or industrial hydrogen-ion determinations. The calomel electrode is similar in principle to that employed with the
June, 1925
INDUSTRIAL AiVD ENGINEERING CHEMISTRY
flow type cell, having a porous cup a t F, a ground-glass plug at E , etc. Electrical connection is made through the tube D. The hydrogen electrode, A , is connected with inner seals through the top and bottom of the calomel cell. Hydrogen enters the side tube and flows around the electrode and out through the holes at the bottom. The platinum electrodes used are readily made, easily replaceable, and cheap. They consist of a piece of No. 18 platinum wire about 6/g inch long sealed into the bottom of the tube C. This type of electrode is quite commonly used a t present in routine measurements and has the additional advantage of coming to equilibrium much more rapidly than the usual Hildebrand type.2 When the body of the cell is protected by a section of rubber tubing it may be supported by a clamp or dipped directly into a beaker with small danger of breakage. I t s convenience in handling, the absence of a potassium chloride reservoir, and its readily replaceable and cheap electrodes make this cell ideal for routine or rough measurement. For industrial uses, where i t is desired to measure the p H of a liquid in a large tank, the cell has been fitted with a 1/4 inch thick rubber tube which extends its entire length, being fitted a t the top with a Bakelite adapter to a hose which contains the electrode leads and a rubber tube for the hydrogen. I n this case the porous cup is fastened on securely by the addition of a rubber tube extending between the top of the cup and the rubber stopper which holds it on. It is evident, of course, that the hydrogen electrode may be replaced by a tungsten electrode. A Rugged Industrial Half Cell
Figure 4 shows a cell which is adapted primarily for industrial work. It is a more rugged and foolproof appliance than the cells previously described. It is entirely surrounded by a Bakelite casing, the upper part of which serves as a protection while the lower part serves as a baffle to protect the electrode and cell when placed, for example, in a n open flume. The internal structure of the calomel cell is of glass and is somewhat similar to that of the flow type cell, the principal difference being a large inner tube at b and the substitution of a porous disk, c, for the ground-glass section in the flow type cell. Electrical connection is made at h. These changes are necessary in order to reduce the internal resistance of the cell, which is increased by its greater over-all length. The introduction of the porous disk permits the addition of a few crystals of potassium chloride at this point, which acts a s a ‘urther precaution against contamination. Th’e porous cup, d, is held on the glass section by a rubber tube. The cup in this cell can be well filled with potassium chloride crystals, and hence the cell requires even less attention than the flow type cell. For flushing out, the whole cell is reaoved by unscrewing it at f. The porous cup is removed fror L the lower end and the rubber stopper at a is loosened unt 1 sufficient solution has run out through the porous disk. Additional solution is then added and the porous CUI is refilled with potassium chloride crystals. ‘Yhen a tungsten electrode is used a t e, we have a combinaticn which requires no supply of gas and no potassium chloride re iervoir and dropping device. The attention which the apparatus requires is usually conditioned by the frequency with which the tungsten electrode needs cleaning or replacement. The frequent expert attention which has been required in other types of calomel cells, when applied to industrial uses, has proved to be one of the great hindrances to a more universal application of these measurements. It is believed that the cells described in this article pave the way to a greater expansion in such applications. Wilson and Kern have recently described a hydrogen electrode similar to this one, THIS J O U R N A L , 11, 74 (1926). 2
639
Some Special Properties of ‘‘Aluminum Paint’ ”
By Junius D. Edwards and Robert I. Wray ALUMINUMCOMPANY OF AMERICA, NEW KENSINGTON, PA.
ROM many standpoints the most interesting proper-
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ties of aluminium powder and aluminium paint are the optical properties. Of the optical properties high reflectivity is the most obvious, but high opacity to light is one of the most useful. Aluminium bronze powder is composed of very small flakes of metallic aluminium, formed by a process of stamping. Therefore, one might expect it to resemble metallic aluminium in being practically opaque. The marked durability of aluminium paint can largely be ascribed to the high opacity of the aluminium powder pigment. That the light transmission factor of a good aluminium paint is practically zero can be ascertained by applying a coat to glass and then examining it in front of an incandescent lamp. Except for possible defects in the film caused by brush marks, the filament will be completely obscured. The metallic flakes of aluminium “leaf” in the vehicle to form a practically continuous and opaque film of metallic aluminium. The very highest opacity in a single coat is only obtained when the powder leafs freely. The so-called leafing is a surface tension phenomenon which brings part of the powder into the surface film and arranges the flat, platelike particles of aluminium parallel to the surface. When the aluminium bronze powder remains in contact with most vehicles for an extended period, the leafing power is gradually destroyed. Sunlight is generally considered an important factor in the destruction of paints. It might be predicted, therefore, that adulterating aluminium powder with a material of high transparency, such as mica, would materially reduce the durability on exposure of paint made with it. Experiments showed this to be the case. Test panels of mild steel were painted with one and two coats of paint made with spar varnish and aluminium bronze powder in the proportion of 240 grams of powder per liter of vehicle (2 pounds per gallon). The powder used in three of the paints contained 10, 25, and 50 per cent, respectively, of ground mica. One panel was also painted with pure mica suspended in spar varnish. Comparatively large amounts of mica can be mixed with aluminium powder without changing its general appearance, as the mica particles themselves are rather bright and shiny. Upon exposure the 100 per cent mica panel showed signs of rusting after a relatively short period of exposure, and all the panels containing mica began to fail before the pure aluminium paint; the order of failure corresponded to the mica content. The addition of small amounts of aluminium powder to paints made with other pigments should materially increase their opacity and durability. White paints in particular experience the deteriorating effect of sunlight, and some of the writers’ first experiments were made with commercial white lead and zinc oxide paints to which had been added varying amounts of aluminium bronze powder. Tests of Light Transmission Factor
I n order to obtain some quantitative figures on the effect of aluminium bronze powder upon the light transmission of these paints, a series of measurements was made, using the reflectometer developed at the Bureau of Standards by Received March 2 7 , 1926. Presented before the Section of Paint and Varnish Chemistry at the 69th Meeting of the American Chemical Society, Baltimore, Md., April 6 to 10, 1925.
