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-
F
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 a t the 69th Meeting of the American Chemical Society, Baltimore, Md., April 6 to 10, 1925.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
640
A. H. Taylor.2 The various paints were applied to glass panels and the light transmission factor of the paints on these panels was measured by Taylor’s method. A commercial white paint of the following composition was used: . White lead.. ................................. Zinc oxide ................................... Silica and silicates. ........................... Linseed oil.. ................................. Japan drier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Per cent 35 22 6 34 3
of A d d i t i o n s of A l u m i n i u m Powder u p o n L i g h t T r a n s m i s s i o n Factor of W h i t e Paint -TRANSMISSION FACTOR-? -1st Series-2nd Serie1 Coat 2 Coats 1 Coat 2 Coats Per cent Per cent Per cent Per cent White paint 26.2 16.3 29.6 16.6 White paint 2.5% aluminiurn powder 11.2 4.5 8.7 1.0 White paint 5 % aluminiurn powder 6.9 1.4 4.8 0.4 White paint 10% aluminiurn powder ... 1.5
