Function of Different Components
Lime. The hydrated lime furnishes the backbone of the absorptive properties of the soda-lime. It constitutes over 50 per cent of the finished dry granule and is responsible in a chemical sense for practically all the gas absorption.
Cement. Cement furnishes a degree of hardness adequate to withstand service conditions. It interferes somewhat with the absorptive properties of the soda-lime and it is an open question whether the gain in hardness produced by its use is valuable enough to compensate for the decreased absorption which results.
Kieselguhr. The loss in absorptive capacity due to the presence of cement is in part counterbalanced by the simultaneous introduction of a relatively small weight though considerable bulk, of kieselguhr. In some cases, there seems to be a reaction between the lime and the kieselguhr, which results in some increase in hardness.
Sodium Hydroxide. Sodium hydroxide has two primary functions in the soda-lime granule. In the first place, a small amount serves to give the granule considerable more activity. The second function is to maintain roughly the proper moisture content. This water content (roughly 13-14 per cent after spraying) is very important, in order that the maximum gas absorption may be secured.
Sodium Permanganate. The function of the sodium permanganate is to oxidize certain gases, such as arsine,[30] and to act as an assurance of protection against possible new gases. The purity of the sodium permanganate solution used was found to be one of the most important factors in making stable soda-lime. It was, therefore, necessary to work out special methods for its manufacture. Two such methods were developed, and successfully put into operation.
Careful selection of other material is also necessary, and this phase of the work contributed greatly to the final development of the form of soda-lime.
CHAPTER XIV
TESTING ABSORBENTS AND GAS MASKS
One of the first necessities in the development of absorbents and gas masks was a method of testing them and comparing their deficiencies. While the ultimate test of the value of an absorbent, canister or facepiece is, of course, the actual man test of the complete mask, the time consumed in these tests is so great that more rapid tests were devised for the control of these factors and the man test used as a check of the purely mechanical methods.
Testing of Absorbents[31]
Absorbents should be tested for moisture, hardness, uniformity of sample and efficiency against various gases.
Moisture is simply determined by drying for two hours at 150°. The loss in weight is called moisture.
The hardness or resistance to abrasion is determined by shaking a 50-gram sample with steel ball bearings for 30 minutes on a Ro-tap shaking machine. The material is then screened and the hardness number is determined by multiplying the weight of absorbent remaining on the screen by two.
The efficiency of an absorbent against various gases depends upon a variety of factors. Because of this, it is necessary to select standard conditions for the test. These were chosen as follows:
The absorbent under test is filled into a sample tube of specified diameter (2 cm.) to a depth of 10 cm. by the standard method for filling tubes, and a standard concentration (usually 1,000 or 10,000 p.p.m. by volume) of the gas in air of definite (50 per cent) humidity is passed through the absorbent at a rate of 500 cc. per sq. cm. per min. The concentration of the entering gas is determined by analysis. The length of time is noted from the instant the gas-air mixture is started through the absorbent to the time the gas or some toxic or irritating reaction product of the gas begins to come through the absorbent, as determined by some qualitative test. Quantitative samples of the outflowing gas are then taken at known intervals and from the amount of gas found in the sample the per cent efficiency of the absorbent at the corresponding time is calculated.
| Per cent efficiency = | p.p.m. entering gas - p.p.m. effluent gas | × 100. |
| p.p.m. entering gas |
These efficiencies are plotted against the minutes elapsed from the beginning of the test to the middle of the sampling period corresponding to that efficiency point. A smooth curve is drawn through these points and the efficiency of the absorbent is reported as so many minutes to the 100, 99, 95, 90, 80, etc., per cent efficiency points.
The apparatus used in carrying out this test is shown in [Fig. 74]. Descriptive details may be found in the article by Fieldner in The Journal of Industrial and Engineering Chemistry for June, 1919. With modifications for high and low boiling materials, the apparatus is adapted to such a variety of gases as chlorine, phosgene, carbon dioxide, sulfur dioxide, hydrocyanic acid, benzyl bromide, chloropicrin, superpalite, etc.
As the quality of the charcoal increased, the so-called standard test required so long a period that an accelerated test was devised. In this the rate was increased to 1,000 cc. per minute, the relative humidity of the gas-air mixture was decreased to zero, and the concentration was about 7,000 p.p.m. The rate is obtained by using a tube with an internal diameter of 1.41 cm. instead of 2.0 cm.