98 PLANT PHYSIOLOGY 



Carbon-dioxide Absorbents and Tests. These are frequently needed 

 in experimentation, and before attempting to apply them to a physiological 

 problem, the student should make himself familiar with their action by 

 actual tests with known quantities of carbon dioxide. 



(i) Caustic potash is the best agent for removing that gas from a confined 

 space, absorbing it after the equation 2K0H + C0 2 =K 2 C0 3 + H 2 0. It 

 may be used in any strength from the weakest solution to the solid, the latter 

 requiring some slight moisture for its operation. As a rule the absorption 

 is the more rapid the stronger the solution, the higher the temperature, and 

 the greater the agitation of the liquid. A useful solution is one of 5 g. com- 

 mercial caustic potash to 15 cc. of water, this strength having the advantage 

 that it is the same as that used later in connection with oxygen absorption 

 (page 104). Of this solution 1 cc. will absorb about 100 cc. of carbon dioxide 

 within three minutes (if agitated), but in practice it is best to allow only half 

 that amount or less (Hempel recommends one-quarter). In making up the 

 solution, advantage can be taken of the fact that the solid sticks in which 

 commercial caustic potash is sold weigh, on the average, about 1 gram per 

 centimeter of length (though the commercial sticks are only from 75-85% 

 pure), and thus the sticks can be measured off accurately enough for practi- 

 cal purposes. A solution should always be made up at least fifteen or twenty 

 minutes (preferably longer) before needed, in order to allow it time to cool, 

 and also to lose the bubbles of air carried into the solution by the solid stick; 

 and it is well to keep a stock solution in bottles tightly stoppered with rubber 

 stoppers. It may most conveniently be applied by use of the reagent tubes 

 later described (page 105). Since it is very diffusible and very disturbing 

 to chemical processes into which it accidentally comes, all dishes which it 

 has touched should be thoroughly washed; and since it is irritating to the 

 fingers (and destructive to fabrics, etc.), it is well to handle the sticks with 

 small tongs kept for the purpose. I have found that these ends may very 

 advantageously be met by having a small wooden tray, about 25 cm.X 15 cm. 

 X3 cm. deep, with handles at the ends, in which the bottle containing the 

 solid sticks (with a screw-eye in the cork for a handle) is kept wired in one 

 comer, and to which the tongs and knife used in handling the substance 

 are attached by light chains. In its bottom is a millimeter, ruler by which 

 the sticks may be measured, and thus roughly weighed. The tray holds 

 also the stock bottles and any other dishes connected with the use of this 

 substance. , 



(2) Soda-lime is a mixture of caustic soda and quicklime, obtainable in 

 tight bottles; it forms an eager absorbent of carbon dioxide, very useful 

 where conditions forbid the use of a liquid. It works somewhat better in 

 lumps of appreciable size. It is much cleaner to use, though less efficient, 

 than caustic potash. 



(3) Lime-water is a saturated solution of calcium hydroxide in water 

 which absorbs carbon dioxide after the equation Ca(OH) 2 + C0 2 =CaC0 3 

 + H 2 0, the calcium carbonate appearing as a white precipitate. The 

 appearance of the precipitate, thus making the absorption of carbon dioxide 

 visible, gives this reagent a great advantage over the caustic potash as a test, 



