MEASURING PHOTOSYNTHETIC ACTIVITY 



241 



would be to make the j^ercentage of oxygen slightly too small. Although 

 throughout the whole research a slightly modified Haldane solution was 

 used, subsequent experiments with the Haldane formula show a some- 

 what larger oxygen percentage. This increased percentage may be due 

 to two causes: (1) the actual absorption of more oxygen, or what is 

 more probable (2) the formation of less or no carbon monoxide. It 

 remains a fact, nevertheless, that the solution as used is without ques- 

 tion suitable for a comparative study of the oxygen percentage of the 

 atmosphere." 



The Haldane potassium pyrogallate solution is more concentrated than 

 the one of Benedict ; the solution recommended by Hempel is more dilute 

 than Benedict's. 



Thus Benedict used two solutions made up according to Hempel : 

 (1) 120 grams of KOH in 80 cc. of water to which was added 5 grams 

 of pyrogallic acid in 15 cc. of water, i.e. 120 grams KOH, 95 cc. HoO, 

 5 grams QHeOg, (2) 130 grams KOH, 70 cc. HoO, 5 grams CgHsOa. 

 Benedict gives analyses of outdoor air and cylinder air made with these 

 four solutions of dififerent concentration of the same method. The 

 averages of his analyses are given in Table 30. 



TABLE 30 



Average Results of Comparative Study of Oxygen Absorption by Potassium 

 Pyrogallate of Varying Concentration (Benedict). 



Outdoor Air 



Hempel 



Solution 1 



20.851 



Hempel 



Solution 1 



20.766 



Hempel 



Solution 2 



20.913 



Benedict 



Solution 



20.938 



Cylinder Air 



Hempel 



Solution 2 



20.830 



Benedict 



Solution 



20.863 



Haldane 



Solution 



20.956 



Haldane 



Solution 



20.877 



It is obvious then that by using the more dilute potassium pyrogallate 

 solutions low values of the oxygen-content of a gas mixture will result. 

 That this is due to small quantities of carbon monoxide seems most Hkely. 

 Whether even with Haldane's solution traces of this gas are formed is 

 not absolutely certain, though comparison with results obtained by other 

 methods would indicate that this could be the case only for exceedingly 

 small amounts. In making up the potassium pyrogallate solution, dif- 

 ficulty may arise from the varying amount of water in the potassium 

 hydroxide ; in dififerent grades this ranges from 5 to 25 per cent. The 

 safest way is to make the solution to a specific gravity of 1.55. 



Temperature also may play an important role in the absorption of oxy- 

 gen by pyrogallate solution: Below 15^ the absorption is slow, but above 

 this temperature there is little difference in the rate of the absorption of 

 oxygen. 



Chromous chloride has been used as an absorbent of oxygen when it 



