848 METHODS OF TvINETIC MEASUREMENTS CHAP. 25 



phate buffer equilibrated with air containing 1-5% CO2; without this 

 high initial concentration, the carbon dioxide suppl.y in the carbonate-free 

 solution will be rapidly exhausted in hght). 



From the point of view of interpretation of manometric data, alkaline- 

 buffered solutions are more convenient than acid solutions, since oxygen 

 alone is responsible for all the pressure changes observed above these solu- 

 tions. 



Ordinaril3% mixtures of 0.1 molar solutions of sodium bicarbonate and 

 potassium carbonate are employed (Table 8. V) . Using one sodium and one 

 potassium salt seems to be somewhat preferable to using two sodium or 

 two potassium salts (c/. page 835). A mixture of 85 parts bicarbonate and 

 15 parts carbonate at 20° C. is in equilibrium with free carbon dioxide at a 

 partial pressure of about 1.9 mm. (c/. Tables 8. II and 8.V; it has to be 

 taken into account that the solubility of CO2 in 0.1 molar salt solution is 

 ~10% lower than in distilled water). Ten cubic millimeters of carbon 

 dioxide can be withdrawn from 1 cc. of this mixture without appreciably 

 altering the partial pressure of CO2 in the gas. Mixtures containing more 

 carbonate and less bicarbonate have higher buffering capacities, and lower 

 concentrations of carbonic acid. 



As mentioned before, the drawback of carbonate buffers is their high 

 pH (about pH 9 for the above-mentioned buffer No. 9). Some algae are 

 damaged by short exposure to even less alkaline buffer mixtures {pR 8.5). 

 Chlorella pyrenoidosa, on the other hand, can be kept even in the more 

 alkaline carbonate mixtures for many hours without signs of damage to the 

 photosynthetic apparatus. Respiration in carbonate mixtures is some- 

 what slower than respiration in neutral or acid media. Despite absence of 

 visible damage, the maximum quantum yield of Chlorella photosynthesis 

 seems to be somewhat lower in alkaline carbonate than in acid (phosphate) 

 buffers; whether the reduction is <20% (Rieke, Emerson and co-workers) 

 or as high as 50% (as suggested by Warburg) is a matter of controversy 

 (c/. chapter 29, pages 1096 and 1107). 



When it is definitely desirable to avoid alkalinity, measurements of 

 photosynthesis usually are made in slightly acid medium (e. g., pH ^ 5, 

 obtained with IVH2PO4), saturating this medium with air enriched with 

 from 1 to 5% carbon dioxide. Both oxygen and carbon dioxide are ex- 

 changed with the gas phase above these solutions, and the observed pres- 

 sure changes must be apportioned to the two gases by means of a rather 

 delicate calculation. 



For each individual 'Warburg vessel," filled to a certain mark with 

 water, a "vessel constant," Kq,, can be determined, giving the increase in 

 pressure caused by the production in this vessel of one cubic centimeter of 

 oxygen; a similar constant, Xco„ can be calculated for carbon dioxide. 



