THE NATURE OF PHOTOSYNTHESIS 81 



Lloyd. ^^ K^ is constant when the total Na-content is kept constant. 



When the latter is varied — changes, and with it K'. 



If c designates the total Na-content in milli-equivalents per liter, War- 

 burg gives the empirical formula 



K^rr: 8739— 1671 log c. (3) 



as holding when c = 100 to 1000 at 25°. 



Although there are no determinations of K^ for temperatures other 

 than 25°, Warburg calculated the CO.-concentration at other tempera- 

 tures. This was done by calculating the change of K^ with temperature 

 from the heat of decomposition of the bicarbonate. Unfortunately the 

 latter are not very accurate. In the dissociation of 2 moles of NaHCOa 

 (aq.) in 1 mole CO, (aq.) and 1 mole NagCOs (aq.) 2028 calories are 

 absorbed. On the further supposition that the degree of dissociation of 

 the salts does not change ^markedly with temperature there is obtained 

 for absolute temperature T 



log K^ ,,, ^ V2XT 2X29S;^ (4) 



2.3 



By means of equations (2) (3) and (4) Warburg has calculated the 

 C02-concentration at three diiTerent temperatures of various mixtures 

 of NaaCOs and NaHCO-.,. These must, of course, be regarded in the 

 light of the assumptions made in the equations. 



In mixtures of this sort many acjuatic plants can carry on photosyn- 

 thetic activity for longer or shorter periods depending upon conditions 

 of light intensity, temj>erature and the nature of the plant itself. In 

 general, the length of time during which plants can survive in the mix- 

 tures depends upon the hydrogen-ion concentration, i.e. the time de- 

 creases with increasing alkalinity. Great care must be exercised in the 

 use of these solutions and preliminary experiments must be carried out 

 to determine whether the plants can live in the solutions without injury.^^ 

 The presence of a living plant in anyone of these carbonate mixtures 

 disturbs the equilibrium either by the liberation of carbon dioxide from 

 the plant in the dark or by the absorption of carbon dioxide from the 

 solution in the light. In the latter case when carbon dioxide is removed 

 from the solution the equilibrium is shifted, so that bicarbonate is de- 

 composed into carbonate and carbon dioxide, and the concentration of all 



'^Seyler and Lloyd, Jour. Chcm. Soc. HI, 138 (1917). See also Kendall, /. 

 Am. Chew. Soc, 38, 1480 (1916). .A.uerbach and Pick, Art. Reichsgcsundh.. 38, 

 274 (1911). For dilute solutions McCoy has shown that in the equation NaHCOs + 

 HOH ^ NaOH -|- H2CO3 the amount of NaOH is about Mio of the amount 

 calculated for a normal hydrolvsis, because of the secondary reaction : NaOH 4- 

 NaHCOs ^ H2O + Na^COs. 



=' Harder, R., Jahrb. wiss. Bat., 60, 538 (1921). 



