G. S. ADAIR 



activity of the protein. The excess coefficient for water, b w , determined 

 by the formula b w = -(55-51/m,°) b t is then positive and it follows that 



d In y p (salt) - + {b w m° t /55-51) d In a° ... .(8) 



d log Y , (salt) = K a dm ( ° + dy .... (9) 



dy = b w (mf /55-51) d log Y ,° ....(10) 



K a = 0-4343 (b J 55-51), a constant over a range where b w is constant. 

 In the molar solution of ammonium phosphates, b, = —32 and 

 b w == +680, and therefore K a = +5-3. 



In comparing measurements of activity coefficients with measure- 

 ments of solubilities, the following equations may be found useful. 

 All terms in equation 1 1 refer to a saturated aqueous solution of the 

 protein. All terms in equation 12 refer to a saturated solution of the 

 protein which contains inorganic salts. 



(^ P ) w (yp)w (solution) = (m'XirX (solid phase) .... (1 1) 

 m p y p (solution) = m'rf p (solid phase) (12) 



If the measurements of solubilities S w and S be expressed in g 

 protein per 100 g of water it follows that 



log (SJS) = log y„ - log T ; + k ... .(13) 



The term k, which may be relatively small, is given by formula 14. 



k = log [(mXWJmfaXl • • • • (I 4 ) 



Haemoglobin crystals in equilibrium with water vapour contain from 



41 to 44 per cent of water 7 and it may be assumed that in the solid 



phase the protein is approximately 0-02 dz 0-002 molal. 



On the hypothesis that the solid phase is like an ' ideal ' mixed 



iO 



in 



-2 -' 



J 

 o 



I - 4 



•5 - 



-6 



004 0-08 012 



Total Phosphate Molality 



Figure 1. Effects of salt con- 

 centration on activity coefficients 

 of haemoglobin. Curve Na7 ox 

 carboxyhaemoglobin in sodium 

 phosphate buffers containing 75% 

 of the disodium salt ; in curve 

 Na5 buffers contain 50% of the 

 disodium salt. Curve N7 sheep 

 carboxy haemoglobin in ammo- 

 nium phosphate buffers containing 

 70% of diammonium salt ; in 

 curve N5 buffers contain 50% of 

 diammonium salt. Curve S shows 

 solubility ratios of horse oxy- 

 oi6 haemoglobin in potassium 

 phosphate buffers pli 6$. 



188 



