  The effects of additions of low-molecular-mass salts on the properties of
aqueous lysozyme solutions are examined by using the cloud-point temperature,
$T_{cloud}$, measurements. Mixtures of protein, buffer, and simple salt in
water are studied at pH=6.8 (phosphate buffer) and pH=4.6 (acetate buffer). We
show that an addition of buffer in the amount above $I_{buffer} = 0.6$ mol
dm$^{-3}$ does not affect the $T_{cloud}$ values. However, by replacing a
certain amount of the buffer electrolyte by another salt, keeping the total
ionic strength constant, we can significantly change the cloud-point
temperature. All the salts de-stabilize the solution and the magnitude of the
effect depends on the nature of the salt. Experimental results are analyzed
within the framework of the one-component model, which treats the
protein-protein interaction as highly directional and of short-range. We use
this approach to predict the second virial coefficients, and liquid-liquid
phase diagrams under conditions, where $T_{cloud}$ is determined
experimentally.
