Salt precipitation during the freeze-concentration of phosphate buffer solutions

Salt precipitation during the freeze concentration of phosphate solutions was investigated by differential scanning calorimetry (DSC), in view of its practical importance in the cryopreservation or freeze-drying of biological materials. It was found that the fraction of salt precipitated depends on the initial salt concentration; it began to decrease with decreasing concentration at approx. 1 M. Salt precipitation also depends on the cooling rate. In some cases, cooling at approx. 10(3) degree min-1 inhibited salt precipitation which had been observed during slow cooling (0.62 degree min-1), without, however, affecting the shape of the ice melting endotherm. In the case of ternary phosphate buffers, the fraction of salt precipitating depends on the salt composition as well as the initial concentration and cooling rate. Near the composition of the ternary eutectic or the composition where two salts are present at the same concentration, salts were prevented from precipitation.