Xciv. the Dissociation of Calcium Citrate

IT is often stated that the addition of a citrate to milk prevents clotting by rennin because an un-ionised calcium citrate is formed, thus removing the calcium ions from solution, these ions being considered essential for the clotting process. The prevention of the clotting of blood by citrates is explained in the same way. It is not the purpose of this paper to discuss the clotting phenomenon, but to record some measurements of the dissociation of calcium citrate. However, it must be pointed out that there is some confusion on the subject. A typical analysis of milk-serum [Van Slyke and Bosworth, 1915] shows a concentration of 0-465 g. of calcium per litre and 2-37 g. of citric acid. The citrate is present in a larger proportion than is necessary for equivalence with the calcium, so if un-ionised tricalcium citrate were formed there would be no calcium ions present. Analyses in this laboratory have shown the above figures to be typical of milks which clot normally with rennin. If the theory mentioned in the first paragraph were quite correct, most milks would not clot, for there is normally enough citrate present to prevent this. Shear and Kramer [1928] and Shear, Kramer and Resnikoff [1929] give results of conductivity titrations of calcium salts with sodium citrate and claim that the results give "direct evidence for the binding of calcium ions by sodium citrate in some kind of soluble complex." They obtained a minimum conductivity when the added sodium citrate was approximately equivalent to the calcium present. As they adjusted the PH of their solutions with uncertain proportions of acid and alkali, it is impossible to make an estimate of the conductivity of the solution due to any calcium ions present. The only safe deduction from their experiments seems to be that calcium citrate is less ionised than calcium chloride or sodium citrate at the same dilution. Stewart and Percival [1928] state that they have found by conductivity measurements that calcium citrate is almost undissociated in solution, but they give no particulars. It was, therefore, decided to publish the following measurements of the conductivity of simple calcium citrate solutions.