Detection of the Soluble Complexes Formed in the Systems Calcium Salt-Citric Acid and Magnesium Salt-Citric Acid

Some water-soluble complexes form in the systems of the aqueous solution between alkaline earth salt used as chemical fertilizer and the solvent containing citric acid. The methods for detecting the formation of their complexes in the extracts of the alkaline earth salts by dilute citric acid solution have been discussed. Preliminary comparison of various physico-chemical methods showed that infrared spectrum, polarographic and conductivity methods are not always convenient to detect directly the complex formed in 2 % citric solution of the salt. Refractive index method, an application of the measurement of dielectric constant, seemed to be adoptable for the above purpose.<BR>From a water-soluble alkaline earth salt, i. e. calcium chloride or magnesium chloride, and citric acid, the mixed aqueous solutions covering various mixing molar ratios of the salt/citric acid were prepared with a fixed total molar concentration which is corresponding to 2-4% citric acid. The refractive indices of these mixed solutions were measured, then the relationship between square of the refractive index and the molar mixing ratio was plotted for each system.<BR>In the system calcium chloride-citric acid or magnesium chloride-citric acid, the plot gave two straight lines bound with a break point at a mixing molar ratio 1 : 1. This break point indicates the existence of a soluble complex, CaCit^- or MgCit^-, in the solution and also a molar composition of its complex. In the aqueous solution system phosphoric acid-citric acid no break point was observed, whereas calcium chloride -phosphoric acid-citric acid system which is substantially equal to the system calcium monohydrogen phosphate-citric acid also gave a break point at the composition of calcium/citric acid in a molar ratio 1 : 1. The above facts could lead to a proof that the refractive index method is convenient and accurate for studying such water-soluble complexes.