Determination of Pyrophosphate by Precipitation with Cadmium and Polarographic Measurement of Cadmium in Precipitate

Abstract

N THE present paper a new method for the determination 1 of pyrophosphate in the presence of varying amounts of orthophosphate is described. The method is of special advantage in the determination of small amounts of pyrophosphate, and may find use in the determination of pyrophosphate in biological material. I n the proposed method the molar concentration of orthophosphate may be 16 times as large as the concentration of pyrophosphate, but even in the presence of 40 times the molar concentration of orthophosphate satisfactory results have been obtained. If larger samples are available, as is usually the case in the analysis of commercial phosphates, the proposed method appears not to be superior to existing methods. The effect upon the proposed method of meta-and (or) polyphosphate, which are often present in commercial phosphates, has not been investigated.Until now no satisfactory method has been described for the determination of small amounts of pyrophosphate in the presence of orthophosphate. Larger amounts of meta-, ortho-, and pyrophosphate present in the same solution can be determined satisfactorily by acidimetric titrations.Gerber and Miles (2, 3) determined the three phosphoric acids by titrating to different pH values using for the determination of orthophosphate the amount of nitric acid liberated by precipitating the phosphates with silver nitrate. Pyrophos hate is further determined according to Britzke and Dragunov (17 by precipitating disodium dihydrogen pyrophosphate with zinc sulfate and titrating the liberated sulfuric acid. In the analysis of mixtures considerable titration error results if the amount of yrophosphate present corresponds to less than 50 mg. of piosphorus pentoxide. When small amounts of pyro-and orthophosphate are present in a solution a separation has to be made by precipitation.Several authors have described the separation of pyrophosphate b precipitation with a zinc or cadmium solution. Kiehl and d a t s (6) found that the precipitate of zinc pyrophosphate was not satisfactorily crystalline but when reprecipitated the pyrophosphate could be determined gravimetrically with an accuracy of 1 per cent. Nylen (9) reported an accuracy of 1 to 2 per cent by the zinc method when working in an acetate buffer with a pH of 5. He found no coprecipitation of orthophosphate when 6 to 15 mg. of phosphorus as pyrophosphate were determined in the presence of 15.5 mg. of phosphorus in the form of orthophos hate. An accuracy of about 1 per cent was also found by Majorsky and Clark (8), who se arated pyrophosphate by precipitation with zinc sulfate a t p 2 2 . 7 to 2.8. Gerber and Miles (S)! precipitating with zinc sulfate at pH 3.0 to 3.3, observed no interference by orthophosphate but a strong interference by ammonium chloride. These authors p i n t out that the conditions for the zinc separation are not yet well enough defined to recommend this method for general application.Hull (4) in qualitative experiments found that precipitation of zinc pyrophosphate is incomplete and that large quan...

Chemistry and Metabolism of the Compounds of Phosphorus

“…The true inorganic phosphate content of the blood has been reported to be much lower than that usually found ; 65 to 70 per cent of the so-called inorganic pho sphate results from the hydrolysis of phosphocreatine (146) . Determination of inorganic pyrophosphate as the cadmium (148) and man��anese (149) salts has been described.…”