The Indirect Colorimetric Determination of Phosphorus with Uranium Acetate and Potassium Ferrocyanide. by Robert B. Gibson and Clarence Estes

The application of calorimetric principles to the determination of phosphorus is especially desirable in view of the long drawn out and laborious gravimetric method now customarily employed. The accuracy of calorimetric methods, the ease with which most calorimetric procedures are carried on and their employment where only small amounts of material are available for analysis would make a reliable calorimetric reaction for phosphorus of the greatest value. Titration with uranium acetate is extensively used for the urinary phosphates and sufficient accuracy is obtained for ordinary metabolism investigations. Titration of the ammonium molybdate precipitate after adding an excess of $ sodium hydroxide or potassium hydroxide (Neumann) is stated by Gregersen’ to be accurate to I milligram of phosphorus when certain technical modifications of the original method are followed. The molybdate reaction for color comparison in the determination of phosphorus has been employed by Schreiner,2 by Woodman and Cayvan and by Veitch.* The intensity of the color developed is influenced to some extent by temperature (Woodman and Cayvan). Silicates give a similar reaction, and their presence in the ordinary laboratory reagents introduces error in the determinations. Veitch has shown that ammonium salts and certain chlorides influence the color of the phosphomolybdate solutions. One of us (Estes)5 has recently reported experi-