The colorimetric determination of sodium

Abstract

No easy, rapid and accurate method of determining sodium in foodstuffs and other biological material is at present available. Such a method should pre- ferably be applicable to the same hydrochloric acid extract of the incinerated material in which calcium, magnesium and potassium are being determined. Many of the published methods are gravimetric [Kramer and Tisdall, 1921] and therefore are not only slow, but require relatively large amounts. Doisy and Bell's [1920] caesium method admittedly presents considerable technical difficulties. Of the other methods examined by us the most promising ap- peared to be those depending on the precipitation of the sodium as the sodium uranyl zinc (or magnesium) acetate [Barber and Kolthoff, 1928; Caley and Foulk, 1929], and the subsequent estimation of the uranium in the precipitate, either by reduction and titration with potassium permanganate, or colori- metrically with potassium ferrocyanide. Barrensheen and Messiner [1927] have described a colorimetric method based on this principle suitable for serum or other biological fluids, but, as described by them, the method is very unsatisfactory. If, for example, the precipitate is washed according to their directions it goes completely into solution during the second washing. Poulsson [1928] modified the method to overcome this obvious defect, but the method is still not directly applicable to acid extracts of incinerated materials. We have therefore subjected the whole technique to a critical study, and have established a method which can be applied to neutral or acid solu- tions of sodium salts. A further communication will shortly be made on the application of the method to blood and serum without incineration.