40 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



4- Sulphur. 



This element is most generally estimated by the same method as 

 the halogens (Carius) ; sulphuric acid is formed and precipitated as 

 barium sulphate. 



It is more convenient to oxidise the substance in a nickel crucible 

 with sodium or barium peroxide ; the contents are acidified with 

 hydrochloric acid and the barium sulphate formed is weighed. Still 

 more convenient is the oxidation mixture used for estimating total 

 sulphur in urine (see p. 542). 



5. Phosphorus. 



Phosphorus is usually estimated in the same way as sulphur by 

 the Carius method, the phosphoric acid formed being precipitated as 

 ammonium magnesium phosphate. 



The most rapid and 'convenient method is that of Neumann. 

 The substance is oxidised in an open flask with a mixture of nitric 

 and sulphuric acids. The phosphoric acid formed is precipitated as 

 ammonium phosphomolybdate and this is then estimated by solu- 

 tion in excess of ^N caustic soda and subsequent titration with '$N 

 sulphuric acid. The difference between -$N NaOH and *5N H 2 SO 4 

 multiplied by I -268 gives the number of milligrams of P 2 O 5 in the 

 given weight of substance taken. 1 This method is described on p. 545. 



Micro-Analyses. 



Minute quantities of substance can be analysed by the methods 

 devised by Pregl. These methods are difficult to perform and require 

 much practice. The full details are given by Pregl in Abderhalden's 

 " Handbuch der Biochemischen Arbeitsmethoden," vol. v., part 2, p. 



Folin has also described a method for estimating nitrogen with 

 special reference to its estimation in urine and blood. Its technique is 

 comparatively simple and is given on p. 558. 



i See J. Physiol., 1906, 33, 439. 



