DETERMINATION OF PROTEIN 17 



and sodium sulphide produced. On adding lead acetate 

 solution to the mixture, a black precipitate of lead 

 sulphide is obtained, and from the depth of coloration 

 a rough indication of the amount of sulphur present 

 in the body under investigation is given. 



10. Quantitative Determination of Protein. 



The conventional method for the estimation of the percentage 

 of proteins present in an organic substance containing them is to 

 determine the percentage of total nitrogen by Kjeldahl's process, 

 and from an assumption that all this nitrogen exists in the 

 substance in the form of proteins, to calculate the amount of 

 the latter bodies from a knowledge of the proportion of nitrogen 

 contained by them. 



This method is, however, open to several objections : — The 

 percentage of nitrogen in different albuminoid substances varies 

 from 14-0 in the case of mucin, to 18-4 in various proteins 

 present in wheat. It is usual, however, to take the mean (16 per 

 cent.) in the calculation of "crude" protein (see p. 150) from the 

 nitrogen percentage, although it can be seen that in many cases 

 an appreciable error will be thus introduced. 



Again, various bodies other than proteins, such as amines, 

 ammonium salts, and other nitrogen-containing substances,^ suffer 

 decomposition during heating with concentrated H2SO4 according 

 to Kjeldahl's process. It will be seen that the nitrogen present in 

 all those bodies undergoing hydrolysis in this way is given in the 

 analysis under the head of crude protein, although these nitrogen- 

 ous substances may possess a very different feeding value, or no 

 feeding value at all (for differentiation between different nitrog- 

 enous compounds in the case of cheese, see p. 209). 



In spite of these obvious objections, however, the method is 

 still largely adhered to in agricultural analysis. 



About 4 grams of ground wheat are heated in a 

 Kjeldahl flask with concentrated HgSO^ in an exactly 



^ Nitrogen in the form of nitrates can also be estimated together 

 with nitrogen in other forms, if salicylic acid be added to the 

 mixture in the Kjeldahl flask, owing to the formation of nitro- 

 salicylic acid, which readily undergoes reduction. 



B 



