48 MEJTHODS OF QUANTITATIVE ANALYSIS. 



collected on the filter, is finally transferred to a Kjeldahl 

 flask and digested with sulphuric acid in the usual man- 

 mer.* From this point on the determination is con- 

 ducted in the same manner as described for total nitro- 

 gen. From the albuminoid nitrogen content thus ob- 

 tained, the percentage of albuminoids is calculated by the 

 formulas given above (see page 45). 

 2. The Ritthausen Method {Modification proposed ly 

 Munk). 



The albuminoid bodies are precipitated from a boil- 

 ing solution by Stutzer's copper hydroxide reagent and 

 the nitrogen estimated in it by the Kjeldahl method. 



The copper hydroxide is prepared in the following 

 manner : 100 grams of crystallized chemically pure cop- 

 per sulphate are dissolved in five liters of water and treat- 

 ed with two and a half grams of glycerine. Dilute sodium 

 hydrate is then added which precipitates the copper as 

 cupric hydroxide. The precipitate is then filtered ofi 

 and washed with water containing 0.5 per cent glycer- 

 ine by decantation until the filtrate is free of alkali. 

 The filtrate must be entirely colorless indicating the 

 absence of copper. If it be still blue more sodium 

 hydrate must be added. The filtered mass is then 

 treated with water containing 10 per cent of glycer- 

 ine and so diluted that it may be readily drawn up into 

 a pipette. The mixture should be kept in a tightly stop- 

 pered blue or brown bottle. The proportion of copper 

 hydroxide per cubic centimeter may be determined by 

 drying a measured volume of the material and weighing 

 the residue. Before using this reagent it should be 

 shaken vigorously in order to mix the precipitate uni- 

 formly. 



*The nitrogen content of Swedish filters is usually small but 

 should be determined in a "blank" and proper deduction made 

 in the calculation. 



