106 ESTIMATION OF TOTAL SOLIDS AND ASH. 



a few minutes only, and are further dried for about an hour in 

 the water-oven or air-bath. They are then weighed as before. 

 The difference between the weight of the basin with the residue 

 and the weight of the basin alone, multiplied by 40, gives the 

 percentage of total solids. 



In the author's experience Stoke's method has a tendency to 

 give results slightly above the truth, but according to Yarrow the 

 difference does not exceed 0-08 per cent. ; it has, however, the 

 advantages of rapidity and of requiring very little attention. 



The fat may be calculated by Tables XX. or XXII., pp. 93, 95. 



Estimation of Ash. The residue of total solids serves excel- 

 lently for the determination of the ash. By igniting over a 

 small Bunsen flame, or an Argand burner, or in a muffle, a white 

 ash can be obtained. The temperature must not be allowed 

 to rise above a barely perceptible red heat, or distinct volatilisa- 

 tion of alkaline chlorides may occur. If the asbestos method 

 of total solid estimation has been used, a somewhat higher tem- 

 perature may be employed than if simple evaporation in platinum 

 has been resorted to. 



A more exact determination is obtained by evaporating a 

 larger quantity of milk than is usually taken for total solid esti- 

 mation 25 to 50 grammes and igniting gently till thoroughly 

 charred ; the mass is extracted with hot water and filtered, the 

 insoluble portion and the filter being (after washing) ignited at 

 a red heat till white ; this will give the insoluble ash. 



By evaporating the filtrate and igniting cautiously at a low 

 temperature, the soluble ash is obtained. The sum of the soluble 

 and insoluble ash gives the total ash ; the results obtained in 

 this way are usually slightly higher (about 0-02 per cent.) than 

 the ash obtained by ignition of the total solid residue. 



If it be desired to examine the ash further, it is desirable to 

 keep the insoluble ash separate from the soluble portion. 



In the solution of the soluble ash the alkalinity may be deter- 



N 

 mined by titration with y= acid, methyl orange being used as 



an indicator ; and the chlorine, by titration with standard 

 nitrate of silver, using potassium chromate as indicator. 



The insoluble ash is dissolved in a slight excess of dilute hydro- 

 chloric acid, and the solution (nearly neutralised with ammonia r 

 if necessary) heated to boiling ; a cold saturated solution of 

 ammonium oxalate is dropped in slowly till the addition of a 

 further drop gives no more precipitate. After standing at least 

 two hours the precipitate is filtered off, washed, and ignited at a 

 low temperature to convert the oxalate into carbonate ; it is 

 best to moisten the ignited precipitate with ammonium carbonate 

 solution and re-ignite at a very low temperature. The pre- 



