458 



PRACTICAL PHYSIOLOGY 



Instead of employing litmus to ascertain when all the ammonia has 

 distilled over, a solution of alizarin may be used. This is yellow 

 in acid solution, and pink to purple in alkali. A drop is allowed to 

 run down the distilling tube, (the end of which is meanwhile removed 

 from the surface of the distillate) so that the alizarin mixes with the 

 hanging drop of the distillate. If this latter still contain ammonia a 

 purple colour is developed, if there be no ammonia the orange colour 

 remains unchanged. 



Titration. Method for standardising a ded-normal solution. It is 

 necessary that a stock solution of very accurately standardised acid 

 be kept, with which other standard solutions may 

 be compared. The best acid to employ for this pur- 

 pose is, I think, sulphuric ; it can easily be obtained 

 pure, it keeps well, and it can be used for all the 

 acidimetric processes necessary in medical chemistry. 

 To standardise it, anhydrous sodium carbonate is 

 employed. About 5 grammes of pure sodium car- 

 bonate are heated to dull redness in a crucible for 

 ten minutes, cooled in an exsiccator, the exact weight 

 taken, some transferred to an Erlenmeyer's flask, the 

 weight again taken, and the amount removed thus 

 estimated. Several accurately weighed samples are 

 removed in this manner, and each is dissolved in 

 distilled water, the solutions being then coloured 

 yellow by methyl orange solution. The acid, the 

 strength of which it is desired to determine, is now 

 run into the sodium carbonate solutions until the 

 neutral point is attained, the exact amount necessary in each case 

 being noted. The data for making the calculation are now at hand. 

 100 c.c. of a normal acid should exactly neutralise 5-3 grm. sodium 



carbonate, i.e. that 100 c.c. of a -^ acid should neutralise 0-53 grm. 



Suppose 0-246 grm. sodic carbonate required 41 -5 c.c. of the acid, then 



0-246: 0-53:: 41-5: z = 89'4 



Again suppose that another sample containing 0-2153 gm. required 

 36'32 c.c. of the acid, then 



0-2153 : 0-53 :: 36-32 : z = 89'4. 



To correct the acid, therefore, place 894 c.c. of the acid in a litre 

 flask (Fig. 272) and fill up to 1000 c.c. with distilled water, when a 



solution will be obtained, 100 c.c. of which will exactly neutralise 

 0*53 grm. sodic carbonate. 



FIG. 272. Flask for 

 accurately measur- 

 ing fluids. 





