6 PUBLIC HEALTH CHEMISTRY 



quantity of the substance under investigation. Exact 

 matching of the tints is arrived at, either by dilution of 

 the stronger, or by putting up several standard tints. 

 Requirements for Volumetric Analysis. 



1. Solution of reagent or test, the chemical power of 

 which is accurately known ; this is called the Standard 

 Solution. 



2. A graduated vessel, from which portions of the 

 standard solution may be accurately delivered : the Burette. 



3. Some indication, unmistakeable to the eye, that the 

 reaction is terminated or concluded : the Indicator. 



The process is called, a litration, i.e., an estimation of 

 the titre or strength of a solution, and the person is said 

 to titrate the solution. 



Volumetric methods may be classified thus : 



1. Neutralization of acids by alkalies, and vice versa 

 acidimetry and alkalimetry. 



2. Reduction or oxidation of substances for example, 

 ferrous sulphate titrated with potassium permanganate 

 illustrates both actions. 



3. Precipitation of some insoluble and definite com- 

 bination for example, precipitation of AgCl in testing 

 for chlorides. 



STANDARD SOLUTIONS. 



A standard solution of any substance is made when a 

 known quantity of that substance is dissolved in a known 

 quantity of water. Then the strength can be expressed 

 definitely as 



So many pounds per gallon, or pint, or ounce ; or 

 grains or 



grammes per litre or cubic centimetre (c.c.) ; or 

 milligrammes ,, ,, 



Thus we might have a standard solution of NaCl of these 

 strengths : (i gallon = 10 Ibs. = 70,000 grs.). 



i Ib. per gallon ; then i fluid grain = o-i grain NaCl. 

 i drachm per fluid ounce ; then i minim = 0-125 gr. NaCl. 

 i gramme per litre ; then i cubic centimetre = o-ooi 

 gramme, or i milligramme of NaCl per c.c. 



