METHODS FOR QUANTITATIVE DETERMINATIONS. 429 



This shows that the gas-volume of 66 c.c. at 54.6 C. and 750 m.m. pressure 

 becomes 54.28 c.c. at C. and 760 m.m. pressure. Knowing the volume at 

 C. and 760 m.m. pressure, and the weight of 1 c.c. of the gas under these 

 conditions, the weight of the total volume is easily found. 



The reduction for temperature and pressure can be made in one operation 

 by using the formula : 



V = v X P X 273 



760 X (273 -f t) ' 



V= volume at C. and 760 m.m. pressure, which is to be found. 



v = volume read at some pressure, p, other than the normal. 



t = temperature in centigrade degrees at which volume v is read. 



Thus, in above case : 



V = 66 X 750 X 273 = ,, 2g 



760 X (273 -f- 54.6) 



Methods of gas-analysis have been adopted by the U. S. P. in the quantita- 

 tive determination of amyl nitrite and ethyl nitrite. The operation is per- 

 formed in an apparatus known as a nitrometer, consisting of two glass tubes held 

 in upright position and connected at the lower ends by a piece of rubber 

 tubing. One of the tubes is open, the other one is graduated and provided 

 with a glass stopcock near the upper end. In using the nitrometer for the 

 analysis of ethyl nitrite the graduated tube is filled with saturated solution of 

 sodium chloride, in which nitrogen dioxide is almost insoluble. Next are 

 introduced through the stopcock the measured (or weighed) quantity of ethyl 

 nitrite with a sufficient amount of solution of potassium iodide and sulphuric 

 acid. By the action of these agents nitrogen dioxide is liberated, and from the 

 volume obtained the quantity of nitrite present is calculated. The decom- 

 position is shown by the equation : 



C 2 H 5 N0 2 -f KI + H 2 S0 4 = C 2 H 5 OH + I + KHSO 4 + NO. 



Water analysis. The objects of water analysis are various. Thus, the 

 analysis may serve to decide the fitness of a water for manufacturing, medici- 

 nal, or household purposes. Accordingly, more or less stress is laid on the 

 exact determination of certain constituents. While the student is referred to 

 special books treating on the different methods of water analysis, a brief outline 

 of the chemical examination of drinking-water is here given. 



It should be remembered that the results obtained by chemical examination 

 only are sometimes insufficient to furnish positive proof of the fitness of a 

 water for drinking-purposes. The reason is that micro-organisms may be 

 present which cannot be detected by chemical means. It is the microscope, 

 aided by appropriate bacteriological methods, which has to be used in such 

 cases, and these methods cannot, of course, be considered in this book. 



Standard of purity. A fixed standard has not as yet been generally 

 adopted for judging the purity of wholesome drinking-water, but most authori- 

 ties agree that the following maxima of admixtures should not be exceeded. 

 They are expressed in milligrams per liter i. e., parts by weight in one million. 

 The following data refer to one liter of water used : 



