CHEMICAL ANALYSIS 7 



The metric system of weights and measures is found to 

 be very convenient for such solutions, because 

 i gramme dissolved in i litre = i milligramme in i c.c. ; or 

 n grammes dissolved in i litre = n milligrammes in i c.c. 



Standard solutions often have their strength expressed 

 in terms of some other substance which they measure 

 usually an elementary substance. For example 



AgN0 8 + NaCl = AgCl + NaN0 3 

 170 (23 + 35-5). 



From this equation we see that 170 parts of silver 

 nitrate precipitate completely 58-5 parts of sodium 

 chloride containing 35-5 parts of chlorine. If we wish 

 to estimate the amount of CI present in a solution of 

 unknown strength, we can titrate with a solution of silver 

 nitrate of known strength that is, a standard solution. 

 What strength shall we make it ? 



a. 170 grm. AgN0 3 in 1 litre of water will precipitate 

 35-5 grm. CI ; then 1 c.c. will precipitate 35-5 mgr. CI. 



b. 17-0 grm. AgN0 3 in 1 litre of water will precipitate 

 3-55 grm. CI ; then 1 c.c, will precipitate 3"55 mgr. CI. 



c. J = 4-78 grm. in 1 litre of water will precipitate 



1 grm. CI; then 1 c.c. will precipitate 1 mgr. CI. 



The strength chosen depends on (1) The simplicity 

 desired ; (2) The strength of solution to be tested ; 

 (3) Whether any one of these strengths will be more 

 useful than the others for other estimations, and so save 

 needless duplication of solutions. 



NORMAL SOLUTIONS. 



These are standard solutions made to a certain strength 

 on the basis of chemical theory and practice. Thus, from 

 the equation 



NaOH + HC1 = NaCl + H 2 

 40 36-5 



we see that 40 parts of sodium hydrate are exactly neutral- 

 ized by 36-5 parts of hydrochloric acid. If therefore we 



