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 



AgNO 3 + NaCl - AgCl + NaNO 3 



17 (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 Cl 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. AgNO 3 in i litre of water will precipitate 

 35-5 grm. Cl; then i c.c. will precipitate 35-5 mgr. Cl. 



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

 3-55 grm. Cl ; then i c.c, will precipitate 3'55 mgr. Cl. 



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



i grm. Cl; then i c.c. will precipitate i mgr. Cl. 



The strength chosen depends on (i) 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 O 



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 



