The Need for Normality

Carl Schaumburg                Von Steuben M.S.C.
                               5039 N. Kimball Ave.
                               Chicago, IL 60625
                               (312) 534-5100

Objective:  

    The materials here will show that not all salts are neutral and will 
demonstrate the necessity for measuring concentration using normality in 
titration experiments. 

Materials needed:          

    Small amounts of solid salts which have a variety of pH's such as NaCl, 
Na3PO4, K2SO4, KNO3, NH4NO3, NH4Cl and whatever others you have available, 
pHydrion paper (1-12), solutions of phenolphthalein, bromthymol blue, alizarin, 
solutions of 0.1M sodium hydroxide, 0.1M hydrochloric acid, 0.1M sulfuric acid, 
0.1M phosphoric acid, burets, pipets and other glassware for titrations. 

Strategy:

    Start by reviewing reactions of acids and bases.  When someone mentions 
neutralization, STOP and discuss this reaction.  Using small amounts of solid 
salts and pHydrion paper discover and discuss that salts are not all neutral.  
When acids react with bases, the final pH will vary. 
    Have students add a squirt of bromthymol blue, phenolphthalein and alizarin 
to separate test tubes.  Add 4 drops of 0.1M HCl to each.  Neutralize the acid 
by adding 3 or 4 drops of 0.1M NaOH to the bromthymol blue until it turns green 
or just barely turns blue.  Counting the drops of base, repeat the process with 
phenolphthalein until a light pink color appears and with alizarin until a 
purple appears.  Discuss!  Indicators change colors at various pH values.  It 
should take more base for the second two and this gives an indication of the 
higher pH at which they change color.  Which indicator will be best for salts 
with pH of 7, 9 or 12? 
    Finally three titrations are done in the normal fashion.  Everyone will use 
10 mL of acid but some groups should use 0.1M hydrochloric acid, others 0.1M 
sulfuric acid and the rest 0.1M phosphoric acid.  All titrations will use 0.1M 
NaOH.  Since all acids were the same concentration, what are predictions of the 
amount of base solution needed?  How much base solution does it actually take?  
Why?  Investigation of the number of H's in the formula of the acid should give 
the clue.  Two H's will neutralize twice the volume of base and three H's three 
times. Lead everyone to see that the concept of molarity is not sufficient for 
many titrations.  A new method of expressing concentration is required - 
normality.  Now the definition, discussion and usual problems sets can be done. 
    
Evaluation:

    A quiz or discussion of the understanding of pH of salts could determine if 
this concept is understood.  Practice titration problems with polyprotic acids 
would quickly show those who do not understand.