Explanation: Relative pH Solution Strength and Normality Conversions


I’ve found the information in TABLE 2 hard to come by.  I don’t know about you, but the importance of a solution’s Normality was not at the front of my mind.  Not to mention what it was, or how to calculate it.   But the information is really quite useful.  Normality is the way a chemist says six of this is equal to a half-dozen of that:  The gram equivalent weight of an acid, base or salt is that fraction of its molecular weight which contains (for acids) or is equivalent to (for bases and salts) one gram atomic weight of displaceable hydrogen.   If you divide a compound’s molecular weight by the number of ions that readily dissociate in water, you get its equivalent weight.

Equivalent Weight  =  Molecular Weight/Valence

In the case of sulfuric acid, two hydrogens separate easily from the sulfate. For sodium hydroxide, the equivalent of one hydrogen separates from the sodium. Their equivalent weights, then, are shown below:

For H2SO4 :  98/2 = 49         For NaOH :   40/1  =  40

A normal concentration, or One Normal Solution (1 N) contains one gram equivalent weight of acid, base or salt for every liter of solution. Using the “this equals that” analogy, 40 gallons of a 1 N sulfuric acid solution would neutralize 40 gallons of a 1 N sodium hydroxide solution.

For most of us involved with water treatment, however, normal solution values are not readily available. Most of the regularly used acids and bases, along with our Jar Testing chemicals, are listed in TABLE 2. To compare these chemicals to one another, I assigned baking soda (0.25N) a value of one. This is convenient for use with our Jar Testing section, since pump sizing and chemical volumes can be easily estimated. In addition, comparisons of relative strength can be made. For example, a one pound per gallon solution of soda ash (approximately 10 %) has a relative strength of 4, while a 25 % solution of sodium hydroxide has a relative strength of 40. You would therefore need 10 times less of the sodium hydroxide solution than the soda ash solution, to do the same job.

One last note of explanation: In the real world, we generally are given percentage concentrations for the chemicals we deal with. When I looked around for a way to convert percentages into Normality, I couldn’t find it. If you can figure out the Specific Gravity and equivalent weight for a chemical, the following formula will do that conversion:

Normality = 10  X   Specific Gravity   X    % Concentration/Equivalent Weight

Note:        Strong Acids HCL
(100 % Ionized) HNO3

Weak Acids C2H4O2