EDTA

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lakshminarayana
lakshminarayana's picture
EDTA

dear sir, can u explain difference between Molarity and normality of EDTA ,in water analysis of my sop  we use Molarity of EDTA instead of normality .maximum acid base titrations we use noramlity in my sop.in whcih cases we use molarity and normality.

Sami Tuomivaara
Sami Tuomivaara's picture
lakshminarayana,

lakshminarayana,

Molarity of a compound is defined as number of particles per volume. Normality of a compound is context-dependent and is always some integer multiple of molarity, so that normality = x * molarity. The "x" depends on the context, but in general it tells what is the stoichiometry in some chemical reaction (if it goes to completion).

Using EDTA as an example, it has 4 acidic groups, so one EDTA molecule can lose 4 protons, and x = 4.

     H4-EDTA <-> EDTA + 4H

When dealing with Calcium chelation, one EDTA molecule can only bind one Calcium, thus x = 1.

     EDTA + Ca <-> Ca-EDTA

Hence, 1 mM (one millimolar) EDTA solution is 4 mN (four millinormal) in acid-base titration and 1 mN (one millinormal) in Calcium titration.

Actually, when you calculate reaction stoichiometries, you use normality all the time, it is just that when you do water analysis (I assume this has something to do about metal chelation), x = 1, and the normality and molarity have the same value.

Cheers,

Metals Man
Metals Man's picture
Normality does not equal

Normality does not equal molarity in this case!

Remember, normality is reaction specific and is based on the number of protons exchanged in the balanced chemical equation.  Even though there is a one to one molecular equivalence, EDTA exchanges 2 protons to neutralize one divalent calcium cation (Ca++), therefore, by definition, EDTA normality is actually twice the molarity (for this particular reaction). 

http://environmentalchemistry.com/yogi/chemistry/MolarityMolalityNormality.html