360
Appendix 1
NORMAL SOLUTIONS
Some of the more important determinations in water chemistry are dependent on the
use of standard solutions. A standard solution contains a known weight of the active
substance dissolved in a definite volume of solution. Methods involving the use of such
solutions are known as volumetric procedures, since the quantitative result is obtained
by the measurement of volumes.
It is convenient to have solutions of known concentrations for use in various
determinations. Such solutions simplify the calculations of results, a decided advantage
in routine analysis. They can best be prepared by diluting portions of stock solutions in
such a manner as to give solutions of the desired strength.
The strength of a standard solution is usually expressed in terms of its normality. A 1
normal solution is one which contains 1 gram-equivalent weight of the active substance
in 1 liter of solution. The gram-equivalent weight of a substance is the weight of that
substance which will react with 1 g of hydrogen.
There are two general types of chemical reactions encountered in the volumetric
determinations used for water analysis: (1) simple neutralization or doubledecomposition reactions, and (2) reactions involving oxidation and reduction.
The reactions of the first type involve a change in position of the various atoms and
groups making up the reacting substances. The following equation illustrates this type
of reaction:
HCl + NaOH~NaCI + H 20
In a reaction of this type, the gram-equivalent weight of each compound reacting is
calculated by dividing the molecular weight of the compound by the number of
replaceable hydrogen atoms or their equivalent in that compound:
Molecular weight of a substance
gram-equivalent
No. of replaceable hydrogen atoms = weight of the substance
or their equivalent
Thus the gram-equivalent weight of HCl is 36.5, since the molecular weight (36.5) is
divided by 1, because there is one replaceable hydrogen. In NaOH, one Na can be
replaced by one H and is equivalent to one atom of hydrogen. Therefore, the gramequivalent weight of NaOH is 40.
The second type of reaction, oxidation-reduction, is illustrated by the following
equation:
2KMn04 + 5H2C 20 4 + 3H2S04~2MnS04 + K 2S04 + 1OC02 + 8H20
A study of the equation will show that there is more involved than a simple
rearrangement of atoms and groups. Mn, for instance, has a positive valence of 7 in
KMn0 4 , but only 2 in MnS04. The Mn has lost 5 positive valences (has been reduced);
since there are 2 Mn atoms, the total loss is 10 valences. Likewise, each C atom in
H 2 C 2 0 4 has a valence of + 3, while each C atom in CO 2 has a valence of + 4. The 10 C
atoms thus gain a total of 10 valences (one for each of the C atoms). The total loss of
positive valence by one kind of atom in this type of reaction must equal the total gain
of positive valences by another kind of atom.
The gram-equivalent weight of a compound entering into an oxidation-reduction
reaction is equal to its molecular weight divided by the change in valence ofthe element
Appendix 1
NORMAL SOLUTIONS
Some of the more important determinations in water chemistry are dependent on the
use of standard solutions. A standard solution contains a known weight of the active
substance dissolved in a definite volume of solution. Methods involving the use of such
solutions are known as volumetric procedures, since the quantitative result is obtained
by the measurement of volumes.
It is convenient to have solutions of known concentrations for use in various
determinations. Such solutions simplify the calculations of results, a decided advantage
in routine analysis. They can best be prepared by diluting portions of stock solutions in
such a manner as to give solutions of the desired strength.
The strength of a standard solution is usually expressed in terms of its normality. A 1
normal solution is one which contains 1 gram-equivalent weight of the active substance
in 1 liter of solution. The gram-equivalent weight of a substance is the weight of that
substance which will react with 1 g of hydrogen.
There are two general types of chemical reactions encountered in the volumetric
determinations used for water analysis: (1) simple neutralization or doubledecomposition reactions, and (2) reactions involving oxidation and reduction.
The reactions of the first type involve a change in position of the various atoms and
groups making up the reacting substances. The following equation illustrates this type
of reaction:
HCl + NaOH~NaCI + H 20
In a reaction of this type, the gram-equivalent weight of each compound reacting is
calculated by dividing the molecular weight of the compound by the number of
replaceable hydrogen atoms or their equivalent in that compound:
Molecular weight of a substance
gram-equivalent
No. of replaceable hydrogen atoms = weight of the substance
or their equivalent
Thus the gram-equivalent weight of HCl is 36.5, since the molecular weight (36.5) is
divided by 1, because there is one replaceable hydrogen. In NaOH, one Na can be
replaced by one H and is equivalent to one atom of hydrogen. Therefore, the gramequivalent weight of NaOH is 40.
The second type of reaction, oxidation-reduction, is illustrated by the following
equation:
2KMn04 + 5H2C 20 4 + 3H2S04~2MnS04 + K 2S04 + 1OC02 + 8H20
A study of the equation will show that there is more involved than a simple
rearrangement of atoms and groups. Mn, for instance, has a positive valence of 7 in
KMn0 4 , but only 2 in MnS04. The Mn has lost 5 positive valences (has been reduced);
since there are 2 Mn atoms, the total loss is 10 valences. Likewise, each C atom in
H 2 C 2 0 4 has a valence of + 3, while each C atom in CO 2 has a valence of + 4. The 10 C
atoms thus gain a total of 10 valences (one for each of the C atoms). The total loss of
positive valence by one kind of atom in this type of reaction must equal the total gain
of positive valences by another kind of atom.
The gram-equivalent weight of a compound entering into an oxidation-reduction
reaction is equal to its molecular weight divided by the change in valence ofthe element