Page 373 - Instrumentation Reference Book 3E

356 Chemical analysis: electrochemical techniques
of pH becomes a less reliable measure of the Table173 pH values of common acids, bases, and salts
concentration of hydrogen ions. In addition, as
the concentration of a solution increases the Compound Molarity pH
degree of dissociation of the electrolyte decreases. Acid benzoic (Saturated) 2.8
A dilute solution of sulfuric acid is completely Acid boric 0.1 5.3
dissociated and the assumption that pH = - log2 Acid citric 0.1 2.1
(H2SO4) is justified. (The 2 occurs because each Acid citric 0.01 2.6
molecule of acid provides two hydrogen ions.) Acid hydrochloric 0.1 1.1
Anhydrous sulfuric acid is only slightly dissociated, Acid oxalic 0.1 1.3
the degree of dissociation rising as the pure acid is Acid salicylic (Saturated) 2.4
diluted. Acid succinic 0.1 2.7
A maximum hydrogen ion concentration Acid tartaric 0.1 2.0
occurs in the neighborhood of 92 percent Ammonia, aqueous 0.1 11.3
4.6
Ammonium alum
0.05
H2S04, but at this concentration, the difference Ammonium chloride 0.1 4.6
between actual hydrogen ion concentration and Ammonium oxalate 0.1 6.4
the activity of the hydrogen ions is large, and the Ammonium phosphate, primary 0.1 4.0
measured pH minimum of about - 1.4 occurs at a Ammonium phosphate, secondary 0.1 7.9
much lower sulfuric acid content. Ammonium sulphate 0.1 5.5
A more reliable indication of the ionic behavior Borax 0.1 9.2
of a solution will be obtained if we define pH in Calcium hydroxide (Saturated) 12.4
terms of the hydrogen ion activity aHf so that Potassium acetate 0.1 9.1
Potassium alum 0.1 4.2
pH = log,, (l/aH') = -log,, aH+ Potassium bicarbonate 0.1 8.2
Potassium carbonate 0.1 11.5
where aH is related to the hydrogen ion concen- Potassium dihydrogen citrate 0.1 3.7
tration cH+ by the equation Potassium dihydrogen citrate 0.02 3.8
Potassium hydrogen oxalate 0.1 2.7
aH+ = fH+cH+ Potassium phosphate, primary 0.1 4.5
where fH+ is the activity coefficient; see Section Sodium acetate 0.1 8.9
8.0
17.1. The pH values of common acids, bases, and Sodium benzoate 0.1 8.3
0.1
Sodium bicarbonate
salts are given in Table 17.8. Sodium bisulphate 0.1 1.4
Sodium carbonate 0.1 1.5
Sodium carbonate 0.01 1 .o
17.4.2 Practical specification of a pH scale Sodium hydroxide 0.1 2.9
As the value of pH defined as - log,, (hydrogen ion Sodium phosphate, primary 0.1 4.5
activity) is extremely difficult to measure, it is neces- Sodium phosphate, secondary 0.1 9.2
sary to ensure that when different workers state a Sodium phosphate, tertiarv 0.01 1.7
pH value they mean the same thing. An operational Sulphamic acjd 0.01 2.1
definition of pH has been adopted in British Stand-
ard 1647:1961. The e.m.f. E, of the cell
quinhydrone. The two bridge solutions may be
Pt HJsoln. Xconc. KC1 solnhef. electrode of any molarity not less than 3.5mol/kg provided
they are the same.
is measured and likewise the e.m.f. Es of the cell
Pt Hz/soln. Skonc. KC1 solnhef. electrode 17.4.3 pH standards
both cells being at the same temperature through- The difference between the pH of two solutions
out and the reference electrodes and bridge solu- having been defined as above, the definition of
tions being identical in the two cells. pH can be completed by assigning at each tem-
The pH of the solution X denoted by pH(X) is perature a value of pH to one or more chosen
then related to the pH of the solution S denoted solutions designated as standards. In BS 1647 the
by pH(S) by the definition: chosen primary standard is a solution of pure
pH(X) - pH(S) = (Ex - Es)/(2.3026 RTIF) potassium hydrogen phthalate having a concen-
tration of 0.05 moyliter.
where R is the gas constant, Tis temperature in This solution is defined as having a pH value of
Kelvins, and F is the Faraday constant. Thus 4000 at 15°C and the following values at other
defined, pH is a pure number. temperatures between 0 and 95 "C:
To a good approximation, the hydrogen elec- Between 0 and 55 "C
trodes in both cells may be replaced by other
hydrogen-responsive electrodes, e.g., glass or pH = 4.000 + 1/2[(t - 15)2/100]