(a) Definition. The pH is defined as the logarithm (to the base 10) of the reciprocal of the hydrogen-ion concentration m moles per liter

pH — lot* + . .

H concentration m moles per liter

figure 4.42 shows pH vs. hydrogen-ion concentration. Points indicate the pH values of various common acids and bases.

All water solutions owe their chemical activity to their relative H+ and OH concentrations In water, the equilib­rium product of the H+ and Oil concentrations is a constant 10 14 at 25°C. When concentrations of H+ and OH in pure water at 25°C are equal, the H concentration is 10 7 and, from the definition, the pH is 7.0 Note that the stale of pH values is not linear with concentration A change of one unit in pH represents a 10-fold change in the effective strength of the acid or base

The pH value depends only on the concentration of hydrogen ions actually dissociated in a solution and not on
the total acidity or alkalinity. Therefore, because dissocia­tion of water increases with temperature and pH is a measure of II concentration only (and not the ratio of H+ to OH ), the pH of pure water increases above 7.0 if the temperature is increased above 25°C. There is no simple way to predict the pH of a solution at a desired temperature from a known pH reading at some other temperature.

(b) Measurement Techniques. Chemical Indicators The pH of a sample may be determined by adding a small quantity of an indicator solution to the sample and comparing the color with that of a color standard. When good color standards are available in steps of 0.2 pH unit and observations are made in a comparator, the limit of accuracy is considered to be 0.1 pH unit. Turbid and colored solutions cannot be observed with accuracy, and indicators are not stable in many strongly oxidizing or reducing solutions. Table 4.21 lists some common pH indicators and their range of use.

Potentiometnc pH Measurement. A potentiometric pH-measuring system consists of (1) a pH-responsive elec­trode, such as glass, antimony, quinhydrone, or hydrogen,

(2) a reference electrode, usually calomel or silver—silver chloride, and (3) a potential-measuring device, such as a pH meter, usually some form of vacuum-tube voltmeter figure 4.43 shows a typical potentiometnc system.

Table 4.22 lists the characteristics of six pH-measuring electrodes. Glass electrodes are electrically sensitive to hydrogen-ion concentration The voltage response to hydro­gen-ion concentration is

E = E° — 0 0591 log H+ (at 25°C)

where E° is the voltage of the particular glass electrode at pi I zero

In Figs. 4.44 to 4.47, some pH-measuring meters are illustrated. The feedback type pH meter (Fig. 4.49) has a circuit capable of good performance if matched tubes are employed to minimize drift The electrodes must be checked periodically against standards for asymmetry.

Figure 4.48 shows the theoretical curve for the pH at 25°C vs. the concentration of ammonia. Figure 4.49 gives the temperature correction for the ammonia curve.

HIGH-IMPEDANCE POTENTIAL MEASURING INSTRUMENT CALIBRATED IN pH UNITS CALOMEL

——————— ГТ——- Ш———— V REFERENCE

/ ELECTRODE

PLATINUM WIRE MERCURY

BUFFER

SOLUTION

CALOMEL

GLASS WOOL LIQUID JUNCTION

ASBESTOS FIBER

Fig. 4.43—A potentiometric pH-measuring system. (From D M Considine, Process Instruments and Controls Hand book, p. 6-106, McGraw-Hill Book Company, Inc., New York, 1957.)

Figure 4.50 shows the theoretical curve for the pH at 25°C vs. the conductivity of ammonia. Conductivity measurements can be used to monitor the pH of the feedwater or the ammonia concentration in the feedwater (Fig. 4.51).

Limitations and Practical Considerations.

1. Glass electrodes can develop cracks, which allow some diffusion between the inner filling solution and the sample. When diffusion occurs responses are erratic and nonrcproducible.

2. Glass is soluble in strongly alkaline solutions and thus has a shorter service life. Special alkali-resistant electrodes should be used for these applications.

3. If the glass becomes coated, the response is sluggish.

4. High sodium-ion concentration for extended periods of time results in loss of sensitivity.

5. Avoid temperature transients.

6. New electrodes should be soaked several hours before use to improve stability.

7. Avoid electrical leakage in the high-impedance input circuit by preventing moisture buildup on the glass elec­trode body and lead, blectrical leakage is sometimes caused by the buildup of humidity and dust inside the instrument case.

8. Grounding problems Many pH meters provide for separate grounding of the amplifier chassis and case. The ground of the amplifier is maintained at the glass-electrode potential by connections with feedback circuits.

9. Shorting of the electrodes causes polarization. The pH reading drifts under these conditions.

10. Colloids are sensitive to salt and may precipitate at the liquid junction as the result of the diffusion of the salt-bridge electrolyte or may form a film on the glass-elec­trode bulb. Slurries cause similar trouble.

11. Glass is attacked by soluble silicates and by acid fluorides. Special alkali-resistant electrodes are available.

12.
Radioactivity in sample solutions may result in ion collection in the high-impedance input circuit, which, in turn, may produce error signals.

13. Glass electrodes respond to high concentrations of sodium, potassium, and lithium ions Sodium-ion correc­tions are usually available from the electrode manufacturer. The need for correcting data for high concentrations of other ions should be investigated.