Q Measuring pH

pH Indicators. The pH of a solution can be measured qualitatively by using pH indicator papers or pH indicator solutions. The pH indicator papers are impregnated with one or more acid-base indicators. The pH indicator solutions have one or more acid-base indicators dissolved in them. 

Acid-base indicators are usually weak acids that ionize as follows. 

pH Meter. A pH meter is an instrument for determining the pH of a small volume of solution simply, quickly, and reproducibly, without changing the composition of the solution. A pH meter is a sensitive electrometer and is comprised of two electrodes and a voltmeter. When the two electrodes are immersed in a sample solution, the circuit is complete. The half-cell potential of one electrode is affected by pH changes whereas the half-cell potential of the other electrode, called the reference electrode is not. The voltmeter senses the potential difference between the electrodes and registers that difference on a scale calibrated directly in pH units. 

There are a great variety of pH meters available covering a range of sensitivities and ease of operation. Most pH meters are very expensive and require tender loving care in their operation. Many pH meters have the general appearance shown in FIGURE Q.l and differ primarily in the 

The first step in the operation of a pH meter is to standardize it. This is accomplished by immersing the electrodes in a buffer solution of known pH and adjusting the meter reading to the pH of that solution. For precise results the pH of the buffer solution should be close to that of the sample solutions being measured, and the temperature of the buffer solution and the sample solutions should be the same. 

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