The pH Meter

How pH is Measured

Today, the pH of a solution is measured either by an indicator dye or by a pH meter and an electrode system whose voltage output is proportional to the active acid ( H₃O⁺) concentration in solution.

Certain organic dye solutions change color over a relatively small pH range. These are called indicator solutions. They can be used to indicate the approximate pH of a solution. By adding a few drops of a phenolphthalein indicator to a solution one can tell if the pH of the solution has a pH greater than 9 by the red color present, or a pH less than 9 by the lack of color. Other dye materials can be chosen whose color changes indicate other pH ranges. For example, phenol red changes at pH 8, bromthymol blue at pH 7, and bromphenol blue at pH 4.

For convenience, these dyes are often deposited on a strip of paper. When a drop of solution to be tested is placed on the paper, the resulting color change is indicative of the approximate pH of the test solution. Dye indicator solutions or paper have the advantage of being quite inexpensive, very portable, and often suitable where only an approximate pH measurement is needed. On the other hand, where precise measurements are needed and / or the solution to be measured is colored, a pH meter is required. Accordingly, pH meter and electrode systems have been developed which respond in a precise manner to the pH of a solution.

To measure pH one can use any number of readily available ($25 – $100 ) pH probes. A pH probe acts like a battery that proportionately generates positive DC voltage for low pH, nothing for pH 7, and negative voltages for high pH values. So, all we have to do is measure this voltage and convert it to pH units.

But there are two problems involved. One problem is that pH is temperature sensitive, with the output voltage ranging from 54 millivolts per pH unit at zero degrees centigrade up to 74 millivolts per pH unit at 100° C.   This means that we have to manually vary the gain or conversion constant of our pH measurement to be able to correct for temperature of the solution being measured.

The second problem is a bit more complex and explains the previously high cost of pH instruments. The source impedance of our pH probe is 15 megohms for the "low–impedance" probes and ranges upwards into hundreds of megohms for special units. In order to measure pH, our voltage amplifier must have an input impedance that is very high compared with 15 megohms. Here is where CMOS electronics has come to the rescue, producing accurate and inexpensive pH meters.

The pH Electrode System

pH electrode systems are always composed of two electrodes, a sensing electrode and a reference electrode. For convenience, these two electrodes can be constructed in one common body which is called a combination electrode. This is the most popular form of the pH electrode system. The sensing electrode contains the specially designed surface whose voltage changes with the pH of the test solution. The reference electrode is used to complete the electrical measuring circuit. Its only function is to give a stable (unchanging) voltage to which the sensing electrode voltage can be compared.

The pH Sensing Electrode

In 1901 a German chemist named Fritz Haber discovered that the voltage at certain glass surfaces changed in a regular manner with the acidity of a solution. Modern pH sensing electrodes are a refinement of this fundamental discovery.

The essential features of a pH sensing electrode are:

The important requirements of this electrode are that ...

2.) the voltage at the internal solution / glass membrane surface ( E² ) remain constant, and ...

3.) the voltage at the glass membrane / test solution surface ( E³ ) changes proportional to the pH of the test solution.

It should be noted that the electrical resistance of the glass membrane is extremely high. Thus, a specialized voltmeter is required to measure the voltage from a pH sensing electrode.

The Reference Electrode

When using a voltmeter to measure the voltage at the pH sensing electrode, the electrical circuit must be completed. The reference electrode performs this function. Just a piece of bare wire could be used to complete the circuit. However, the voltage at its surface would change in an unpredictable fashion with time and test sample composition. Accordingly, a reference electrode is a wire which has been terminated with the proper choice of metal and surrounded by the proper metal ion solution, so as to give a constant voltage independent of time and test sample composition.

The essential features of a reference electrode are:

Electrode Lead
Electrode Cap
Reference Metal Wire
E¹ Reference Metal Ion Solution
E² Salt Bridge Solution
E³ Liquid Junction

The important requirements of this electrode are that the voltages E¹, E², and E³ remain constant with time and test sample composition.

The Combination Electrode

The combination electrode is a version of the pH electrode system in which the pH sensing electrode and the reference electrode are combined into one common body. All comments applicable to the individual electrodes are also applicable to their combination.

The advantages of this form of the electrode system include handling convenience and rugged construction. The single body construction also allows one to measure the pH of small sample volumes, as well as the pH of surfaces, such as soil and skin.

The pH Meter

A pH meter is a specialized voltmeter which has two fundamental requirements. First, it must be able to function accurately when measuring the voltage of extremely high resistance electrodes. Second, one must be able to change its sensitivity as a voltmeter to correspond to the pH / voltage characteristics of the electrode system.

Most modern pH meters use all solid-state electronics with very high input resistance or impedance characteristics. These meters measure the voltage of the pH electrode system while drawing extremely little current. Fortunately, the voltage change of a pH electrode varies linearly with pH units. At room temperature, a change of 1 pH unit causes a voltage change of about 60 millivolts (mV) or 0.060 volts. At O° centigrade (temperature at which water freezes) 1 pH unit change causes a 54 mV change. At 100° C. a 1 pH unit change causes a 70 mV change. Thus, a properly designed pH meter will have a temperature dial which varies the sensitivity of the meter to match the voltage from the electrodes.

Occasionally, specialized sensing electrodes fall short of delivering the full voltage which theory would predict. Accordingly, very versatile pH meters will also have an additional sensitivity control, called a slope control. This control, like the temperature dial, allows the analyst to vary the sensitivity of the meter to match the voltage from the electrodes.

The pH Standard

The voltage from the pH electrodes at any given pH value can be predicted approximately. However, for highest accuracy, the pH electrode system can be dipped into a solution of known pH and then the meter adjusted to correspond to this pH value. This adjustment is called standardizing the pH system. The solution used is called a pH standard buffer solution. The chemical composition of pH standard buffer solutions have been defined by the U.S. National Bureau of Standards. Such solutions may be prepared by a competent chemist or technician. They are also available from most pH meter manufacturers.

pH Value 25° C. Composition

  1.68 – Potassium Tetroxalate ( 0.05M )
  3.56 – Potassium Hydrogen Tartrate ( Saturated )
  4.01 – Potassium Hydrogen Phthaiate ( 0.05M )
  6.86 – Potassium Dihydrogen Phosphate ( 0.025M )
  9.18 – Borax ( 0.01M )
12.45 – Calcium Hydroxide ( Saturated )

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