pH Meters

 

pH meters are used to measure the acidity (or alkalinity) of a solution by measuring its hydrogen ion concentration. Such instruments are extremely important in both industry and academia and have a very wide field of application.  They consist basically of two half cells joined by some form of electrolyte bridge and a high impedance input voltmeter. A diagram showing the basic components of a pH meter is shown in figure 6.

 

Figure 6. The Basic pH Meter

 

The first pH meters that became commercially available were those produced by Beckman Instruments and by the Danish firm Radiometer in the mid "thirty's". The design and development of these meters were provoked by needs of the fruit farmers who found that it was essential to be able to measure the acidity of their fruit juice products. In the original model, the electrode that measured the hydrogen ion concentration was a thin walled glass bulb that was porous to hydrogen ions and generated about 0.06 V per pH unit. In a somewhat different form, the same type of electrode is used in some instruments today. The reference electrode can be one of a number, including the calomel electrode and the silver chloride electrode (both pf which will be described later) but there are a number of proprietary electrodes that have been developed with special attributes such as the capability of measuring the pH of non-aqueous liquids. The meter is a high input resistance (20-1000 MW) voltmeter containing appropriate operational amplifiers and the output is usually displayed in units of both volts and pH units. Most instruments are fitted with temperature compensation sensors and they are calibrated using solutions of known pH. An example of an ISFET pH Tester available from Pulse Instruments that employs non-glass electrode technology is shown in figure 7. The meter is compact, waterproof, has a rapid response time and can measure samples as small as a single drop. Single point calibration at a pH of 7.0 is normally all that is necessary but a model is also available for higher accuracies that employ three-point calibration at pH values of 4.0, 7.0, and 10.0. The measuring electrode consists of a silicon chip pH sensor that is virtually unbreakable and eliminates the need to use fragile glass electrodes.

 

There are a multitude of different types of pH meters provide by instrument companies established in almost every country in the western world. They vary widely in price but not so widely in accuracy and reliability as their specifications are mostly very similar irrespective of their cost.

 

 

Figure 7. The ISFET pH Tester Available from Pulse instruments.

 

Physically their design can range from pen-like portable devices to very complicated and sophisticated laboratory instruments that can be interfaced with computers. The less expensive models sometimes do not have automatic temperature compensation.