pH METERS 41 



(2) Degree of accuracy required. — Only rarely in biological work is an accuracy 

 greater than pH 0-02 required, and for many routine purposes such as media produc- 

 tion the changes produced by heat sterilisation, loss of carbon dioxide, etc., are so 

 great that an accuracy of pH 0-05 is ample. For economy automatic temperature 

 regulation may be omitted when accuracy greater than pH±0-2 is not required. 



(3) Power supply. — If the convenience of being able to plug in directly to electric 

 power mains as sole power supply is required, it is necessary to have in the meter or 

 externally a constant voltage transformer to eUminate the effect of variation in the 

 mains voltage and also frequency stabilisers to avoid difficulties due to variations in 

 the supply frequency, which occur when there are heavy demands on the electricity 

 mains. The use of certain electrical apparatus in neighbouring laboratories has been 

 reported as troublesome. As an alternative, chargeable accumulators may be used, 

 high capacity car accumulators being satisfactory. Care in the maintenance and 

 charging of the batteries is necessary and drifts in potential may occur if they are 

 not kept in a state of constant charge. With the low-consumption thermionic valves 

 available to-day it is possible to use dry batteries of the electric bell or deaf- aid type 

 in many meters. These may last for months and meters can thus be made self- 

 contained and light in weight for portability. It is advisable to keep a replacement 

 battery in stock and to replace the meter battery when potential drift is noted. 



(4) pH readings. — In some instruments a null-point balance method is employed, 

 a dial is rotated to bring the galvanometer needle back to zero and the pH value is 

 read from the calibrated dial. A " magic eye " cathode tube can be used instead of a 

 galvanometer for obtaining null-point balance. In other instruments the throw on the 

 galvanometer needle is proportional' to pH differences and the galvanometer dial 

 is caHbrated in pH units. In some instruments the whole pH range can be covered 

 on one dial, but in instruments reading to greater accuracy it may be necessary 

 to have a range-change switch to cover the whole pH scale. This is particularly 

 annoying when the scale-change is in the neighbourhood of pH 7-0, which is about 

 half way along the pH range, but is also in the middle of the biologically impor- 

 tant range, and constant switching in the middle of readings may be necessary. 



(5) Glass electrode and cell. — The glass electrode is generally in the form of a 

 thin bulb blown in the end of a glass tube. The electrical resistance of the glass 

 membrane is high (5-500 megohms), requiring specially designed electrometers, 

 precautions against short circuits, and shielded leads. In the early days of the glass 

 electrode both the capillary electrometer and Lindemann electrometer were used as 

 null-point instruments, but the development of thermionic electrometer valves has 

 made possible the use of relatively simple galvanometer milliammeters. Sometimes 

 a long capillary tube of glass is used instead of the thin bulb membrane. To complete 

 the cell any half-cell may be employed ; this used to be a calomel electrode, but now 

 the silver : silver chloride electrode is gaining popularity. Usually the half-cell is 

 filled with saturated potassium chloride solution which also serves to minimise 

 liquid : liquid junction potentials. 



(6) Special biological proble7ns. ^The conventional glass electrodes may be found 

 unsuitable for some purposes and special modifications may be necessary. The usual 

 apparatus is difficult to sterilise after use with cultures of pathogenic bacteria and the 



