pH tester with Temperature  Ref: hi-98127-033001001
	The pH concept was introduced in 1909 by the Danish chemist Sorensen, and is a measure of the acidity of alkalinity of aqueous solutions. pH is defined as the negative logarithm of the hydrogen ion activity. Although we normally refer to Hydrogen ion activity, in reality it is the solvated form of the Hydrogen ion or the Hydronium ion which is being referred to. Solutions which have a pH close to 7 are termed neutrals. Those with higher pH values are called alkaline or base, whereas those lower than 7 are termed acidic. Since the Hydrogen ion activity seldom attains values higher than 1 or lower than 10-14, then pH scale is restricted to the range 0 to 14. There is occasional need to read pH values above 14 or bellow 0 and some pH meters are capable of doing so if required. pH is measured by using 2 electrodes which have (in recent years), been combined to form a single probe which is called a combination pH electrode. This consists of a pH Half Cell (or Measurement Cell), whose voltages varies proportionally to the Hydrogen ion activity of the solution being measured, and a reference electrode (known as the reference Half Cell), which provides a stable and constant reference voltage. The measurement part of an electrode consists of a thin membrane of Hydrogen sensitive glass blown on the end of an inert glass tube. This tube is filled with electrolyte, and the signal is carried through a Ag/AgCl wire. A similar tube (but without the Hydrogen sensitive glass) makes up the reference part of the electrode. A small whole is formed in the reference electrode tubing and a filter inserted. This filter allows the current to pass through and thus forms a circuit to the wire within the measurement cell via the Hydrogen sensitive glass bulb. The meter measures the difference between the pH Half Cell and the Reference Half Cell in millivolts DC. This millivolt reading is displayed in pH units. Unlike other types of sensor or probe, a pH electrode has a certain life expectancy, after which it will fail. This is due to the constant ion-exchange reactions at the hydrated phase boundary which causes the alkaline constituents to be continuously depleted. As this occurs on both sides of the glass, there comes a time when the electrode fails. With proper care and maintenance, an electrode should last more than 2 years, although exposure to high contaminants or temperature extremes will shorten the electrode life. On a day-to-day basis the factors that affect the pH electrode performance are more basic. The Hydrogen ion sensitive glass bulb at the end of the electrode needs to be kept hydrated - this is best achieved using a storage solutions ( part number HI-70300) or tap water. Distilled or De-ionised water should not be used. Additionally the electrode should be kept clean. If either the reference cell filter (known as the liquid junction), becomes blocked, or the glass bulb is scratched or dirty, the electrode will either be slow to read or may fail altogether. In this case the electrode can usually be re-activated by using a specialist cleaning solution - HANNA part number HI-7061L.