pH Meter uses and Calibration

In this page you will find information on how pH meters work how to use a pH meter, how to calibrate a pH meter and lots of other interesting information.

Contents:0) The pH scale
1) How a pH meter works

2) Calibration of a pH meter, How to re-calibrate

3) Types of pH meter avalible

4) What are pH meters used for?

5) Interesting info about pH
0) pH Scale

1) How a pH meter works:

The meter circuit is fundamentally no more than a voltmeter that displays measurements in pH units instead of volts. The input impedance of the meter must be very high because of the high resistance—approximately 20 to 1000 MΩ of the glass electrode probes typically used with pH meters. The circuit of a simple pH meter usually consists of operational amplifiers in an inverting configuration, with a total voltage gain of about -17. The inverting amplifier converts the small voltage produced by the probe (-0.059 volt/pH in basic solutions, +0.059 volt/pH in acid solutions) into pH units, which are then offset by 7 volts to give a reading on the pH scale. For example:

  • At neutral pH (pH 7) the voltage at the probe’s output is 0 volts. 0 * 17 + 7 = 7.
  • At alkaline pH, the voltage at the probe’s output ranges from > 0 to +0.41 volts (7 * 0.059 = 0.41). So for a sample of pH 10 (3 pH units from neutral), 3 * 0.059 = 0.18 volts), the output of the meter’s amplifier is 0.18 * 17 + 7 = 10.
  • At acid pH, the voltage at the probe’s output ranges from -0.7 volts to < 0. So for a sample of pH 4 (also 3 pH units from neutral, but in the other direction), 3 * +0.059 = +0.18 volts, the output of the meter’s amplifier is -0.18 * 17 + 7 = 4.

The two basic adjustments performed at calibration (see below 2) set the gain and offset of the inverting amplifier.

2) Calibration of a pH meter and how to re-Calibrate
Calibration with at least two, but preferably three, buffer solution standards is usually performed every time a pH meter is used, though modern instruments will hold their calibration for around a month. One of the buffers has a pH of 7.01 (almost neutral pH) and the second buffer solution is selected to match the pH range in which the measurements are to be taken: usually pH 10.01 for basic solutions and pH 4.01 for acidic solutions . The gain and offset settings of the meter are adjusted repeatedly as the probe is alternately placed in the two calibration standards until accurate readings are obtained in both solutions. Modern instruments have completely automated this process and only require immersing in each solution once, or at worst, twice.

The calibration process correlates the voltage produced by the probe (approximately 0.06 volts per pH unit) with the pH scale. After calibration, the probe is rinsed in distilled, deionized water to remove any traces of the buffer solution, blotted with a clean tissue to absorb any remaining water which could dilute the sample and thus alter the reading, and then immersed in the sample. Between uses, the probe tip, which must be kept wet at all times, is typically kept immersed in a small volume of storage solution, which is an acidic solution of around pH 3.0. Alternatively, the pH 7.01 calibration solution can be used, but this results in a need for more frequent calibration. In an emergency, tap water can be used, but distilled or deionised water must never be used for longer-term probe storage as the relatively ionless water ‘sucks’ ions out of the probe, which degrades it.

Occasionally (about once a month), the probe should be cleaned using pH-electrode cleaning solution; generally a 0.1 M solution of Hydrochloric Acid (HCl) is used [2], having a pH of about 1.

Re-calibration:

If you have a pH meter with one calibrating point at pH 7.0, like the MM Instruments pH001 re-calibration is simple. A re-calibration of a pH meter would be performed if at pH7.0 the meter reads pH6.2 or any reading other than 7.0 in a accurate buffer solution. If at pH7.0 buffer the meters display says say 6.2 for example you would simply adjust the calibration screw on the meter whist the meter is still in the solution untill you reach pH7.0 on th display. Please be aware that a buffer solutions value can be slightly affected by the temperature of the solution. If you use MM Instruments buffer solution a temperature reference guide is provided with every buffer so you can see the accuracy of the buffer at different temperatures, other competators may not!.

MM Instruments Buffer Solution

If you are strugguling with the calibration of you meter just contact us by email and we will respond and rosolve any trouble you may have.

3) Types of pH meter

pH meters range from simple and inexpensive pen-like devices to complex and expensive laboratory instruments with computer interfaces and several inputs for indicator (ion-sensitive, redox), reference electrodes, and temperature sensors such as thermoresistors or thermocouples. Cheaper models sometimes require that temperature measurements be entered to adjust for the slight variation in pH caused by temperature. Specialty meters and probes are available for use in special applications, harsh environments, etc. Pocket pH meter are readily available today for a few notes that automatically compensate for temperature (ATC) like most MM Instruments pH testers.

4) What pH meters are used for, pH meters are used all around the world for testing all sorts of things including:

Hydroponics, Aquitics, Soils, Catering, Swimming pools, Lakes, Ponds, Beer Making, Wine making, Reptile keeping and a very important factor in our ever changing world, THE ENVIORONMENT


5) Interesting info about pH

Blood pH: Although many people are unaware of the fact, maintaining the acid/base balance of your blood is actually vital to your survival. If the pH of your blood drops below 7.2 or rises above 7.6, then very soon your brain will no longer be able to function normally and you will be in dire straits. As luck would have it, although you cannot consciously detect your blood pH, the human body does in fact have an elegant but effective means of coping with every change in pH, large or small. This relies on three interlinking objects: buffers, the lungs and the kidneys

Acid Rain:

Rain is always slightly acidic because it mixes with naturally occurring oxides in the air. Unpolluted rain would have a pH value of between 5 and 6. When the air becomes more polluted with nitrogen oxides and sulphur dioxide the acidity can increase to a pH value of 4. Some rain has even been recorded as being pH2.

The effect of acid rain:

Acid rain can be carried great distances in the atmosphere, not just between countries but also from continent to continent. The acid can also take the form of snow, mists and dry dusts. The rain sometimes falls many miles from the source of pollution but wherever it falls it can have a serious effect on soil, trees, buildings and water.

The principal cause of acid rain is sulphuric and nitrogen compounds from human sources, such as electricity generation, factories and motor vehicles. Coal power plants are one of the most polluting. The gases can be carried hundreds of kilometres in the atmosphere before they are converted to acids and deposited. In the past, factories had short funnels to let out smoke, but this caused many problems; thus, factories now have longer smoke funnels. However, this causes pollutants to be carried farther, causing greater ecological damage.

Forests all over the world are dying, fish are dying. In Scandinavia there are dead lakes, which are crystal clear and contain no living creatures or plant life. Many of Britain’s freshwater fish are threatened, there have been reports of deformed fish being hatched. This leads to fish-eating birds and animals being affected also. Is acid rain responsible for all this? Scientists have been doing a lot of research into how acid rain affects the environment.