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pH and how to measure it
Knowing the pH value of a solution or fluid is very important for many chemical and analytical tasks and its measurement determines any follow up measurements.

Taking a pH measurement often seems to be trivial, which is the reason why pH measurements are frequently not questioned.  But to make a useful pH measurement close attention must be paid to the measurement's details.  To make a proper pH measurement and avoid errors you must first be familiar with the basics of pH measurement.

The elementary questions are:
  • What defines the pH-value?
  • How do I measure the pH-value?
  • Where and why are pH measurements made?
What does the pH value of a pH measurement mean?
The water molecule has the property of dissociating into two ionic components in aqueous solutions.

H2O <-> H+ + OH-

The H+ ion is termed hydrogen ion or proton, the OH- ion hydroxide ion.

The pH value describes the activity of hydrogen ions in aqueous solutions typically on a scale of 0 to 14.  Based on this pH scale, liquids are characterized as being acidic, alkaline or neutral: a solution which is neither acidic nor alkaline is neutral.  This corresponds to a value of 7 on the pH scale.  Acidity indicates a higher activity of hydrogen ions and a pH measurement value lower than 7.  Alkaline solutions are characterized by a lower hydrogen ion activity or higher hydroxide ion activity, respectively and a pH measurement value above 7.  The graph below uses examples to illustrate the pH scale.

pH measurement scale

The pH scale is logarithmic.  A difference of one pH measurement unit represents a tenfold, or ten times increase or reduction of hydrogen ion activity in the solution.  This explains how a solution's aggressiveness increases with the distance from the neutral point.

One of the keys to understanding pH measurements is the term "activity", because the activity is temperature dependent it is not the same as the solution's concentration.  Activity, a, is defined as the product of the activity coefficient, y, which is always smaller than 1, and the actual concentration, c, of the concerned compound (a=y * c).

Activity is the effective concentration of a chemical compound, or more precisely its particles in the solution.  In a real solution the activity is constantly smaller than the actual concentration.  This is true because only in an ideal (infinitely thinned) solution the soluted particles do not affect each other.  In this case they are spread apart because many molecules of the solvent are between them.  The difference between activity and concentration becomes apparent in real solutions of ions, because ions interact with each other as a result of their electric charge.  To describe or calculate the characteristics of a solution as exactly as possible the activity and not the concentration must be used in the mass action law.

How do I measure the pH value?
The pH value can be measured using electrochemical measuring systems, litmus paper, or indicators and colorimeters.  The easiest way to take a pH measurement is to use litmus paper or a colorimeter.  The advantage of this type of pH measurement is that the pH range is well known and they are easy to apply.  Unfortunately in many cases litmus paper and colorimeters are not accurate enough to make high quality pH measurements, because the pH value transition point depends on the user.

Another pH value measurement possibility is amperometry.  The advantage of amperometry as a pH measurement method is that it is simple to use.  In amperometric pH measurements hydrogen generation occurs on a noble metal.  When combined with a less noble metal a power distributing galvanic cell is formed.  Because hydrogen ions are generated the cell's current depends on the pH value.  The disadvantages of this method is that differences in the sample composition create very large errors in pH measurements and the method cannot deliver dependable results in extremely concentrated acids and bases due to effects related to the pH glass membrane.

In special cases the pH value measurement can be made using conductometry (conductivity measurement).  With this pH measurement method any membrane effects are minimized because of the measurement technique.  The advantage of this pH measurement method is that it is relatively easy to use.  The disadvantage is that a conductivity measurement measures all ion activity not just hydrogen ion activity.  Additionally this pH measurement is only reproducible and safe at low ion concentrations.

A relatively new method for pH value measurement is the use of an ion selective field effect transistor (ISFET).  Briefly the ISFET is a transistor with power source and drain, divided by an isolator.  This isolator (gate) is made of a metal oxide where hydrogen ions accumulate in the same way as an electrode.  The positive charge that accumulates outside the gate is 'mirrored' inside the gate by an equal negative charge generates.  Once this happens the gate begins to conduct electricity.  The lower the pH value the more hydrogen ions accumulate and the more power can flow between source and drain.  The ISFET sensors, similar to glass pH electrodes, act according to the Nernst equation.  The advantage of an ISFET is that they are very small.  The actual field effect transistor is only 0.2 mm2.  The disadvantage of using an ISFET for pH measurements is that they have comparatively short durability and low long-term stability with a typical use life cycle being in range of weeks.

The most common method of pH value measurement is the use of pH measurement electrodes, like the IoLine series from SI Analytics.  These pH measurement devices are electrochemical sensors which consist of a measuring electrode and a reference electrode.  The pH measurement electrode is made of special glass which, due to its surface properties, is particularly sensitive to hydrogen ions.  The pH measurement electrode is filled with a buffer solution which has a pH value of 7.  When placing the pH measurement electrode into a test solution, the change in voltage is measured by the pH electrode by comparing the measured voltage to the stable reference electrode.  This change is recorded by the pH meter, such as the pH3110 Field pH Meter, Lab 850 Benchtop pH Meter, or ProLab 1000 Professional Bench Top pH Meter, and converted into the pH measurement value displayed.

Of these pH value measurement methods, at this time, the best one is the use of pH electrodes.  There is no other pH measurement system that provides as good reliability, accuracy, and speed of pH measurement across the complete pH range.  The minimal disadvantage of using glass pH electrodes for pH measurement method is the fact that glass electrodes are delicate and should be handled with care.  This disadvantage is overcome by using gel filled pH electrodes in applications where the electrodes must be more robust.

Where and why are pH measurements made?
pH value measurements are made in a wide variety of applications:
  • Pharmaceutical and chemical industry
  • Quality control
  • Controlling of chemical reactions
  • Food production: quality characteristics (pH curve of meat after slaughtering), storage life (reproduction of harmful bacteria depends on pH value), control and monitoring of micro biotical food production
  • Quality control at dairies: fresh milk has a pH-value of 6.6 - 6.8. Storing: pH value decreases, milk becomes sour, coagulation at a pH value of 4.7
  • Quality control and efficient production in bakeries: dough needs a low pH value to rise, bread size depends on its pH value, at higher pH values bread is very firm and does not rise much
For general information on calibrating a pH measurement electrode please click here.
For information on different pH measurement electrode calibration methods and the difference between pH solutions please click here.
For specific information on pH maintenance please click here.
For specific information on pH glass electrode types please click here.
For information on pH electrode reference systems and diaphragms click here.
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