Waterguide - pH value
What is pH value?
pH is the abbreviation for “potential of hydrogen”. The pH value is defined as the negative decadic logarithm and is the opposite of the pOH value. The more hydrogen ions in a solution, the lower the pH value. The scale of the pH value (can be seen in the figure below) ranges from 0 – 14, with 0 being strongly acidic and 14 being strongly basic. The range around pH 7 is considered neutral. Drinking water has a pH value of 6.5 – 9.5.
How is the pH value calculated?
To calculate the pH value, you need to know whether the solution is a strong/weak acid solution or a strong/weak base solution. The general formula for calculating the pH value is as follows:
The following demonstrates the relationship between the pH value and pOH value:
The pKa or pKb value is important for acids and bases. The higher the pKa value in an acid, the stronger the acid (dissociates very strongly in water). The lower the pKb value of a base, the stronger the base. They are generally constants that do not have to be additionally calculated.
The acid constant KA and the base constant KB can be calculated using the law of mass action:
How is the pH value metrologically determined?
The easiest way of determining this is via visual or colourimetric colour changes using indicator dyes. Adding a liquid indicator dye to the solution causes it to change colour. By comparing the colour change with a scale (see image above for colours), you can more or less estimate the range of the pH value.
Another method that is similar to using indicator dyes is to use measuring strips. These can be simply torn off the roll and inserted into the solution. The measuring strip changes colour in the solution. The pH value range is read in the same way as with the indicators, using a scale.
It is also possible to use a pH meter (potentiometer), which provide an accurate, highly precise pH value measurement. The pH meter consists of an indicator electrode in a glass sphere filled with a buffer solution, a potential difference measuring device and a digital display. The glass sphere is immersed in the solution that is to be measured, and hydrogen ions attach to the glass sphere. The pH value is output on the display based on the potential difference between the inside and the outside of the glass sphere.
Significance of pH value for technology and environment
In the environment:
It is important for plant life that the soil is slightly acidic. If the soil is in the neutral or basic range, plants are unable to take up iron through their roots, which leads to iron deficiency. The iron deficiency can then lead to growth problems of the flora.
The pH values in the human body are different. For example, the blood requires a pH value of 7.35 – 7.43, the skin needs a pH value of 5.5 and saliva a pH value of 6.5 – 7.2.
As mentioned above, drinking water needs a pH value of 6.5 – 9.5. Lower pH values would cause copper and lead to dissolve from pipes and enter the drinking water. These two metals are toxic for humans. This pH value range is also important for the lime-carbonic acid equilibrium.
In technical plants, the pH value can lead to undesirable changes in the state of aggregation. Furthermore, in certain reactions, the pH value can function as the catalyst and influence the reaction speed.
The pH value is particularly important in waste water or water treatment processes. In certain pH value ranges, some substances automatically precipitate and can be removed through sedimentation. Some treatment steps also require an acidic or basic pH value in order to prevent unwanted reactions due to a substance in the waste water.
Important pH values in water
Rainwater: pH 5.6
Mineral water: pH 4.5 – 5.6 (slightly acidic because it contains carbonic acid)
Seawater: pH 8.0