So far, we have recognized pH and specific gravity as ways to determine chemical concentration in cleaning solutions. This blog will explore additional techniques that may also be used for this purpose.
Pure water is a poor conductor of electricity. As chemistry is added to water, it’s ability to conduct electricity is increased. Conductivity is a measure of a liquid’s ability to conduct electricity. In an earlier blog, conductivity and resistivity were used as an example of two methods used to measure essentially the same parameter in different ways but with one being more sensitive than the other. In the case of determining chemical concentration in cleaning solutions, we are dealing with materials that are relatively good conductors of electricity so conductivity is usually a more appropriate and sensitive measure than is resistivity.
Conductivity is nearly always measured using an electrical probe immersed directly in the cleaning solution. The probe is wired to a meter which provides an analog or digital readout. The unit of measure for conductivity is Siemens per meter which is abbreviated S/m. Frequently the “/m” is dropped and measurements are expressed in Siemens (S). In many cases, conductivity is much lower than 1 S/m and is therefore expressed in either mS/m (m = milli or 1/1,000) or µS/m (µ = micro or 1/1,000,000).
The conductivity of a liquid is usually directly (although not necessarily linerally) related to its concentration. However, once the concentration reaches a certain point, the solution is considered “saturated.” The conductivity of a saturated solution will not change as more chemical is introduced. As with pH, one must make sure that conductivity is an appropriate way to measure chemical concentration for a particular chemical and establish that the effectiveness of cleaning is, indeed, directly related to its concentration.
Although we usually think of titration as a method of determining the pH of a solution (acid or base), it can also be used as a test for a number of chemical components present in a cleaning solution as well. As a measuere of these components, titration can indicate concentration. Measuring concentration by titration will be specific to a particular chemistry depending on the component being measured. The following example from a product data sheet is typical.
In this case, the chemical supplier offers a kit (consisting of a small vial and the required reagents) or a method that can be used by anyone familiar with chemistry. Notice that the “home-brew” procedure requires a means to measure pH while the titration kit uses an indicator that changes color when the “end point” has been reached.
Note – The procedure above references a “N” solution. N stands for “Normal” which is a measure of the concentration of an acid in water. It is based on the number of Hydrogen atoms available per molecule in the acid. A 1.0 N solution of HCl would be 1 gram of HCl in one liter of water. In the case of H2SO4, 1 gram of H2SO4 in one liter of water would be a 2N solution because two hydrogen atoms are available from each molecule. Solutions with specific concentrations are available from chemical suppliers.
As with pH and specific gravity, there are devices which can automatically perform conductivity measurements. Automatic titrators are also available. The next blog will discuss the benefits and possible problems with automatic chemical makeup using these devices.
– FJF –