The hardness of water has traditionally been identified by the “soap” test. Hard water reacts with soap to form a precipitate (aka soap scum). This precipitation reduces the effectiveness of the soap and is seen as not being able to get the soap to lather with water as with washing your hands. Treated hard water, as with a water softener, will lather very easily since the ions that contribute to hardness are replaced with sodium or potassium as the water is treated.
Water hardness is primarily the quantitative measure of calcium and magnesium ions in water. In water, the total sum of the Ca2+ and Mg2+ ions is a parameter called "total hardness" which allows you to classify the degree of hardness of the water. Other ions are known that certain other ion species are known to contribute to the overall water hardness including iron, zinc, and manganese. The measure and subsequent control of water hardness is essential for detergents to work, to prevent scaling in boilers and cooling towers, and for basic home use.
Hardness in the forms of calcium, magnesium and total can also be measured photometrically with reagents. Photometric analysis is based on the Beer-Lambert principle of absorbance. Photometric analysis products include handheld chemical test kits, colorimeters, portable and benchtop photometers, and spectrophotometers. Photometric methods include reagent chemistries based on the Calmagite and EDTA colorimetric methods as found in the Standard Methods for the Examination of Water and Wastewater. Total hardness is an adaptation of EPA Method 130.1.
Hardness is commonly expressed as mg/L of calcium carbonate (CaCO3). The portable and benchtop photometers allow hardness to be expressed as French degrees (of), German degrees (odH) and English degrees (oE). For water conditioning companies the chemical test kit can measure as grains per gallon (gpg).
Below are the most common products used for measuring water hardness.
Titration systems with precision dosing pumps are available to measure hardness. These systems can use either the calcium ISE or a photometric probe to determine the endpoint and results can be displayed in a variety of formats based on the specific need.
Ion-Selective Electrodes and Photometeric Probes
Potentiometric titration systems can use a calcium ISE or a photometric probe for the titration of hardness. The use of a titration system allows for a precision measurement that can be automated for multiple measurements.
The calcium ISE is a liquid membrane sensor in which an ionophore is embedded in a matrix that reacts with calcium in a lock and key fashion. The calcium ISE is available as either a single half-cell or combination design complete with a reference cell.
A photometric probe is a sensor that generates a steep millivolt potential when a solution with an indicator changes color when adding a titrant. Calcium hardness is determined with EDTA as a titrant and calconcarboxylic acid (Patton-Reeder) indicator. In this complexometric reaction calcium binds to the indicator to change the color of the solution from blue to red. As EDTA is added to the solution the hardness binds with it instead of the indicator. The solution will change from red to blue when there is no free hardness available to react with the indicator. The photometric probe has a wavelength of 625 nm in which there is high transmittance when the solution is red but has a low transmittance when the solution turns blue.
Spectrophotometers are available that have hardness methods in several ranges pre-programmed into the meter. The spectrophotometer offers the highest precision due to the quality of the optical system that has a wavelength accuracy of +/- 1.5 nm. The spectrophotometer allows for custom methods and can display hardness results in a variety of chemical forms and units of measure.
Benchtop photometers include multiparameter versions for water quality, wastewater and swimming pool analysis. Each photometer is customized with the parameters used by a specific industry. All benchtop have a digital pH electrode input allowing it to be used as a traditional pH meter. All benchtop photometers can display results in multiple chemical forms.
Single parameter photometers are available to measure calcium, magnesium and total hardness. Multiparameter portables are available that measure total hardness along with other key water quality parameters including iron and manganese. The portable photometers have a CAL Check feature that provides performance verification and, if needed, recalibration using the CAL Check standards. The portable meters are available as a meter only option or as a kit. The kit version include a rugged carrying case and CAL Check standards.
Checker HC, like a chemical test kit, are reagent based in which there is a color change based on concentration. The handheld colorimeters, like portable and benchtop instrumentation, use the Beer-Lambert principle to determine the color change. The change in color is not as subjective as interpretation with the naked eye.
Chemical Test Kits
Chemical Test Kits (CTK) are available as a single parameter that can measure total hardness as mg/L (ppm) or grains per gallon (gpg) as calcium carbonate. Multiparameter CTKs are also available that include the hardness parameter for water quality, marine science, environmental, and water treatment.
Reagents include the reagents used with Checker HC and photometers.
Solutions for hardness measurement include ISE standards and ISA for calibration and measurement. Other solutions include the CAL Check standards for verifying meter performance and, if needed, the calibration of portable photometers. Each CAL Check standard is supplied with a Certificate of Analysis stating the accuracy and traceability of the standard.
Accessories include spare cuvettes and caps for photometers, magnetic stirrers to ensure adequate mixing when using an ISE and the spare parts for the calcium ISE.