Water hardness is mainly caused by calcium and magnesium ions. Over time, excessively high water hardness can negatively affect water systems and pipes. Accurate and reliable monitoring of water hardness is important for protecting company assets. Controlling the water hardness can decrease the risk of clogging and improve heat transfer.

ASTM D8192 describes the photometric titration of the total, calcium, and magnesium hardness in water with an optical sensor for objective endpoint indication, increasing precision and reliability. The method is suitable for both colored and colorless samples such as groundwater, surface water, wastewater, and drinking water. Using a fully automated OMNIS system equipped with an Optrode ensures that the sample preparation and analysis are repeatable. This increases the precision and reliability, allowing accurate determination of these parameters.

This application is demonstrated on wastewater (moderate yellow hue), tap water, and leachate. No sample preparation is required.

The determination is carried out on an automated system consisting of an OMNIS Sample Robot S, OMNIS Dosing Modules, and an OMNIS Advanced Titrator equipped with an Optrode (Figure 1).

In the first titration, the total hardness is determined. The calcium hardness is determined in a separate titration. The magnesium hardness is subsequently calculated from the difference of the two titration results.

An appropriate amount of water sample is pipetted into the titration beaker. For the total hardness determination, Eriochrome Black T indicator solution and a buffer solution (pH 10) consisting of sodium tetraborate and sodium hydroxide are added to the sample. For the calcium hardness determination, sodium hydroxide and hydroxyl naphthol blue indicator solution are added to the sample. The prepared samples are then titrated with standardized EDTA until after the equivalence point.

Determination of the water hardness according to ASTM D8192 using the Optrode ensures objectivity when determining the endpoint in comparison to the subjectivity of the human eye. This results in increased precision and reliability when monitoring water hardness in colored and colorless samples.

Thanks to its glass shaft, the Optrode is very easy to clean and 100% solvent resistant. Additionally, the Optrode is maintenance-free. Its space-saving design allows it to be used conveniently in automated systems.

Using a fully automated system further increases the precision and reliability of the determination as all sample preparation steps can be automated. Furthermore, the modularity of the OMNIS system allows the analysis of various other parameters, such as alkalinity or conductivity on one system. Efficiency can be even further improved with the ability to run up to four determinations in parallel on the same OMNIS system. This is regardless of whether it is the same parameter or different parameters.