The HI38033-100 are high quality reagents that are pre-measured, allowing for users to achieve fast and accurate total hardness measurements with the chemical test kit. These reagents follow the EDTA titration method to determine total hardness concentration as calcium carbonate (CaCO3). 

For determination of total hardness, the sample solution is first buffered to pH 10. The calmagite color indicator is added to form weak complexes at this basic pH with magnesium and calcium, forming a red colored complex. Addition of the EDTA titrant solution also forms complexes with the remaining free magnesium and calcium ions. Once all of the free magnesium and calcium ions have been complexed, the excess EDTA preferentially removes the ions bound with the calmagite color indicator, forming a stronger complex. The removal of these metal ions from the indicator causes a color change from red to blue, indicating the endpoint of the titration. The number of milliliters of titrant used to reach the appropriate endpoint will determine the total hardness as mg/L CaCO3. These reagents are designed to be used with samples that have an expected total hardness range of 0 to 30 gpg (grains per gallon) CaCO3.