Hardness Testing

Hardness is usually expressed as CaCO₃. Several methods of hardness testing can be used to analyze hardness. Some of methods of hardness testing that are commonly used as follow:

1. Hardness testing with Total Method

This method is used to calculate hardness based on concentration of divalent ions in water such as Ca²⁺, Mg²⁺, and SR²⁺. Calculation of hardness based on concentration of divalent ions can be calculated by using following equation:

Hardness as CaCO₃ (mg/l) = M²⁺ (mg/l) x (50 / equivalent weight M²⁺)

Where M²⁺ = divalent ion metal

Example: From the analysis of water is known that water contain ions (mg / l)and only divalent cations  Ca²⁺, Mg²⁺, and SR²⁺  that are calculated as the cause of hardness, so total hardness based on concentration and equivalent weight of each divalent  ion of hardness as following table:

Figure 1: Table of Hardness Testing with Total Method

2. Hardness testing with EDTA (Ethylenediamine Tetra Eacetic Acid) Titration Method

Complexometri titration reaction includes the formation of complex ions or formation of neutral molecules that dissociate in solution. Thus fundamental requirement of complex formation is high level of solubility. End point titration is determined by metal indicator or by potentiometer and spectrophotometer.

This method uses solution of EDTA (ethylenediamine tetra eacetic acid) as the standard solution. Metal indicator is used to determine the end point of titration. The indicator is usually used is Eriochrome Black T (EBT).

Eriochrome Black T as indicator will form complex compound entirely with EDTA which is added, in other words, the addition of excess EDTA solution is indicated by changes in solution color from red to blue. This reaction is running complete at pH 8-10. Buffer solution is added to maintain pH solution. Ca²⁺ and Mg²⁺ will form complex compounds in red wine color with EBT.

M²⁺ + EBT →(M EBT)_{complex compounds in red wine color}

Changes will more clearly when the pH gets higher, but high pH can cause hardness ions loss from solution, due to precipitation of Mg(OH)₂ and CaCO₃. At pH> 9, CaCO₃ already begin to form.