Calcium hardness in water is typically measured using a titration method involving EDTA (ethylenediaminetetraacetic acid). This method relies on the selective complexation of calcium ions with EDTA at a specific pH.
Here's a breakdown of the process:
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Sample Preparation: A known volume of the water sample is collected.
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pH Adjustment: Sodium hydroxide (NaOH) is added to the sample to raise the pH to a highly alkaline level, usually between 12 and 13. This high pH is crucial because it helps prevent magnesium ions from interfering with the titration by precipitating them out of the solution as magnesium hydroxide [Mg(OH)₂]. At this pH, EDTA will selectively bind with calcium ions (Ca²⁺).
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Titration: The prepared sample is then titrated with a standardized EDTA solution. EDTA acts as a chelating agent, forming a stable, colorless complex with calcium ions. The EDTA is slowly added to the water sample while the mixture is continuously stirred.
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Endpoint Detection: An indicator dye, such as murexide or calcon, is added to the sample before the titration begins. This indicator changes color when all the calcium ions have reacted with the EDTA. At the equivalence point (endpoint), where the EDTA has complexed with all the calcium ions, the indicator undergoes a distinct color change, signaling the end of the titration.
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Calculation: The volume of EDTA solution used to reach the endpoint is recorded. Using the concentration (molarity) of the EDTA solution and the volume used, the concentration of calcium ions in the original water sample can be calculated. The calcium hardness is usually expressed in milligrams per liter (mg/L) as calcium carbonate (CaCO₃).
In summary, the steps are:
- Add sodium hydroxide to raise pH to 12-13.
- Titrate with EDTA solution.
- Use an indicator dye to identify the endpoint.
- Calculate calcium hardness based on the volume of EDTA used.
This titration method is a standard procedure for determining calcium hardness in water and provides accurate and reliable results.
