To prepare a sample of hard water, you can dissolve a known quantity of calcium carbonate (CaCO3) in a controlled manner. Here's a step-by-step method:
Materials Needed:
- Pure, dry Calcium Carbonate (CaCO3)
- Dilute Hydrochloric Acid (HCl) - Use caution when handling acids
- Distilled Water
- Beaker
- Water bath or hot plate
- Volumetric flask (1 Liter)
- Pipette
Procedure:
- Dissolve CaCO3: Accurately weigh 1 gram of pure, dry CaCO3. Carefully add it to a beaker containing a minimum quantity of dilute HCl. The HCl will react with the CaCO3, dissolving it. Add HCl slowly and in small amounts to avoid excessive bubbling. The reaction will produce calcium chloride (CaCl2), water (H2O), and carbon dioxide (CO2).
- Evaporate to Dryness: Place the beaker on a water bath or a hot plate set to low heat. Evaporate the solution to dryness. This step is crucial to remove excess HCl, which would interfere with subsequent hardness measurements.
- Dissolve Residue: Once the residue is completely dry, dissolve it in distilled water.
- Make up to Volume: Transfer the solution quantitatively (ensuring all the residue is transferred) into a 1-liter volumetric flask. Add distilled water to the flask until the solution reaches the 1-liter mark. Mix thoroughly.
Calculation:
Each 1 ml of this solution contains 1 mg of CaCO3 equivalent hardness. This is because you dissolved 1 gram (1000 mg) of CaCO3 in 1 liter (1000 ml) of solution.
Important Considerations:
- Purity of CaCO3: The accuracy of your hard water sample depends on the purity of the CaCO3 used. Ensure you are using a reagent-grade chemical.
- Concentration of HCl: The concentration of the dilute HCl is not critical, but it should be sufficient to dissolve the CaCO3. Using concentrated HCl is generally not recommended. Start with a lower concentration and add more as needed.
- Complete Dissolution: Ensure that the CaCO3 is completely dissolved before evaporating the solution. If any solid remains, the final concentration will be inaccurate.
- Storage: Store the prepared hard water sample in a clean, airtight container to prevent contamination.
Example:
If you need a lower concentration of hard water, you can dilute this stock solution. For example, to prepare 1 liter of hard water with a concentration of 500 mg/L (ppm) as CaCO3, you would take 500 ml of the stock solution (1000 mg/L) and dilute it to 1 liter with distilled water.
