Methods Manual for Salt Lake Studies/Hardness

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Authors: PSJ Coleman

Contents

[edit] Hardness overview

"Hardness" is a measure of calcium and magnesium in brine or water. The following titration, based on APHA methods 2340C, 3500-Ca D and 3500-Mg E (Eaton et al, 1995) provides a measure of Ca and Mg separately, and also a combined measure of "hardness" .

[edit] Equipment and reagents required

  • beaker for sample analysis,
  • pipettes for measuring sample & reagent volumes,
  • burette tube or Dr Schilling self-zeroing burette for titre,
  • sample,
  • distilled water,
  • normal HCl,
  • normal KOH,
  • calcium indicator,
  • EDTA titre,
  • solochrome black indicator

[edit] Sample to be tested

Between 10 mL and 100mL of brine, depending on salinity. Use 10mL for brines approaching the saturation point for salt, ranging up to 100 mL for brackish waters. Test a blank (deionised water) alongside the sample.

[edit] Determining sample calcium content

  • Put sample in beaker and make up to 100mL with deionised water
  • Add 5 mL of Normal (N) HCl
  • Add 20 mL of Normal (N) KOH
  • Add 10 mL of Calcium indicator
  • Fill the Burette tube to zero with EDTA
  • Stir the sample for approximately 1 minute
  • Add EDTA to sample until the vortex changes colour from orange to yellow, then add EDTA dropwise until the entire beaker changes colour. To prevent over titration, wait for initial colour change to take effect before adding more EDTA.
  • Record EDTA used from burette tube in the lab notebook.

[edit] Ca Calculation

Correct EDTA use for any blank value obtained. Calculate Calcium concentration for a 10 mL sample by multiplying mL of .02 N EDTA by 40.08 and report as mg/L Ca. For larger or smaller samples than 10mL, correct for the sample size.

[edit] Determining sample magnesium content

  • Using the same sample that you used to determine the calcium,
  • Add 25 mL Normal HCl to sample to decolorize the sample
  • Add 5 mL Ammonia buffer to sample
  • Add approximately 10 drops of Solochrome Black indicator to sample
  • Stir for 1 minute
  • Fill Burette tube to zero level with EDTA
  • Add EDTA to sample until the vortex changes colour from purple to blue then add EDTA dropwise until the entire beaker changes colour. To prevent over titration, wait for initial colour change to take effect before adding more EDTA.
  • Record EDTA used from burette tube in the lab notebook.


[edit] Mg Calculation

Correct EDTA use for any blank value obtained. Calculate Magnesium concentration for a 10 mL sample by multiplying mL of .02 N EDTA by 24.3 and report as mg/L Mg. For larger or smaller samples than 10mL, correct for the sample size.

[edit] Total hardness calculation

If you need the results as total hardness, rather than as separate Ca and Mg, the following calculation will provide hardness as CaCO3 mg/L

Hardness mg equivalent CaCO3 mg/L = 2.497[Ca mg/L] + 4.118[Mg mg/L][1]

Record results in the laboratory daybook

[edit] Things to be aware of with this method

Contaminated reagents or deionised water can invalidate results. Test against Calcium and Magnesium standards monthly and test a blank in every analytical run. Record the results of the blank and use in calculation.

Old or out-of-date reagents may produce inaccurate results. Always check the age of reagents. Exhausted ammonia buffer produces a wine colour that is diagnostic.

Acids and bases can cause burns when splashed on skin or clothing. Take care not to splash chemicals. Use safety glasses. Any splashes should be immediately rinsed with water.

[edit] References

  1. ↑ [1]
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