TEST METHOD: CALCIUM AND MAGNESIUM IN WATERS (hardness)

Date updated: 12 December 2009 (addedmore advice on sample sizes)

RISK ASSESSMENT

Assess Safety, Health, Quality and Environmental aspects of each specific step.

TASK SAFETY REQUIREMENTS

Safety boots are mandatory

Safety glasses to be worn whenever handling reagents

 SPECIFIC JOB STEPS

RISK ASSESSMENT

RISK CONTROL

Equipment and reagents required:

Beaker, pipette, burette tube, sample, distilled water, normal HCl, normal KOH, calcium indicator, EDTA, solochrome black indicator

Amount of sample to be tested: 30mL NaCl stock solution (200g/L), 10 mL for crude dissolving brine, 10mL Ca/Mg standard, 10 mL for irrigation bores, 1 or 2 mL for seawater and hypersaline brines, 20mL for relatively hard bore water used for drinking, or 100mL fresh natural waters or normal tap water.

 

Contaminated reagents or deionised water

 

Acids and bases can cause burns when splashed on skin or clothing.

 

Test against standards monthly, test a blank in every analytical run. Record standard results on the appropriate standards chart. Record blank and use in calculation. Exhausted ammonia buffer produces a wine colour that is diagnostic.

Take care not to splash chemicals. Any splashes shall be immediately rinsed with water.

Determine sample calcium content:

  1. Put sample in beaker and make up to 100mL with deionised water
  2. Add 5 mL of Normal (N) HCl
  3. Add 20 mL of Normal (N) KOH
  4. Add 10 mL of Calcium indicator
  5. Fill the Burette tube to zero with EDTA
  6. Stir the sample for approximately 1 minute
  7. Add EDTA to sample until the vortex changes colour from orange to yellow, then add EDTA dropwise until the entire beaker changes colour.
  8. Record EDTA used from burette tube on scrap paper.

 

Overtitration

 

Wait for initial colour change to take effect before adding more EDTA.

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.

   

Determine sample magnesium content

  1. Add 25 mL Normal HCl to sample
  2. Add 5 mL Ammonia buffer to sample
  3. Add approximately 10 drops of Solochrome Black indicator to sample
  4. Stir for 1 minute
  5. Fill Burette tube to zero level with EDTA
  6. Add EDTA to sample until the vortex changes colour from purple to blue then add EDTA dropwise until the entire beaker changes colour.
  7. Record EDTA used from burette tube on scrap paper.

 

Overtitration

 

Wait for initial colour change to take effect before adding more EDTA.

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.

Client may want results as total hardness, rather than as separate Ca and Mg Hardness mg equivalent CaCO3/L = 2.497[Ca, mg/L] + 4.118[Mg, mg/L]
Record results in the laboratory daybook    

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