Hardness is the sum concentration of magnesium and calcium ions. Things such as iron and sodium ions contribute a negligible amount to hardness, except in salt water. Carbonate hardness forms when different types of alkalinity such as carbonates, bicarbonates and hydroxides combine with metals. Non carbonate hardness results when metals combine with anions--negatively charged ions--that are not alkaline. This is a straightforward method using a titration of a weak acid to determine the concentration—or lack of—carbonates.
- Skill level:
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Things you need
- Glass or plastic bottles
- pH meter
- Burret with 0.02 N of sulphuric acid
- 1 100ml Erlenmeyer flask
- Phenolphthalein indicator
- Sulphuric acid
- 1 100ml beaker
With gloves on, collect water sample in plastic or glass bottle and store it at 4 degrees C for no longer than 24 hours.
Pour 50ml of sample into Erlenmeyer flask and measure pH using the meter.
Add six drops of the phenolphthalein indicator. If the colour does not change to pink, all alkalinity is bicarbonate and non-carbonate alkalinity is zero. If it does turn pink, go to Step 4.
Slowly add the 0.02 N of sulphuric acid from the burret to the flask while simultaneously swirling the flask until the water is colourless and immediately stop adding the acid.
Measure the amount of sulphuric acid used from the burret.
Multiply the millilitres of sulphuric acid used by 20 to get the mg/l of P alkalinity. This is the amount of non-carbonate hardness present in your sample.
Pour tested sample into beaker and dispose of sample in hazardous waste receptacle.
Tips and warnings
- Always calibrate pH meter using buffers 4 and 7 before starting the test.
- P alkalinity and non carbonate hardness are used interchangeably.
- As stated by a University of Arkansas website: If the initial pH of a sample is less than 8.3, there is only bicarbonate alkalinity and no P alkalinity.
- To determine total (T) alkalinity there are additional steps to take.
- Always use gloves with this test.
- Non carbonate hardness can't be removed by boiling water.
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