Analysis of water hardness by automated titration
A
Metrohm UK product story
Edited by the Laboratorytalk editorial team
Oct 3, 2007
The major contributors to the hardness of a water sample are generally regarded as dissolved calcium and magnesium ions, which combine with soap to form insoluble precipitates
Hardness is the soap-consuming or soap-precipitating power of a water sample.
Hard water is not considered desirable as it normally requires increased amounts of soap to generate lather or it produces lime scale on industrial or domestic appliances and water pipes.
There are two forms of hardness: carbonate or temporary hardness associated with carbonates and bicarbonates of calcium, and magnesium and non-carbonate or permanent hardness associated with sulphates and chlorides of calcium and magnesium.
Thus, the sum of the concentrations of the calcium and magnesium ions is used to calculate the hardness of a water sample, commonly expressed as CaCO3, the calculation for which is as follows:.
Hardness as mg/L CaCO3 = 2.5[Ca mg/L] + 4.2[Mg mg/L].
Alkalinity can also be expressed as CaCO3.
When they are both in these units they can be directly compared with each other.
It can be useful to do this in order to determine the amount of permanent or non-carbonate hardness that may be present along with how much temporary or carbonate hardness is present.
When hardness > alkalinity, then the alkalinity result corresponds to the amount of carbonate hardness present.
The difference between the hardness result and the alkalinity result corresponds to the amount of non-carbonate hardness present.
When hardness < or = alkalinity, then all the hardness is present as carbonate hardness.
Hardness can be determined by a complexometric titration, and if required other measurements such as conductivity, pH, and p and m alkalinity can be added too.
The fully automated and PC-controlled water analysis system based on the 836 Titrando and the 815 Robotic USB sample processor XL.
In particular it contains the new quick aliquoting device 6.9958.007, an overflow burette allowing quick and precise dosing of 100ml sample after conductivity measurement, necessary for alkalinity determination and the water hardness titration.
The Mati 1 has all it takes to perform 59 water determinations fully automated and includes all necessary accessories and sensors.
It is a ready-to-use system that fulfills the following requirements: conductivity, pH, p-and m-value, hardness (Ca/Mg), quick aliquoting device with a high sample throughput and it is PC controlled.
The water analysis involves the determination of alkalinity and water hardness.
The Aquatrode Plus electrode is used as indicator electrode for the alkalinity determination; the water hardness titration is indicated by a Ca-ISE combined with the reference system of the Aquatrode Plus.
The conductivity measurement is carried out with the 712 Conductometer by means of a conductivity cell installed on the second tower.
The signal supplied by the conductivity meter is evaluated by the 836 Titrando.
The standard rack used accommodates 59 beakers with a volume of 120ml.
The samples are stirred by an 802 rod stirrer.
The new overflow burette transfers the sample to the external titration vessel using the built-in pumps of the 815 Robotic USB Sample Processor XL.
After the titration the membrane pumps of the 815 Robotic USB Sample Processor XL guarantee efficient aspiration of the spent solution and thorough rinsing of the titration equipment.
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