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Metrohm has launched a technical guide that describes a variety of water analysis techniques.

A complete tap-water analysis includes the determination of the pH value, the alkalinity and the total water hardness.

Both the pH measurement and the pH titration by means of a standard pH electrode suffer from several drawbacks.

First, the response time of several minutes is too long and, above all, the stirring rate significantly influences the measured pH value.

The Aquatrode Plus, with its special glass membrane, is said to guarantee rapid and precise pH measurements and pH titrations in solutions that have a low ionic strength or are weakly buffered.

Total water hardness is ideally determined by a calcium ion-selective electrode (Ca ISE).

In a complexometric titration, calcium and magnesium can be simultaneously determined up to a calcium/magnesium ratio of 10:1.

Detection limits for both ions are in the range of 0.01mmol/L.

Since drinking water is essential for humans, it is not surprising that the analysis of drinking-, ground- and surface water is continually increasing in importance.

Among other things, a complete water analysis involves the determination of pH value, alkalinity and total hardness.

The most precise way to measure the pH is the potentiometric determination using a pH glass electrode.

However, the pH measurement in poorly conducting or weakly buffered aqueous solutions such as drinking water places high demands on a pH glass electrode.

Additionally, the measured pH value often depends on the stirring speed.

Alkalinity – the capacity of bases to neutralise acids – is a sum parameter, comprising the water’s carbonate, hydrogen carbonate, hydroxide, borate, silicate, phosphate, ammonium and sulphide concentrations.

It is determined by titration with an acidic titrant.

Drinking water and ground water generally contain only carbonates with trace amounts of the other bases.

The total hardness is the sum of calcium, magnesium, barium and strontium ions.

Since in drinking water only calcium and magnesium are present in significant amounts, the hardness is entirely determined by their content.

The total hardness can be determined by potentiometric titration with ion-selective electrodes.

This presentation deals in detail with the determination of the pH value, the alkalinity and the total hardness of water samples.

Moreover, it addresses some problems that can occur during these measurements and offers viable solutions.

For the determination of the carbonate alkalinity, EN ISO 9963-2 (1996-02) prescribes a SET titration with the titrant c(HCl) = 0.05 mol/L to pH = 5.4.

A prerequisite for a correct titration is a rapid response of the glass membrane and an accurately measured pH value at the start and end of the titration.

However, standard pH glass electrodes in ion-deficient solutions, such as drinking water, have response times of several minutes.

An additional problem, which is often underestimated, is the dependence of the measured pH value on the stirring speed.

The potentiometric determination of the total hardness is best performed with a calcium ISE.

A curve with two potential jumps is obtained.

The first potential jump corresponds to the calcium content and the second to the sum of calcium and magnesium.

Different titrations are performed in the concentration range between 0.01-10mmol/L.

Even in the low-concentration range, the Ca ISE is said to provide reliable results.

While the calcium concentration is held constant at 2.5mmol/L, different calcium/magnesium ratios are analysed.

Magnesium is still detectable at a Ca/Mg ratio of 10:1.

Nevertheless, at these ratios, high-standard deviations are observed.

The precision of the magnesium determination can be significantly improved by spiking the sample with a magnesium standard.

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