Measure calcium and magnesium - 0 to 100 ppb in BRINE

CALCIUM and MAGNESIUM may foul ion-exchange membranes used in the electrolytic manufacture of caustic and chlorine from brine solutions. These ions must be minimized and closely monitored to prevent membrane plugging, and frequent analysis of the brine feed solution prevents the membrane plugging, costly maintenance and down time.

TYTRONICS’ SENTINEL automatically measures calcium and magnesium in brine as frequently as every 10 minutes. The analyzer uses a batch sampling technique that allows the use of 1/4" tubing for the sample lines; consequently, small amounts of solids do not affect instrument performance.

The instrument is fully microprocessor controlled and menu-driven with malfunction diagnostic features. The optics bundle is designed with a dual wavelength configuration. The measurement wavelength is 650 nm and the reference wavelength is 810 nm. The use of a reference wavelength allows for correction of any sample turbidity and coating on the colorimetric probe. An auto blank feature compensates for sample background color. Because of these features, weekly instrument calibration is sufficient. Calibration is automatically performed to insure instrument accuracy.

The Tytronics'® Sentinel series of analyzers has been designed to provide:

• Continuous on-line analysis of process streams

• Maximum reliability

• Ease of maintenance

• Self-checking for easy diagnostics

METHOD OF ANALYSIS

A colorimetric method based on Hydroxy Naphthol Blue (HNB) is used. HNB, which is blue in alkaline solutions, forms a red complex when combined in alkaline solutions with calcium and magnesium. The red complex is reverted by the addition of EDTA to re-form the blue color of HNB alone. The method requires the measurement of two absorbancies.

• The first measurement, A₁, is made after adding KOH and HNB to the sample.

• The second measurement, A₂, is made after the addition of EDTA.

• A linear relationship is observed between the difference in absorbance, A₂ – A₁, and concentration in the range of 0 to 100 ppb calcium and magnesium.

The benefit of using the absorbance difference is that results are not affected by small changes in dye concentration and alkalinity in the incoming brine. The HNB method is sensitive to Sr, Ba, Al and Fe⁺³. The table below gives the error in Ca and Mg at different concentrations of Sr, Ba, Al and Fe⁺³.

The HNB is dissolved in 40% ethylene glycol.

• The mixed HNB is stable for at least one month at room temperature.

• Both the potassium hydroxide and the EDTA solutions are stable.

Use our Tytronics Sentinel for measuring calcium and magnesim in brine