November 2006Analytical Instrumentation & Environmental Monitoring
The purity and the colour tone of the textiles depend on the accurate measurement of the dye pH value. By controlling the pH value, undesirable reactions may be avoided and a highly defined product obtained.
Azo-dyes are produced through several stages in a reactor which is usually rubber coated. During the first stage the pH value is low (between pH 0 and pH 2) and the temperature less than 20°C. During the second stage the pH value is slightly raised (pH 3 to pH 5) and the temperature increases to between 40 and 50°C. During the last reaction stage the pH value is increased to 10 and the temperature is up to 90°C.
The measuring problems are caused by precipitated dyes and azonium salts which contaminate the junction. A variety of intermediate products may react with the reference electrolyte and compound the problem. The wide pH and temperature range of this process also puts special demands on the pH electrode, especially if repeatable accuracy at high pH and high temperature is required.
This problem has been solved by the patented pH differential technology from HACH which uses three measuring electrodes instead of the two in conventional pH sensors. The process electrode and the reference electrode measure the pH differentially with respect to a third ground electrode. This provides high accuracy, reduces reference junction fouling, and virtually eliminates ground loops. The benefit is greater reliability with less downtime and maintenance and makes these pH electrodes the preferred solution for this problem as well as for all harsh pH applications.
For more information contact Steve Herbst, PREI Instrumentation, +27 (0) 11 448 2172.
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