Chlorine Monitor without reagents

Measurement of free chlorine in potable water traditionally requires the use of reagents. The Q45H62 from ATi allows you to measure free chlorine in water with pH from 6 to 9.2, in low and high conductivities without buffers. This dramatically reduces total life costs.

Chlorine Monitor without reagents

The problem
Many water companies want to measure free chlorine residuals without the need for chemical buffering traditionally associated with such measurements. Acetate and phosphate buffers are expensive and environmentally unfriendly. Buffer delivery systems are maintenance intensive and have fairly costly consumables.

One proposed solution
Amperometric cells and most polarographic probes only respond to hypochlorous acid, (HOCl). HOCl dissociates into hypochlorite (OCl-) in a pH dependant manner. This is why most monitors need chemical buffering in most applications. The typical pH of water measured on a water treatment works may range from 7 to 9.2. Chemical buffering reduces the pH to between5 and 6 and ensures that all the residual chlorine is present as HOCl.

If an HOCl monitor is combined with a pH monitor it is proposed by some manufactures that the output of the HOCl monitor can be compensated for with reference to the HOCl v pH dissociation curve.

Number of curves
There is not a single HOCl v pH curve! This is a problem. See Appendix 1049 and 1050 From Handbook of chlorination, 2nd Edition, Geo. Clifford White, Van Nostrand Reinhold.)
Effect of temperature
At pH 7 with constant ionic strength of 40ppm

5oC HOCl = 83%
25oC.HOCl = 75%

At pH 8.5 the effect is even more dramatic
5oC HOCl = 10%
25oC.HOCl = 15%

I.e. at higher pH the error can be as much as 2.5% of measured signal per oC.
Effect of ionic strength
At pH 7 with constant temperature of 5oC

40ppm, HOCl = 75%
40,000ppm, HOCl = 60%
Low signals
Another problem is the low raw signals at higher pHs. At a pH of 8.5, not untypical on a water treatment plant, the %HOCl can be as low as 5% of the HOCl at pH 6. A correction factor of twenty would have to be applied. Even if the pH signal were 100% accurate the chlorine value would be noisy.

Errors due to errors in pH measurement
This is a large potential source of error. Most water companies would be very happy if ther pH monitors were within 0.1pH units of the true value. If an accuracy of +/- 0.1 pH is assumed it is possible to look at the kind of errors that could occur.

At 25oC, pH 8.5, 40ppm total dissolved solids.

% HOCl at 8.5 = 10%
% HOCl at 8.4 = 11.5%
%HOCl at 8.6 = 8.5%

The correction factor applied to the HOCl measurement would be times 10 from the pH monitor. The actual factor needed could be between 8.7 and 11.8. The error in the chlorine measurement due purely to the error in the pH would be +/- 18%.

For a typical final water free chlorine residual of 1.00ppm it is possible to calculate the errors possible:

Real pH of 8.5, %HOCl = 10% = 0.1ppm, correction factor = x10, displayed value = 1.00ppm
Measured pH of 8.4, %HOCl = 11.5%, correction factor = 8.70, displayed value = 0.87ppm
Measured pH of 8.6, %HOCl = 8.5%, correction factor = 11.76, displayed value = 1.18

This means that if the error in pH is +/- 0.1pH units the error band for the chlorine residual measurement is 0.31ppm.
The error due to inaccurate pH measurement is actually bigger than the error caused by measuring the free chlorine without compensation or buffering.

Errors due to response time differences between pH and free chlorine measurement
Free chlorine sensors have a faster response time than pH sensors. This is especially true in low conductivity water or in situations with low sample flow. If pH is varying, the HOCl sensor will respond to these changes more rapidly than the pH sensor. The effect of this is to cause over and under compensation of the HOCl signals. (See ATi PAN A15-62-1(6-99))

Combined effect
The combined effect of temperature, ionic strength, errors in pH measurement, response time differences and errors in the actual HOCl itself measurement severely limits the use of pH corrected HOCl monitors. An accuracy/ reproducibility of better than 20-25% would be difficult to achieve in any situation other than one in which pH was already stable.

The ATI pH corrected solution for variable pH
The ATi Q45H62 pH corrected chlorine monitor is designed to address the above problems.

Enhanced response
The ATi A10 sensor has a much higher response at elevated pH values. By correct selection of materials and by design the A10 sensor has for example a response at pH 8.1 of >80%. This compares to?

Multiple curves
The Q45H62 features a multi pH calibration that tunes the response of the monitor to the water conditions. In most case however a single pH calibration is sufficient.

pH errors
The Q45H62 features a sensitive low conductivity, fast response pH as standard to reduce the errors due to pH. Reproducibility is the main concern with pH. The way the Q45H62 is calibrated ensures that even if there is a small error with the pH measurement it does not affect the corrected Chlorine signal errors

The ATI A10 sensor is resistant to many of the common contaminants found in water treatment, iron, manganese, poly phosphate etc. The chlorine signal is more reliable which in turn leads to more reliable corrected chlorine readings.

The ATI solution for stable pH
The response of the A10 sensor at pH as high as 9.5 is such that in many case it can be operated without reagents or pH correction. If the pH of a process is controlled and does not vary by more than around +/- 0.1pH units around a set point the enhanced response mean that the Q45H62 can operate reagent and correction free.

Q45H62 typical installations
The monitor is a small footprint monitor that require little routine maintenance. It has very low running costs and is supplied with 3-10 years of consumables

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N.B. The information contained in this entry is provided by the above supplier, and does not necessarily reflect the views and opinions of the publisher