YSI 600DW-B Sonde - Manual (Page 293)

Frequently Asked Questions About YSI Drinking Water Systems Appendix H
amount of free chlorine and a large amount of combined chlorine. Total Chlorine is defined as the sum
of the free and combined chlorine concentrations.
How are combined and total chlorine measured?
Combined chlorine is usually not measured directly. Instead, the free chlorine is measured by a color-
forming chemical test under one set of conditions and the total chlorine is measured colorimetrically under
a slightly different set of conditions. Since Total Chlorine = Free Chlorine + Combined Chlorine, the
Combined Chorine is calculated as the difference between the Total and Free Chlorine values.
Can the YSI Free Chlorine Sensor measure Total Chlorine?
No. It is very important to be aware of the fact that the YSI 6572 free chlorine sensor and its electronics
measure only free chlorine – the sensor is unresponsive to combined chlorine (chloramines). This makes
the sensor of questionable utility for measuring “chlorine” in distribution systems which use ammonia as
part of the treatment process and thus have predominantly combined chlorine in their drinking water.
Can the YSI Free Chlorine Sensor function in water with high pH?
Yes, unless the pH is much higher than normal for most drinking water sites. From empirical testing, it
appears that the YSI free chlorine sensor is less sensitive to OCl
- than it is to HOCL and as the pH rises,
there is no question that the amount of HOCl relative to OCl- is reduced significantly. This means that the
sensitivity of the sensor to the mixture of HOCl and OCl
- is reduced at high pH, but the sensor has been
shown empirically to have enough overall sensitivity to be functional at pH values up to ca. 9.3 and data to
support this claim is available on request from YSI. Note, however, that at pH values above 8.8, there is a
possibility that calibration errors can occur. If you see these errors, contact YSI Technical Support for
advice on whether to override them.)
The YSI philosophy relative to the use of the free chlorine sensor at high pH is to calibrate the sensor under
the these conditions of reduced sensitivity in the high pH water and then assume that minor variations in
pH during the monitoring study will have only a minimal effect on accuracy. This philosophy has been
adopted rather than compensating for changes in pH using the on-board pH sensor in the sonde. It was
decided not to do pH compensation because chlorine is usually the most important parameter to the
drinking water industry and the accuracy of this parameter would be grossly compromised if a pH sensor
failed while providing compensation data during long term monitoring studies. In other words, we feel
that the risk of pH probe failure or significant drift is more likely to be a problem to users than the smaller
inaccuracies in the uncompensated pH data which might result from small changes in pH. Note that the
quoted accuracy specification for water which has a pH of greater than 8.5 is only valid if the pH changes
less than 0.3 pH units from the pH at the time of calibration. In addition, users should note that it will not
be possible to make free chlorine measurements with the YSI free chlorine sensor under any conditions if
the pH exceeds ca. 9.3.
The plot below shows that the chlorine sensor can function well in water of high pH (average of 8.7) as
long as it is calibrated versus a DPD analysis in the high pH water. Note that the final QC point which was
taken in water at a pH of 7.5 and shows a positive error relative to the DPD analysis since the sensitivity of
the sensor to free chlorine had increased significantly as predicted once the sensor was transferred to the
lower pH water.
YSI Incorporated Drinking Water Monitoring Systems Operations Manual H-7