YSI 600DW-B Sonde - Manual (Page 219)

Principles of Operation Section 5
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. 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. It is 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.
FREE CHLORINE SENSOR -- GENERAL
The sondes employ an amperometric sensor to measure free chlorine which is similar in design and theory
to the patented YSI Rapid Pulse system for the measurement of dissolved oxygen. It is important to note
that, while the Rapid Pulse dissolved oxygen sensor is effectively flow independent, the analogous free
chlorine sensor possesses significant flow dependence (20-30 % in stagnant water). This characteristic
means that you must use the free chlorine sensor in flowing water utilizing YSI flow cells in order to
realize the stated accuracy specification.
The Rapid Pulse free chlorine system utilizes a sensor that is similar to other membrane-covered, steady-
state amperometric free chlorine sensors. The system still measures the current associated with the
reduction of free chlorine which diffuses through a hydrophilic membrane, and this current is proportional
to the free chlorine content in the flowing drinking water being evaluated. The user will note from
examination of the 6572 free chlorine probe that the sensor consists of three electrodes (a cathode, anode,
and reference electrode) while steady state amperometric free chlorine probes usually have only two
electrodes (a cathode and a combined anode-reference electrode). In addition, the geometry of the sensor is
novel, consisting of a thin linear platinum cathode placed between two silver rectangles which serve as
anode and reference electrodes. These sensor changes are necessary to implement the new Rapid Pulse
method for free chlorine measurement as described in the following section.
FREE CHLORINE SENSOR -- METHOD OF OPERATION
Standard amperometric free chlorine sensors are continuously polarized at a voltage sufficiently negative to
cause free chlorine (hypochlorous acid + hypochlorite ion) to be reduced at the platinum cathode and silver
metal to be oxidized to silver chloride at the anode. The voltage must also be sufficiently positive to
prevent the reduction of oxygen during the measurement sequence. The free chlorine diffuses through a
hydrophilic polyvinylidene membrane and is reduced at the cathode. The current associated with this
process is proportional to the free chlorine concentration outside the membrane. As this electrochemical
reaction proceeds, free chlorine is consumed (or depleted) in the medium, resulting in a decrease in
measured current (and apparent free chlorine content) if the external solution is not stirred rapidly. This
flow dependence is minimized (but not eliminated) by rapidly and reproducibly polarizing (on) and
depolarizing (off) the probe electrodes during a measurement sequence. The Rapid Pulse system thus
measures the charge or coulombs (current summed over a specific time period) associated with the
reduction of free chlorine during a carefully controlled time interval. The coulombs due to charging of the
cathode (capacitance), but not to reduction of free chlorine, are subtracted during integration after the
cathode has been turned off. The net charge, like the steady state current in a standard system, is
proportional to the free chlorine concentration in the medium. Note that, as described above, the flow
dependence of the sensor is not completely overcome by this method (as it is in Rapid Pulse dissolved
oxygen sensors) because of the different diffusion characteristics of free chlorine through the hydrophilic
polyvinylidene membrane versus oxygen through a Teflon membrane.
YSI Incorporated Drinking Water Monitoring Systems Operation Manual 5-6