YSI 600DW-B Sonde - Manual (Page 218)

Principles of Operation Section 5
59). Determination of the slope of pH vs. mv plots at temperatures different from T
c
is thus relatively
simple. In order to establish the intercept of the new plot, the point where plots of pH vs. mv at different
temperatures intersect (the isopotential point) must be known. Using standard pH determination protocol,
the sonde software assigns the isopotential point as the mv reading at pH 7 and then calculates the intercept
using this assumption. Once the slope and intercept to the plot of pH vs. mv are assigned at the new
temperature, the calculation of pH under the new temperature conditions is straightforward, and is
automatically carried out by the sonde software.
MEASUREMENT AND CALIBRATION PRECAUTIONS
1- When filling the calibration cup prior to performing the calibration procedure, make certain that the level
of calibrant buffers is high enough in the calibration/storage cup to cover at least ½ inch of the pH probe
and the temperature sensor of the 6560 probe.
2 - Rinse the sensors with deionized water between changes of calibration buffer solutions.
3 - During pH calibration, allow the sensors time to stabilize with regard to temperature (approximately 60
seconds) before proceeding with the calibration protocol. The pH readings after calibration are only as
good as the calibration itself.
4 - Clean and store the probe according to the instructions found in Section 2.10, Care, Maintenance and
Storage of this manual.
5.6 TEMPERATURE
The sondes utilize a thermistor of sintered metallic oxide that changes predictably in resistance with
temperature variation. The algorithm for conversion of resistance to temperature is built into the sonde
software, and accurate temperature readings in degrees Celsius, Kelvin, or Fahrenheit are provided
automatically. No calibration or maintenance of the temperature sensor is required.
5.7 FREE CHLORINE
CHLORINE CHEMISTRY
When chlorine gas (Cl
2
) is added to water, a reaction (called disproportionation) occurs to form
hypochlorous acid (HOCl) and hydrochloric acid (HCl). The hypochlorous acid is a strong oxidant and the
active species in most drinking water disinfection processes rather than gaseous chlorine (Cl
2
). One
molecule of HOCl has the same oxidizing capacity as a molecule of Cl
2
. Thus, the formula for “free
chlorine” is really HOCl rather than Cl
2
. (A portion of the free chlorine will also be in the form of
hypochlorite ion OCl - , with the amount depending on the pH of the sample.) However, convention
dictates that free chorine be reported in units of “mg Cl
2
per liter” even though the active compounds in
solution are HOCl and OCl - . Thus, YSI free chlorine readings are expressed in units of “Cl2 mg/L” rather
than “HOCl/OCl - mg/L” to be consistent with standard reporting convention.
There are two categories of “chlorine” in most treated drinking water – free chlorine (HOCl + OCl
- ) and
combined chlorine which is made up of species formed in the reaction of free chlorine with ammonia
which is present in the water. Combined Chlorine is thus made up of a general class of compounds called
chloramines with formulae such as NH
2
Cl, NHCl
2
, and NCl
3
. In water which is treated just with chlorine,
there will be a small amount of combined chlorine from the natural ammonia in the water and a large
amount of free chlorine; in water which is treated with both free chlorine and ammonia there will be a small
YSI Incorporated Drinking Water Monitoring Systems Operation Manual 5-5