Hach AutoCAT 9000, Инструкция по эксплуатации

Страница: 153 / 176

Amperometric Titration Theory
Amperometric Titration Theory
50081_Titration.fm
Page 151
1.4 Shift of Endpoints Due to Sample Iodine Demand
Certain organic compounds exert an “iodine demand” which shifts the titration end point,
even if the back titration procedure is used. An example of this effect is shown in
Figure
4 . If the sample contains suspended particles, generated iodine readily adsorbs into the
particles, shifting the measured current readings. In addition to adsorption, iodine can
react with dissolved organic matter in the sample.
For samples with a high iodine demand, it may be difficult to determine the end point.
Continuing the titration past the end point helps determine the end point more accurately.
Also, a rapid titration will minimize iodine demand. Sample dilution with chlorine
demand-free water also minimizes iodine demand, although this may cause chlorine loss
and decreases sensitivity.
1.5 Order of Reagent Addition
Measuring chlorine in saline, estuary, or seawater is very difficult with any of the
available methods. The literature concerning the amperometric determination of total
chlorine in salt water is somewhat confusing. Several studies indicate the order of
reagent addition (KI and buffer) may underestimate the total chlorine concentration
when determined amperometrically.
The chemistry of chlorine in sea water is extremely complex. Saline waters usually
contain an appreciable chlorine demand, due in part to oxidation of carbon and nitrogen-
containing compounds. Bromide, usually present in seawater, oxidizes to bromine when
chlorine is added. Also, the concentration of chlorine-containing and secondary oxidants
produced by chlorination depends on the characteristics of the water being chlorinated.
Salinity, organic load, water temperature, and incident sunlight influence the level of
chlorine-containing compounds and oxidants.
There is general agreement that in the amperometric determination for total chlorine in
saline waters, iodide reagent should be added before, or simultaneously with, the pH 4
buffer. If the saline sample is buffered prior to addition of the iodide, the total oxidant
concentration may be underestimated.
Figure 4 Iodine Demand (back titration curve)
TITRANT VOLUME (mL)
CURRENT (µA)