Testing for chlorine.
Test strips may be suitable for quick and convenient testing. Hach free and total chlorine test strips test in the range 0 - 10 ppm and show both free and total chlorine on the same strip. See details at Hach free & total chlorine test strips. Lamotte also make test strips that measure chlorine, this time free chlorine (contact Apps Labs for details).
The only disadvantage of some test strips is that they have low resolution at low chlorine levels eg below 0.5 ppm. In addition the step size may be too coarse for some applications. However Macherey Nagel chlorine sensitive test strips provide better resolution at low values. See details at Macherey Nagel (total) chlorine sensitive test strips.
Many test kits that measure chlorine in the below 10 ppm range use the DPD method. These usually only require addition of one reagent for free chlorine and another for total chlorine. Using a test kit for chlorine can seem tricky at first especially if the kit tests for free and total chlorine and in different ranges. Usually though, there will be complete instructions with each kit. After the first couple of tests the process will become easier.
A test kit that shows free chlorine in the range 0 to 2.5 or 3.5 mg/L is suitable for finding disinfection levels in drinking water or pools. But kits with a step size of 0.5 mg/L are best for pools.
Because the efficiency of chlorination depends on pH, it is useful to test pH when testing for free chlorine. Some chlorine kits and photometers also test for pH. If the kit does not test for pH then pH test strips are a simple way to do this. Because small pH changes are important choose test strips with a 0.3 or 0.5 pH unit step. Macherey Nagel pH-Fix test strips with range 4.5 - 10 are suitable because they have a 0.5 pH unit step size.
For drinking water chlorination higher accuracy is preferred. Also a higher resolution in the low range is useful to test for residual chlorine. An example of this type of kit is the Hanna Instruments HI38017 dual range free kit. This tests free and total chlorine separately. See details for this kit at HI38017 dual range chlorine kit.
These types of test kits are quite accurate but they are visual ie require subjective assessment of colours. For more frequent use or for multiple samples an instrument method may be more convenient. There are instruments (photometers) that give a reading after reacting the sample water usually with the same chemicals used in the kits. These are more expensive initially but the advantage is higher accuracy and convenience. If you will be doing a lot of tests then a photometer will end up being good value. Some photometers also test pH. An example is the HI93710 free and total chlorine and pH photometer. See details for this photometer at HI93710 free & total chlorine and pH photometer.
High range chlorine
Even though the upper chlorine limit for the DPD method is around 10 mg/L, some kits get around this limitation by automatically diluting the sample as it is tested. Examples are the Chemetrics Vacuettes ampoules system. These ampoules contain all reagents and diluents. When the stem of the ampoule is broken in the sample, a small sub sample is drawn into the ampoule and reacts to form a coloured complex. The sub sample is automatically diluted as it enters the ampoule so much higher chlorine levels can be tested - up to 5000 ppm for both free and total chlorine.
Iodometric (titration) method. For higher range total chlorine, Hanna Instruments offer the HI38023 kit. The range is 10 - 200 mg/L and the kit will perform approx 100 tests. See Apps Labs for details.
For testing high range chlorine, Hach Company offers the model CN-21P kit. This kit gives total chlorine from 10 - 200 mg/L as Cl2.
High range test strips. Both Lamotte and Hach make high range chlorine test strips. Both report free chlorine as Cl2 in ppm or mg/L. The Lamotte test strips test free chlorine from 0 - 800 ppm. See details at High range free chlorine test strips.The Hach test strips test free chlorine from 0 - 600 mg/L. See details at High range chlorine test strips.
Testing for iodine and bromine.
Total chlorine test kits can also be used for testing the other halogens iodine and bromine. For example iodine can be tested if it can be assumed there are no other halogens or strong oxidants present. To get a value for I2 (iodine) multiply the tests result by 3.58. To get Br2 (bromine) multiply by 2.25. Hach (2006) Products for Water Analysis.
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