There are a number of chemical re-agents to test for chlorine which you can choose from. The most common ones are the orthotolidine (OTO) and the diethyl-p-phenylenediamine (DPD) tests. Both are readily available. OTO tests are usually available in a liquid form and the DPD tests are available in tablet or liquid form. Before looking at each of the tests, it is important to understand some chlorine chemistry. Although both tests rely on colour comparisons with colour standards to achieve test results, there are differences.
Definitions:
1. Chlorine - chemical of choice used for the disinfection of pool water. The most common chemical used for this is calcium hypochlorite.
2. Chlorine demand - the amount of chlorine that must be added to water at any given time to react with all of the naturally occurring chemicals and bacteria in the water.
3. Combined residual - some chlorine will combine temporarily with chemicals in the water, in particular ammonia, which is occurring naturally in the water and results from people using the water. These combined residuals are also known as chloramines.
4. Residual chlorine - when enough chlorine has been added to react with all of the chemicals in the pool water, any chlorine left is residual chlorine. This is the chlorine that does the disinfecting.
5. Free Residual or Free Available Chlorine - refers to the chlorine that remains after chlorine demands and the combined residual levels of chlorination have been reached. The free residual is what actually does the most disinfecting of pool water because it is not attached to other chemicals present in the water. This is the chlorine type that does the disinfecting of pool water, so that it is a measurement that is extremely important. Without any free residual, the chlorine that may be present in the water is not doing any disinfecting.
6. Total Chlorine Residual - sum of the free and combined chlorine residuals.
Therefore, it is the free available chlorine test that is the most important. It is also the test that is required under the Public Pools Regulations of Ontario (O. Reg. 569/90).
OTO Versus DPD Tests
Test |
Advantages |
Disadvantages |
OTO |
• cheap cost |
• gives free chlorine reading only if water is below 35oF (2oC); otherwise reading is a combined chlorine reading |
DPD |
• gives accurate free chlorine reading quickly and accurately-unaffected by water temperature
• legal under Public Pools Regulations (O. Reg. 569/90)
• most kits using DPD also have quick accurate tests for combined and total chlorine content |
• expense |
|
