Catherine Allen

While Chlorine is one of the most widely used test parameters to determine water quality throughout the world, do we really understand the mechanism that Chlorine uses to achieve disinfection or removal of Biofilm? Or do we just accept and test? Understanding the background of the way in which Chlorine interacts with other components of the water system is fundamental to choosing the correct test for the application.

Since its identification in 1894, as a potential additive to water to eliminate germs, our understanding of Chlorine’s behaviors and conditions needed to be effective have developed.

Early control of the Chlorine level was based on adding a fixed amount of Chlorine to a given volume of water, but as knowledge of disease control grew it soon became apparent that knowing the Chlorine residual levels at all times was more useful. The first test used to do this was a cumbersome iodiometric test using starch iodide. By 1908 this had been widely replaced by Ortho-Tolidine (OTO).

The popularity of the OTO method grew rapidly in the early part of the 20^th Century as it was a good control method for the time. It was reliable and cost effective. It could be used by non-technical personnel and was readily available.

It was not until the mid 1930’s that scientists gained an understanding of the break through point and Chlorination, which created the need for a test method able to differentiate between Free, Total and Combined chlorine.

In 1957 the DPD Method was introduced and, for many, has become the standard test used for Chlorine.

Over the last 100 years there have been several methods tried, tested and used.

In this paper we will discuss all the current methods being used, their application, interferences, pros, and cons and will unravel some of the mystery surrounding the reagent names and techniques.

Catherine Allen is an Application Specialist and Product Manager at Tintometer Ltd, focused on products for use in the Industrial Water Treatment Market. Catherine is a graduate of Newcastle University having received a BSc hons degree in Applied Biology.

After graduation Catherine immediately started work as a Technical Specialist for a test kit manufacturer working in both laboratory and sales roles.  Now, with over 21 years’ experience in water testing, Catherine is often consulted with for her knowledge about test kit and product design for Industrial Water testing applications.