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Chlorine Measurement and It’s Advances

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February03

An abundance of water has literally given Earth its name ‘The Blue Planet’. But it almost comes as a surprise to learn that only one percent of all the water is available for human consumption – and that one percent often contains a host of microorganisms which may cause life-threatening waterborne diseases such as cholera, typhoid and dysentery.

The chlorination of water for human consumption is almost 100 years old. It has been practiced since 1908 to contain and eliminate waterborne disease outbreaks.

Wastewater chlorination was pioneered two years later in 1910 in Philadelphia, and soon implemented in many other cities in the United States based on this early success.

The filtration and disinfection of drinking water can in large part be credited for the 50 percent increase in life expectancy in this century.

Today, 98 percent of the 55000 public water supplies in the U.S. are disinfected with chlorine or chlorine-based compounds.

According to the survey results, chlorine-based products remain the most widely used disinfectants by water treatment professionals, largely due to their effectiveness, cost and residual properties.

While the benefits of chlorine for the safety of modern civilization are indisputable, issues about its toxicity and chronic health effects remain subject to many discussions.

The application of chlorine to water and wastewater results in the formation of disinfection byproducts (DBP). The nature of biological treatment (prior to chlorination) and the presence of ammonia can have a substantial impact on the extent of byproduct formation.

Chlorine itself is already perceptive at levels of 0.2 – 0.4 ppm (mg/L). Quantities of 1 – 15 ppm may cause mild to moderate irritation of the respiratory tract, prolonged exposure to 30 – 60 ppm is followed by immediate chest pain, dyspnea and toxic pneumonitis. The lethal concentration is about 600 ppm for ten-minute exposure.

The Environmental Protection Agency has therefore established a set of stringent guidelines whenever chlorine treated waters are discharged into natural environments.

Free chlorine levels which are toxic for fish vary from 0.03 to 0.05 ppm, depending on exposure time and fish species. For this purpose, dechlorination of excess chlorine is normally performed prior to discharge to help prevent harm to aquatic life and also to reduce the formation and impact of DBP’s.

Nowadays, the water processing industry is facing pressures on two fronts – (1) EPA regulations lowering the allowable level of DBPs and (2) constituent concerns as a result of regulations, activist groups and media reporting. These pressures require water professionals to continually review their disinfection practices and communicate to the public on the benefits and risks associated with chlorination.

With these new issues at hand alongside ongoing compliance to disinfection standards for drinking water and effluent guidelines for wastewater treatment, adequate laboratory- and field methods for residual chlorine determination are essential for reliable chlorine and DBP monitoring.

Chlorine is typically added to water as chlorine gas or as sodium- or calcium hypochlorite. The active species formed is hypochlorous acid, commonly referred to as ‘free available chlorine’. It is a strong oxidizing agent and readily reacts with nitrogenous compounds as ammonia, nitrites and amino acids.

The products of this reaction are so-called combined chlorine compounds, such as mono-, di-, or trichloramines which are also potent biocides but less effective than hypochlorous acid itself.

The chlorination of water to the extent that all nitrogenous compounds are converted to trichloramines is referred to as ‘break point chlorination. For best disinfection efficiency as well as control of taste and odor, the goal is to keep the system slightly above this break point.

The oldest and most widely used analytical method is the so-called DPD (N,N-diethyl-p-phenylenediamine) method, first introduced in 1957. When DPD reacts with chlorine, the primary oxidation product is a stable cationic radical also referred to as ‘Würster dye’ which was first observed and published in 1931. Excess chlorine transforms the characteristic red dye into an unstable colorless imine, therefore limiting its use at higher chlorine concentrations.

The DPD reaction has been incorporated as Colorimetric Method 4500-Cl G into the Standard Methods for the Examination of Water and Wastewater. The first test kit based on the DPD reaction was introduced in 1973, as the need for accurate chlorine measurements for on-site analysis became more important. However, early field kits lacked reading accuracy as the colored solution had to be compared by the operator to a set of colored standard images.

In the mid-seventies the residual chlorine electrode was introduced. The new device was basically a combination-extension of the iodide electrode with a platinum element, capable of accurately measuring the redox potential difference between iodide and iodine which is proportionally generated by the active chlorine in solution. This potentiometric technique was hailed in a number of evaluations as a rapid, simple means for determining low-level active chlorine. However, the electrode with associated meter were too expensive and the liquid standard calibration procedure too inadequate for practical field analysis which became more and more important in the seventies and eighties.

Portable Colorimeters, turned out to be a much better alternative for field use and on-site analysis. In recent years, the availability of more compact light sources, faster microprocessors and electronics enabled further evolution of portable field units into small handheld colorimeters. The concept permitted accurate on-site testing and eliminated the human error associated with color matching. A number of instrument manufacturers followed this path and introduced similar technology.

To date, colorimetric analysis remains the predominant technique for relative accurate and cost-effective chlorine determinations. Admittedly, the DPD method – although now firmly established – is not a complete bias-free technique as it has to be compensated for color, turbidity and non-linearity of the calibration.

A handheld sensor in a pen format which features the established potentiometric redox electrode technology is the latest in state-of-the-art chlorine measurement technology. As a non-optical method, it is insensitive to sample color and turbidity. Similar to the line of handheld colorimeters, this pen-probe has been particularly designed for rapid field analysis with dry reagent tablets for sample preparation. This chlorine electrode can be used in accordance to Standard Method 4500-ClI (Iodometric Electrode Technique) for quantitative chlorine analysis.

In view of the relative instability of chlorine and chloramines in aqueous solutions along with the availability of more reliable handheld colorimeters and, lately, electrodes in pen format, many professional water operators have shifted their routine to rely on on-site analysis. With a shrinking accuracy ‘gap’, operators can now afford to use their costlier bench-top instruments operating in controlled lab environments as ‘referee’ methods to verify and confirm obtained field readings.

For more information on these types of Chlorine Measurement devices; please contact Konie Chen at 781/434-3950

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