PIPETTING AND TITRATION

Pipetting and Titration

Pipetting and Titration

Introduction

Chlorine reacts with natural water and wastewater constituents to produce a variety of compounds with a variety of disinfection abilities. Free available chlorine (FAC) is a strong disinfectant. The presence of free chlorine, i.e., a nonzero measured FAC residual, is usually regarded as a sign of adequate disinfection. In fact, US Environmental Protection Agency regulations1 allow for the replacement of up to 75 percent of the required microbiological assays with free chlorine measurements in some finished drinking waters. Proposed regulations exempt drinking water utilities from filtration requirements if measured free chlorine residuals (incorporated into concentration X time values) are adequate. These regulations emphasize the need for accurate FAC measurements. If FAC measurements are interfered with by poorer disinfectants, the measured residuals will not ensure adequate disinfection or bio fuelling control. Inorganic and organic chloramines are much weaker disinfectants than free chlorine. A positive interference occurs if a portion of the free chlorine measurement is due to chloramines. This interference would lead to an overestimation of the true FAC concentration and could result in an incorrect assessment of the microbiological quality of the treated water

Discussion

Alternative Indicators

A number of new indicators were tried with the result that two were found which may be used as substitutes for phenolphthalein and methyl orange. An added advantage of these two indicators is that both have the same colour changes. Six drops of one indicator in 75 e.c. of solution gives a fairly deep blue in the presence of sodium hydrate and carbonate and on titration with hydrochloric acid retains this colour until the hydrate is all neutralized and the carbonate converted into bicarbonate when it changes at the neutral point to a muddy green and then with a slight excess of acid to a lemon yellow. The addition of three drops of the second indicator will now change the solution to a deep blue, which continues until the bicarbonate has all been destroyed, when the solution shows the same intermediate change as before and becomes a lemon yellow again when a slight excess of acid is present.

These indicators are among the nine recommended by Clark & Lubs for the colorimetric determination of hydrogen ion concentration. The first indicator, thymol blue (thymol sulfon phthalein) is prepared by introducing 1 decigram of the substance into a Florence flask and then adding 4.3 c, e. of n/20 sodium hydroxide. The solution is best heated by introducing the flask into hot water and agitating until the indicator is all dissolved. When solution is complete, the volume is made up to 250 i.e., with distilled water.

The substitute for methyl orange is brom phenol blue (tetra bromo phenol sulfon phthalein). This indicator is made up in the same way except that 1 decigram requires only 3.0 c.o, of n/20 sodium hydroxide.

According to the standard methods for amperometric titration of chlorine (4500-Cl D, Standard Methods1), a standardized titrant solution of phenyl arsine oxide (usually 0.005 64N) is reacted with the sample solution to an endpoint ...