INTRODUCTION

Chloride, a naturally occurring element found most commonly in natural waters. It is often found as a component of salt (sodium chloride) or in combination with potassium or calcium. The weathering of soils, salt-bearing geological formations, salt used for road de-icing, contributions from wastewaters and in coastal areas, intrusion of salty ocean water into fresh groundwater are the main sources of chloride in water. Chlorides can corrode metals and affect the taste of food products. It is not usually harmful to humans; however, the sodium part of table salt has been linked to heart and kidney disease. Chloride is basically fully absorbed in normal individuals until the content is too high. Presence of Sodium chloride in water gives salty taste at 250mg/l, calcium or magnesium chloride does not impart any taste until it reaches 1000mg/l.

The measured chloride ions can be used to know salinity of different water sources. For brackish water (or sea water or industrial brine solution), it is an important parameter and indicates the extent of desalting of apparatus required. It also interferes with COD determination and thus it requires a correction to be made on the basis of amount present or else a complexing agent, such as HgSO₄ can be added. Further, chloride ions are used as tracer ions in column studies to model fate of different contaminants in soil and liquid media.

Effects on humans :

A normal adult human body contains approximately 81.7g chloride. Based on a total obligatory loss of chloride of approximately 530mg/day, a dietary intake for adults of 9mg of chloride per kg of body weight has been recommended (equivalent to slightly more than 1g of table salt per person per day). For children up to 18 years of age, a daily dietary intake of 45mg of chloride should be sufficient. A dose of 1g of sodium chloride per kg of body weight was reported to have been lethal in a 9-week-old child. Little is known about the effect of prolonged intake of large amounts of chloride in the diet.

Principal involved :

The Mohr's Method uses silver nitrate for titration (normality: 0.0141) (method applicability: 0.15 to 10 mg/l chloride ions). This corresponds to 1ml of 0.0141 equals to 1mg chloride in solution. The silver nitrate solution is standardized against standard chloride solution, prepared from sodium chloride(NaCl). During the titration, chloride ion is precipitated as white silver chloride.

(Eg.1) : Ag⁺ + Cl^- ⇔ AgCl (Solubility product constant, Ksp=3×10^-10)

The indicator (potassium chromate) is added to visualize the endpoint, demonstrating presence of excess silver ions. In the presence of excess silver ions, solubility product of silver chromate exceeded and it forms a reddish-brown precipitate (Eq.2). This stage is taken as evidence that all chloride ions have been consumed and only excess silver ions have reacted with chromate ions:

(Eg.2) : 2Ag⁺ + CrO₄^2- ⇔ Ag₂CrO₄ (Ksp=5×10^-12)

This method can be used to determine the chloride ion concentration of water samples from many sources such as seawater, stream water, river water and estuary water. If the solutions are acidic, the gravimetric method should be used.

Relevant Indian Standard for Chloride in Water Test :

1. IS 3025 (Part 32) -1988: Method of Sampling and Test (Physical and Chemical) for Water and Wastewater- Chlorides, First Revision.