How to Determine Chlorides in Water Sample

One of the most common inorganic anion in water and waste water is chloride (CL- ion). The salty taste produced by chloride concentrations in potable water varies depending on the chemical composition of the water. If the cation is sodium, some waters containing 250 CI- mg/I may have a detectable salty taste. When calcium and magnesium are the dominant cations, the typical salty taste may be absent in waters containing up to 1000 mg/L.

Because sodium chloride (NaCI) is a common dietary ingredient that passes through the digestive system unchanged, the chloride concentration in waste water is higher than in raw water. Because of salt water leakage into the sewerage system along the seacoast, chloride concentrations may be high. Industrial processes may also increase it. The standard test method is based on EPA instructions.

Method Selection

The following methods for determining chloride are presented:

  1. Argentometric method: When 0.15 to 10 mg chlorine are present in the portion titrated, it is suitable for use in relatively clear waters.

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  1. It is easier to detect through mercuric nitrate method.
  2. The potentiometric method It is appropriate for coloured and turbid samples.

Argentometric Method

In a natural or slightly alkaline solution, potassium chromate can indicate the end point of the chloride silver nitrate titration, as silver chloride is quantitatively precipitated before red silver chromate is formed.

Reagents:

  1. Potassium Chromate Indicater or solution of potassium chromate: Dissolve 50 gm potassium chromate in a small amount of distilled water.
  2. Common silver nitrate titrant (0.0141 N). Distilled water is used to dilute 2.395 g AgNO3 to ml. Validate against NaCI.
  3. Sodium chloride standard (0.0141). Dilute 1000 ml with 824 mg NaCI (dried at 110°C) in distilled water.

Procedure

  1. Preparation of the sample Use a sample of 100 ml or a suitable portion diluted to 100 ml. If the sample is highly coloured, add 3 mL of AI(OH)3 suspension, mix, allow to settle, and filter. If the pH is not in the neutral range (7-8), adjust it with H2S04 or NaOH (with the help of pH meter).

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  1. Titrate with standard silver nitrate after adding 2 drops of pot. chromate (0.0141 N). The appearance of brick red indicates the end point of chloride ion.
  2. Note the amount of silver nitrate consumed and calculate the chloride content of water sample.

Significance

  1. Water sample analysis is required to determine whether a sample is suitable for drinking, construction, hydro-testing, or some industrial process. Sands with a high chloride content are not recommended for use in sandblasting or construction.
  2. Waters with a chloride content greater than 250 mg/1 are considered unfit for human consumption. Sodium chloride gives water a salty taste, and too much chloride causes cardiovascular disease.
  3. Chlorides are also corrosive and cause water to become permanently hardened. Waters containing more than 2000 mg/I of chloride are also not recommended for many construction purposes.
  4. Chloride corrodes steel and reduces the strength and solidity of concrete.

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  5. In reasonable concentrations, chlorides are not toxic to humans. They give water a salty taste at concentrations above 250 mg/1, which many people dislike. As a result, chlorides are typically limited to 250 mg/1 in public-use supplies.
  6. Prior to the development of bacteriological testing procedures, chemical tests for chloride and nitrogen in various forms served as the foundation for detecting waste water contamination of ground water.
  7. The chloride content of agricultural crop irrigation waters is generally controlled along with the total salinity of the water.
  8. Evapotransporation tends to increase chloride and salinity at the root zone of irrigated plants, making it difficult for crops to take up water due to osmotic pressure differences between the water outside the plants and the water within the plant cells. As a result, chlorides and total salinity concentrations at or below drinking water standards are typically specified for water used to irrigate salt sensitive crops.
  9. In prawn culture waters, salinity (a measure of chloride concentration) should be between 10 and 30 ppm (gm/litre).

Interferences

Substances found in normal quantities in potable water will not interfere. Equivalent chloride concentrations are bromide, iodide, and cyanide. Sulfide, thiosulfate, and sulfite ions interfere but can be removed with hydrogen peroxide treatment. Orthophosphate concentrations greater than 25 mg/I interfere by precipitating as silver phosphate. Iron concentrations greater than 10 mg/1 interfere by masking the end point.

Precautions

  1. A uniform sample size, preferably 100 ml, must be used to ensure that the ionic concentration required to indicate the end point remains constant.
  2. The pH must be between 7 and 8 because Ag+ precipitates as AgOH at high pH levels and Cr204- is converted to Cr207- at low pH levels.
  3. To provide a specific concentration of Cr04′, a specific amount of indicator must be used; otherwise, Ag2CrO4 may form too soon or too late.

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