The alkalinity of water and wastewaters is the capacity to neutralize acids. Alkalinity is the term used to describe the concentration of hydroxide ions in water.
The general formula for alkalinity is Ca x 2 + Mg x 2 H 2 O where Ca and Mg are the cations and x is the degree of hydroxylation. Calcium reacts with water to produce ions that may cause alkalinity.
Alkalinity is usually expressed in mg/l or meq/l of CaCO 3. The pH scale is a logarithmic scale used to measure acidity. basicity of a solution and it also represents alkalinity.
What causes high alkalinity in water?
The alkalinity of natural waters may be attributed to the presence of salts of weak acids. Examples of these are bicarbonates, phosphates, silicates, and borates. These induce buffer capacity and resist the lowering of H+ ions.
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Surface waters containing algae and also water treated by the soda-lime process may contain considerable quantities of carbonate and hydroxide alkalinity. The alkalinity of water is due to Hydroxides, carbonates, and bicarbonates.
What happens if alkalinity is high?
Highly alkaline waters may lead to caustic embrittlement. It may cause the deposition of precipitates and sludges in boiler tubes and pipes. Bicarbonates of calcium and magnesium induce temporary hardness in water. The temporary hardness of water is due to the presence of carbonates and Bicarbonates of calcium and magnesium. The presence of these carbonates and bicarbonates also produces high alkalinity values.
Types of Alkalinity
1. Hydroxide only
2. Carbonate only
3. Bicarbonate only
4. Hydroxide and carbonates and bicarbonates
The line diagram explains the variation of types of alkalinity and acidity on pH.
Principle of alkalinity determination:
The type and extent of alkalinity present in a water sample may be determined by titrating an aliquot of the sample with standard acid to the phenolphthalein endpoint, (P) and then continuing the titration to the methyl orange endpoint (M).
Reagents required for measuring alkalinity of water
1. Standard HCL or H2SO4 (N/50)
2. Phenolphthalein indicator
3. Methyl orange indicator
Procedure for calculating alkalinity of water
1. Transfer 100 ml of the water sample into a conical flask and add 2 drops of phenolphthalein indicator. Titrate the sample with standard HCl or H2SO4 solution until the pink color disappears. Note the titer value as the phenolphthalein endpoint (P).
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2. Add 2 to 3 drops of methyl orange indicator to the same solution and continue the titration until the sharp color changes from yellow to orange. Note the total titer value from the beginning of the experiment as the methyl orange endpoint (M).
The general equation of carbonate titration is caco3 + hcl.
3. Calculate the alkalinity of water using this formula:
Significance of alkalinity in water
1. Large amounts of alkalinity impart a bitter taste to water.
2. The principal objection to alkaline water is the reactions that can occur between alkalinity and certain cations in water. The resultant precipitate can foul pipes and other appurtenances of water distribution systems like valves.
3. Highly alkaline water is usually unpalatable and consumer acceptance decreases. Water having an alkalinity of less than 200 mg/L as CaCO3 is desirable for drinking.
4. To neutralize acids produced during flocculation, the sample should be alkaline, as otherwise, further flow formation (Either Al(OH)3 or Fe(OH)1 slowly ceases. For example, alkalinity of about 4.5 mg/L as CaCO3 is essential for every 10 mg/L dosages of alum and basicity of h2so4.
5. To find out the quantity of lime and soda ash required for the removal of hardness, alkalinity should be found. The purity of lime also can be analyzed by finding alkalinity values.
6. Wastewater containing excess caustic (hydroxide) alkalinity is not to be discharged into natural water bodies or sewers. Excess alkalinity in water is harmful for irrigation which leads to soil damage and reduces crop yields. Water having an alkalinity content of less than 250 mg/I is desirable for domestic consumption.
7. Excess alkalinity gives a bitter taste to water. It reacts with cations and forms precipitates and damages pipes etc. Some alkalinity is required in drinking water to neutralize the acids such as lactic acid and citric acid, produced in the body.
Thus it acts as a buffer. As such, waters having no alkalinity (such as rainwater or distilled water) cannot be used for drinking.
What is the difference between pH and alkalinity?
Water’s alkalinity is a measure of its ability to neutralize acids. The concentration of hydroxide ions in water is referred to as alkalinity. The pH of a solution indicates its acidity. The ph scale diagram runs from 0 to 14, with 7 representing neutral.
What is the total alkalinity of water?
Water’s total alkalinity is the sum of its hydroxide and carbonate alkalinity. It is measured in milligrams per liter of CaCO3. The pH scale is a logarithmic scale that is used to determine acidity. It represents the basicity of a solution as well as its alkalinity.
What is caustic alkalinity in water?
Caustic alkalinity in the water samples is referred to as hydroxide alkalinity. This is a result of the presence of hydroxide ions. It is usually determined by the method of periodic acid titration.
Titration with standard acid can be used to determine the alkalinity of water. The technique is known as direct titration. The method is also referred to as the direct method. An aliquot of the water sample is titrated with standard acid until the phenolphthalein endpoint is reached in this method. Adding 2-3 drops of methyl orange and re-titrating with standard acid until the endpoint is reached completes the reaction.