Alkalinity refers to the capability of water to neutralize acid. This is really an expression of buffering capacity. A buffer is a solution to which an acid can be added without changing the concentration of available H+ ions appreciably. It essentially absorbs the excess H+ ions and protects the water body from fluctuations in pH, the buffering system is carbonate-bicarbonate (H2CO3, HCO3, and CO3). The presence of calcium carbonate or other compounds such as magnesium carbonate contribute carbonate ions to the buffering system. Alkalinity is often related to hardness because the main source of alkalinity is usually from carbonate rocks (limestone) which are mostly CaCO3. If CaCO3 actually accounts for most of the alkalinity, hardness in CaCO3 is equal to alkalinity.
Measuring alkalinity is important to determining a river’s ability to neutralize acidic pollution from rainfall or snowmelt which is measured by a pH. It’s one of the best measures of the sensitivity of the river to acid inputs. Alkalinity comes from rocks and soils, salts, certain plant activities, and certain industrial wastewater discharges. Total alkalinity is measured by collecting a water sample, and measuring the amount of acid needed to bring the sample to a pH of 4.2. At this pH all the alkaline compounds in the sample are “used up” which is being determined by the use of titration.
A titration is a technique where a solution of known concentration is used to determine the concentration of an unknown solution. Typically, the titrant which is the know solution is added from a burette to a known quantity of the analyte which is the unknown solution until the reaction is complete. Knowing the volume of titrant added allows the determination of the concentration of the unknown. Often, an indicator is used to usually signal the end of the reaction, the endpoint.
An indicator is a chemical reagent used to recognize the attainment of end point in a titration. After the reaction between the substance and the standard solution is complete, the indicator should give a clear colour change to show the end point of the solution in titration, where as an end point is a stage which shows the completion of a particular reaction and equivalence point is a stage in which the amount of reagent added is exactly and stoichiometrically equivalent to the amount of the reacting substance in the titrated solution. The end point is detected by some physical change produced by the solution, by itself or more usually by the addition of an auxiliary reagent known as an ‘indicator’. The end point and the equivalence point may not be identical. End point is usually detected only after adding a slight excess of the titrant.
Materials and methods
0.1M Sulphuric acid
Methyl red indicator
100ml of a sample was measured into a 250ml Erlenmeyer flask and placed onto a stir plate, the initial pH of the sample was measured and if the pH of the sample was below 8.3, several drops of methyl red indicator was added into the sample to turn it yellow then titrated with 0.1M of H2SO4 until the colour changed to pink or red and recorded the total volume of the acid used for the titration. If the pH of the sample was above 8.3 , several drops of Phenolphthalein indicator were added into the sample and the colour turned into pink then it was titrated with 0.1M of H2SO4 until the colour changed from pink to clear and recording the volume of the acid used for titration then after the same pH of the sample was used with methyl red indicator and titrated with 0.1M of H2SO4 . The procedure was repeated three times for concordant values, requiring calculations for both phenolphthalein alkalinity and total alkalinity.
Table 01: volume of Sulphuric acid used during titration of three different samples, where each sample was titrated three times.
Sample no: Volume of sample(ml) Volume of H2SO4 for 1st titration(ml) Volume of H2SO4 for 2nd titration(ml) Volume of H2SO4 for 3rd titration(ml) Average titre(ml) Alkalinity CaCO3 mg/l
1 100ml 0.9ml 0.5ml 0.2ml 2 100ml 0.1ml 0.2ml 0.4ml 3 100ml 0.3ml 0.3ml 0.7ml 100ml 0.3ml 0.7ml 0.6ml
Two of the most common titrations are acid-base and redox titrations. In an acid-base titration one of the solutions is an acid and the other a base. one is placed in a flask. The other is placed in a burette, from which it is dripped into the flask until the titration reaches its end point. A suitable indicator needs to be chosen such that the end point shows accurately that all of the solution in the flask has reacted with the solution being dripped into it – the point at which this happens is called the equivalence point. In a perfect titration the end point and equivalence point will be identical. If the indicator is not chosen well, the end and equivalence points will differ, and the titration will not produce accurate data about the solution of interest