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The below mentioned article includes a list of five easy experiments on water.
1. Experiment to determine pH of the water and to identify the water quality by pH:
Requirements:
Water sample, broad range and narrow range pH paper strips, pH paper strip booklet, BDH Universal Indicator, test tube.
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Method:
As already explained in Exercise No. 28, pH of water sample may be determined by using pH paper strip method or Universal Indicator method. In the “pH paper method’, the strip of broad range of pH paper is immersed in the water sample.
Colour of pH paper strip changes. Compare this colour with the standard colour chart printed on pH paper strip booklet. Narrow range pH paper strip is dipped in the water sample and compared with the standard colour chart for determining correct pH of the sample.
In the “Universal indicator method”, take about 100 ml of water sample in a test tube and add in it the BDH Universal Indicator. Colour of the water changes. Compare this colour with the chart given on the bottle of the indicator and note the pH.
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Determination of water quality by pH:
pH is a quantitative expression for acidity or alkalinity of a solution. It denotes the concentration of hydrogen or hydroxyl ions. If the pH is 7, the water or solution is neutral. The water or solution is acidic if its pH is less than 7, and alkaline if it is more than 7. The pH scale ranges from 0 to 14.
The tap water, found normally in the pipelines has pH range of 7-8.5. Less than 7 pH of the water indicates it corrosiveness. Water with a pH more than 8.5 indicates sediment deposits and difficulty in chlorination.
2. Experiment to find out the total hardness of the water:
Requirements:
Water sample, conical flask, Erichrome Black T indicator, ammonia buffer, EDTA solution.
(a) Preparation of erichrome black T indicator:
It is prepared by dissolving 0.6 gm Erichrome Black T and 100 gm NaCl in 20 ml distilled water. Warm it and use.
(b) Preparation of ammonia buffer:
It is prepared by mixing 16.9 gm NH4CI in 143 ml of liquid ammonia and diluting the mixture to 250 ml by adding distilled water.
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(c) Preparation of EDTA solution:
It is prepared by dissolving 4 gm disodium ethylene-di-amine-tetra-cetate-dihydrate in 800 ml of distilled water.
What is hardness of water?
The characteristic of water which checks lathering of soap is known as hardness. It is usually due to the salts of calcium and magnesium dissolved in water. The carbonate hardness is caused by carbonates and bicarbonates whereas non-carbonate hardness is caused by chlorides and sulphates of calcium and magnesium.
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Method:
1. In a conical flask take 100 ml of water sample and add in it a few drops of Erichrome black T indicator and 1 ml of ammonia buffer.
2. Shake the conical flask well and titrate it with EDTA solution immediately. Mix the EDTA solution until the colour changes from red to blue.
3. Calculate the total hardness by the following formula:
Results:
Usually the hardness is expressed in mg per litre of CaCO3. If the hardness ranges between 0- 50 mg/l the water is said to be very soft. Water is moderately soft if the hardness is between 51 and 100 mg/l; slightly hard if it is between 101 and 150 mg/l; moderately hard if it is between 151 and 250; and hard if it is above 250 mg/l.
3. Experiment to determine the Biochemical Oxygen Demand (BOD) of water:
Requirements:
pH meter, water sample, 1N acid and 1N alkali solutions (1 N sulphuric acid is prepared by adding 2.8 ml of conc. H2SO4 to 100 ml of BOD-free distilled water, whereas 1 N sodium hydroxide is prepared by adding 4 gm of NaOH in 100 ml of distilled water), BOD bottles (6), allylthiourea, BOD incubator, BOD-free distilled water, pipette, flask and all requirements of Exercise No. 31 of this article.
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Method:
(BOD or Biochemical Oxygen Demand is a method of determining the amount of organic compounds in sewage as estimated by the volume of oxygen required by bacteria to metabolize it under aerobic conditions. It provides us an idea of the organic pollution. In case the organic matter is very high in the sewage or polluted water, more amount of oxygen will be used by bacteria to degrade it.
In a water sample, BOD is usually measured by incubating the sample at 20°C for five days in the dark under aerobic conditions. By BOD estimation, the degree of pollution in the water reservoir or stream is determined).
BOD can be determined in the following manner:
1. With the help of pH meter, adjust the pH of the water sample to neutrality by adding 1 N alkali or 1 N sulphuric acid.
2. Fill the water sample in all the six B OD bottles and add 1 ml allylthiourea in each bottle.
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3. Determine the amount of dissolved oxygen in three bottles by the method explained in Exercise No. 31 of this article. Take the mean of three readings and treat them as D2.
4. To the remaining three BOD bottles incubate at 20°C in a BOD incubator for 5 days. After 5 days, estimate the amount of dissolved oxygen concentration in all these three incubated samples of water by the same method as explained in Exercise No. 31 of this article. Take the mean of these three readings also and treat them as D2.
Results:
Calculate BOD of the water by the following formula:
BOD in mg/1 (5 days at 20°C) = D1 – D2 where
D1 = Initial dissolved oxygen (mg/1) in sample.
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D2 = Dissolved oxygen (mg/1) after 5 days incubation.
4. Experiment to determine Chemical Oxygen Demand (COD) of water:
Requirements:
Water sample, conical flasks (6), distilled water, 0.1 N potassium dichromate solution (prepared by dissolving 3.676 gm of K2Cr2O7 in 1 litre of distilled water), water bath, potassium iodide solution (10%), 2M sulphuric acid (prepared by adding 10.8 ml conc. H2SO4 in 100 ml distilled water), 0.1 M sodium thiosulphate (prepared by dissolving 15.811 gm sodium thiosulphate in 2 litres of distilled water), starch solution (1%), water blanks, titration assembly.
Method:
(Chemical Oxygen Demand or COD is the quantity of oxygen required during chemical oxidation of organic matter. It determines the amount of oxygen required for chemical oxidation of organic matter in water which is formed due to decaying vegetable matter and animal or human wastes.
COD-test has an edge over BOD-test because results in this test may be obtained within a few hours as compared to BOD-test which requires 5 days). COD may be determined by several methods, of which one is under mentioned:
1. Pour 50 ml water sample in three separate conical flasks, and also run simultaneously three distilled water blanks as standards.
2. In each of the six flasks add 5 ml of potassium dichromate solution and keep all the flasks in water bath at 100°C for one hour. Allow the samples to cool down for about 15 minutes.
3. In each flask add 5 ml of potassium iodide solution (10%) and 10 ml of H2SO4.
4. Titrate the solution of each flask with sodium thiosulphate (0.1M) until the appearance of pale yellow colour.
5. Now add 1 ml starch solution in each flask. The solution turns blue coloured.
6. Titrate it again with sodium thiosulphate (0.1M) until the blue colour disappears completely.
Note your observations as under:
Observations:
Before starting the titration, note the reading on the burette. Also note the reading at the end point. Calculate the volume of sodium thiosulphate used for each sample and find out the mean value of the three readings for each sample. Also note the volume of sodium thiosulphate used for distilled water blank standards.
Note the readings in the following Table 4.18:
Results:
Calculate the COD (mg1-1) per litre of the water sample by following formula:
COD of water sample mg/l =8 × Z × (Y – X)/W
where
W = Volume of water sample taken (in ml)
X = Volume of titrant used for blank (in ml)
Y = Volume of titrant used for water sample (in ml)
Z = Concentration of titrant (M mol/litre).
5. Experiment to estimate the salinity (amount of chlorides) of different water samples:
Requirements:
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Water sample, conical flask, potassium chromate indicator solution, standard silver nitrate solution, pipette, beaker, etc.
Preparation of Reagents:
(a) Potassium chromate indicator solution:
Dissolve 50 gm. K2CrO4 in a little amount of distilled water and add in it AgNO3 solution until a red precipitate is formed. Wait for about 12 hours and add distilled water to make the solution 1 litre.
(b) Standard Silver Nitrate Solution (0.0141 N):
Dissolve 2.395 gm. AgNO3 in a little amount of distilled water and add more distilled water to make the solution 1 litre.
Method:
1. Take 50 ml water sample in a conical flask and adjust its pH between 6-8. Add 1ml K2CrO4 indicator solution in the sample.
2. Titrate with standard silver nitrate solution until a pinkish yellow or red colour is obtained.
3. Repeat the titration with reagent blank and calculate the chloride in mg/litre by the following formula:
Chloride (mg/litre = A – B) × N × 35.45 × 1000/… ml sample
where,
A = Amount of AgNO3 (in ml) used for sample.
B = Amount of AgN03 (in ml) used for blank.
N = Normality of AgNO3.
