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  Section: Plant Protocol » Environmental Science Methodology
 
 
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Determination of pH of Water

 
     
 
Aim

To determine the pH of given samples using
(1) universal indicator
(2) pH paper, and
(3) digital pH meter.

Principle
pH value of water indicates the hydrogen ion concentration in water and concept of pH was put forward by Sorenson (1909). pH is expressed as the logarithm of the reciprocal of the hydrogen ion concentration in moles/ litre at a given temperature. The pH scale extends from 0 (very acidic) to 14 (very alkaline) with 7 corresponding to exact neutrality at 25°C. pH is used in the calculation of carbonate, bicarbonate and CO2, corrosion and stability index etc. While the alkalinity or acidity measures the total resistance to the pH change or buffering capacity, the pH gives the hydrogen ion activity. pH can be measured colorimetrically or electrometrically.

Colorimetric method is used only for rough estimation. It can be done either by using universal indicator or by using pH paper. The hydrogen electrode is the absolute standard for the measurement of pH. They range from portable battery operated units to highly precise instruments. But glass electrode is less subjected to interferences and is used in combination with a calomel reference electrode. This system is based on the fact that a change of 1 pH unit produces an electric charge of 59.1 mV at 25C.

Apparatus

1. pH meter with electrode
2. Beaker
3. Thermometer
4. Colour comparator with discs
5. Cuvettes

Reagents (» click to check the preparation of reagents)
1. Buffer solutions
2. pH paper
3. Universal indicator

Procedure
(a) Using Universal Indicator
  1. 10 mL of sample is taken in a cuvette.
  2. Another 10 mL sample is taken in another cuvette and 0.2 mL of universal indicator is added and placed in the hole provided for.
  3. A colour disc corresponding to this indicator is inserted into the comparator and the disc rotated such that the 2 circles indicate identical colours.
  4. The reading is noted.
  5. The procedure can be repeated using an indicator whose range is near the value obtained.
  6. The exact pH is obtained.
(If comparators are not available, compare the colour with colours given in the chart.)

(b) Using pH Papers
  1. Dip the pH paper in the sample.
  2. Compare the colour with that of the colour given on the wrapper of the pH paper book.
  3. Note down the pH of the sample along with its temperature.
(c) Using pH Meter
  1. Follow the manufacturer's operating instructions.
  2. Dip the electrode in the buffer solution of known pH.
  3. Switch on the power supply and take the reading. Standardize the instrument using the calibrating knob.
  4. After cleaning, again dip the electrodes in the buffer solution of pH 7. Note the reading. If it is 7, the instrument is calibrated. If not, correct the value and is manipulated so that the reading in the dial comes to 7.0.
  5. A solution whose pH is to be found is taken in a beaker and the temperature knob is adjusted such that the temperature of solution is same as that in dial.
  6. The electrode is washed with distilled water and reused with the solution and then it is dipped in the solution.
  7. The reading on the dial indicates the pH of the solution.

Results
 
     
 
 
     



     
 
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