Chemical parameters-pH

pH 
  • In aquaculture systems, the pH factor plays an important role. The ‘soil pH’ and the ‘water pH’ need to be tested in culture systems before and after stocking. Also, it is essential to determine if the site is suitable for aquaculture.

Water pH

  • The pH factor of water can be tested using ‘pH paper’, or ‘pH meter’ or by using an ‘electronic pH pen’. However, use of pH meter is advisable due to its accuracy than other methods. pH meters are available with digital and electronic dibble modules.

Principle

  • The pH of a sample of water can be measured using a pH meter, which employs the glass electrode. The pH meter should first be calibrated with a set of standard buffer solutions (pH 4, 7 or 9) before determining the pH of the water sample. The glass electrode and the reference calomel electrode or a ‘combined glass electrode’ are used to determine the pH of a solution.

Standard buffer solutions

  • Potassium tetroxalate: 3.175g of KH3 C4O8 2H2O is dissolved in water and made up to 250ml.
  • Potassium acid tartrate: 250 ml distilled water is taken in a glass stoppered bottle. Excess KHC4 H4O6 is added and the stoppered bottle is shaken vigorously for about 10 min to obtain a saturated solution. The solution is filtered and preserved by adding 0.1 g thymol to it.
  • Potassium acid phthalate: 2.552 g of KHC8H4O4 is dissolved in water and is made up to 250 ml.
  • Potassium dihydrogen phosphate : 0.860 g of KH2PO4 and 0.88755 Na2HPO4 are dissolved in water and the solution is made upto 250 ml
  • Borax: 0.952g of Na2B4O710H2O is dissolved in water and is made up to 250 ml.

Procedure

  • The glass electrode and the calomel electrode are washed gently with distilled water and then with the solution for which the pH is to be measured. One of the standard buffer solutions is taken in a dry beaker and then the two electrodes are held inside the buffer. The electrode bulbs should be immersed in the liquid without touching the bottom or the sides of the beaker. After 3 minutes the pH meter is adjusted to read the pH for that buffer solution. The calibration is now checked by reading the pH of another buffer solution


Buffer solution

Temperature ( 0 C)

20

25

30

35

40

0.05M Potassium tetroxalate solution

1.68

1.68

1.69

1.69

1.70

Saturated solution tartrate

3.56

3.56

3.55

3.54

3.54

0.05M Phthalate solution

4.00

4.01

4.01

4.02

4.03

0.025M Phosphate solution

6.88

6.86

6.85

6.84

6.84

0.01M Borax

9.22

9.18

9.14

9.10

9.07

  • The beaker containing the standard is then removed and the electrodes are washed with distilled water and finally with the water sample whose pH is to be measured. The water sample is taken in a dry beaker and the electrodes are immersed inside the water sample. The pH of the water sample is noted after about 3 minutes. Aeration and agitation of the sample water are avoided during the measurement of pH. The calibration of the pH meter should be done by a buffer solution whose pH is close to that of the sample water.

Note

  • pH values are temperature-dependent. Therefore, the container of the sample may be maintained in a constant temperature water bath (250C) when the pH is recorded. Similarly, the buffer solutions can also be maintained in the water bath when the pH meter is calibrated.
  • When a water sample is brought to the laboratory, its pH should be determined immediately. Delaying the measurement may involve change in the pH value of the sample because of loss of dissolved gases and also chemical changes. The best pH value of a water sample is obtained by determining this value at the time of sample collection on the spot itself using a battery operated portable pH water meter or pH pens that are available commercially.

pH_Meter pH_pen

PH meter

PH pen

pH meter pH pen

Soil pH

  • Soil pH testing is necessary to confirm the suitability of soil for farming practice. It is normally tested with the help of a battery operated, cone shaped, digital pH meter. By inserting the cone shaped metallic tip into the soil, the display meter shows the pH of the soil at that point. If the soil is found to be hard, the distilled water can be let into the area, and after soaking the soil, the pH reading can be taken.

Solid pH meter

Solid PH meter

Sampling protocol

  • Number of soil sample/ hectare
    • Existing culture pond (to check pH between crops) = 4 Nos.
    • New area (for site selection) = 8 Nos.
  • Depth at which soil sample is to be drawn
    • Existing culture pond = Top 15 cm
    • New areas = From the depth of 1 meter
Last modified: Wednesday, 4 April 2012, 11:31 AM