Suspensibility

SUSPENSIBILITY

Principle

           A known quantity of pesticide is shaken with a known quantity of standard hard water for a limited time and from the quantity of the material settled at the bottom of the cylinder, the suspensibility of the material is evaluated.

Materials required

           (i) 100 ml beaker, (ii) 250 ml graduated cylinder with glass stopper, (iii) Water bath, (iv) Vacuum pump, (v) Nozzle with glass tube, (vi) Balance and (vii) Standard hard water (hardness 342 ppm CaCO3). Dissolve 0.302 g of CaCl2 and 0.139 g of MgCl2 and make up the volume to 1 litre with distilled water.

Procedure

• Transfer an accurate quantity of material into a 100 ml beaker so that it forms a uniform suspension of the required concentration (for material like gamma isomer of BHC, Malathion, Zineb, Diazinon and Sulphur 0.5 per cent; DDT and Thiram 2.5 per cent, Dialdrin and Copper oxychloride 0.625 per cent etc.).

• Add 30 ml of standard hard water at 30 ± 1oC.

• Make it into uniform slurry and allow it to stand for 15 minutes at 30oC.

• Transfer the slurry into a 250 ml graduated cylinder.

• Wash the residue if any in the beaker with small quantities of standard hard water collecting the washings in the cylinder.

• Make up the volume of the contents to 250 ml using standard hard water.

• Close the cylinder with the stopper and invert it sharply through 30 complete cycles within one minute.

• Take care to see that the temperature of the contents is maintained at 30 ± 1oC during the period.

• Allow the cylinder to stand at rest for 30 minutes in a thermostatic waterbath at 30 ± 1oC or on a table if the room temperature is around 30 ± 1oC. Protect the table against any vibration.

• At the end of the settling period (i.e. 30 minutes) dip the nozzles of glass tube in the supernatant liquid contained in the cylinder and withdraw 225 ml of the suspension, within 10-15 seconds using a vacuum pump.

• During its withdrawal there should be very minimum disturbance of the contents. This could be achieved by keeping the nozzle of the glass tube just below the sinking top level of the suspension (if excessive flocculation occurs during the test, the material should be rejected. Excessive flocculation in this context means either curdling up of the suspension in the entire column or the appearance of transparent liquid accompanied by the total or partial curdling up).

• Determine the weight of the sediment in the cylinder and calculate the per cent suspensibility.

Calculation

Weight of the material taken for preparing suspension=W1 g

Wt. of the sediment remaining in the cylinder after the drawal of 225 ml of the suspension=W2 g

Therefore suspensibility per cent by wt.=

Things to learn:

Why the suspensibility test is carried out?