Acidity and alkalinity

ESTIMATION OF ACIDITY AND ALKALINITY OF PESTICIDE

The pesticide formulations may often develop acidity or alkalinity on storage due to manufacturing process and sometimes due to addition of emulsifying agents, wetting agents and filler materials. The acidity or alkalinity may lead to degradation of active ingredients and caking in case of dusts. Normally the acidity is expressed as percentage of H2SO4 and alkalinity expressed as percentage of NaOH.

Qualitative test for acidity or alkalinity

           Take 0.5g of pesticide materials in a test tube and add about 1ml of distilled water and mix it thoroughly. Test the mixture for acidity or alkalinity with litmus paper. Change of red litmus to blue indicates alkalinity and blue litmus to red indicates acidity.

Quantitative determination of acidity

1)     Wettable powders

Materials required

    i). 0.02N NaOH ii) Methyl red iii) Acetone iv) 0.02N H2SO4 v) Conical flask, vi) filter paper, vii) Burette and viii) Balance

Procedure

v  Transfer exactly 10 g of pesticide material into a dry conical flask and add 25ml of acetone and mix thoroughly.

v  Warm gently till the active ingredient completely dissolves i.e. for 10-15 min.

v  Add 25 ml of water and allow to stand for an hour. Filter the supernatant aqueous extract.

v  Titrate the filtrate immediately against standard NaOH using methyl red as indicator.

v  The end point will be the change of colour from pink or red to yellow. Simultaneously carry out a blank with 50ml aliquot by adding 25ml water.

Quantitative determination of alkalinity

Procedure

Ø  Transfer exactly 10 g of water dispersible material into a dry conical flask and add 25ml of acetone and mix the content thoroughly.

Ø  Warm the flask gently for 10-15min so as to dissolve the active ingredient and add 25 ml of water and allow to stand for an hour.

Ø  Filter the supernatant aqueous extract.

Ø  Titrate the filtrate immediately against standard H2SO4 using methyl red as indicator.

Ø  The end point will be the change of colour from yellow to red or pink. Simultaneously carry out a blank with 25ml acetone and 25ml water.

ii) Liquid formulations

·       Weigh accurately 10 g of material, transfer it to dry conical flask and add 100ml of water.

·       Titrate the content of the flask with standard acid or alkali. Depending upon the original alkalinity or acidity using methyl red as indicator.

·       Carryout blank determination using 100ml water.

Calculation

Acidity determination

Volume of standard NaOH consumed in sample titration=V1 ml

Volume of standard NaOH consumed in blank  titration=V2 ml

Normality of standard NaOH=N

Weight of sample taken=W  g

Acidity percentage (as H2SO4) by mass=4.9X (V1-V2) N/W

If blank shows alkaline reaction neutralize with standard H2SO4 and calculate the acidity as given below

Acidity percentage (as H2SO4) by mass=4.9X (V1N1+V2N2)/W

Where,

V1 = Volume of standard NaOH consumed in sample titration

N1 = Normality of standard NaOH

V2 = Volume of standard H2SO4consumed in blank titration

N2 = Normality of standard H2SO4

W = Weight of sample taken

Alkalinity determination

Volume of standard H2SO4 consumed in sample titration=V1 ml

Volume of standard H2SO4consumed in blank  titration=V2 ml

Normality of standard H2SO4=N

Weight of sample taken=W  g

Alkalinity percentage (as NaOH) By mass=4.0X (V1-V2) N/W

If blank shows acidic reaction, neutralize with standard NaOH and calculate the alkalinity as given below

Alkalinity percentage (as NaOH) By mass=4.0X (V1N1+V2N2)/W

Where,

V1 = Volume of standard H2SO4consumed in sample titration

N1 = Normality of standard H2SO4

V2 = Volume of standard NaOH consumed in blank titration

N2 = Normality of standard NaOH

W = Weight of sample taken

I.S.I. Specification for acidity and alkalinity

Powder formulations: 1%

Liquid formulations   : 1.5%