IS: 3708 (Part 4) – 1986
DETERMINATION OF VOLATILE FATTY ACID
1. SCOPE
1.1 This standard prescribes a method for the determination of volatile fatty acid number of
natural rubber latex which contains preservative agents and has been subjected to same type
of concentration process.
1.2 This method is not necessarily suitable for lattices from natural sources, sources other than
Hevea brasiliensis and is not applicable to compounded latex, vulcanized latex, artificial
dispersions of rubber or synthetic rubber lattices.
natural rubber latex which contains preservative agents and has been subjected to same type
of concentration process.
1.2 This method is not necessarily suitable for lattices from natural sources, sources other than
Hevea brasiliensis and is not applicable to compounded latex, vulcanized latex, artificial
dispersions of rubber or synthetic rubber lattices.
2. TERMINOLOGY
2.0 For the purpose of this test, the following definition shall apply.
2.1 Volatile Fatty Acid (VFA) Number of Latex - the number of grams of potassium hydroxide
equivalent to the volatile fatty acid in latex containing 100g of total solids.
2.1 Volatile Fatty Acid (VFA) Number of Latex - the number of grams of potassium hydroxide
equivalent to the volatile fatty acid in latex containing 100g of total solids.
3. OUTLINE OF THE METHOD
3.1 The latex is coagulated with ammonium sulphate and the resultant serum is separated and
acidified with sulphuric acid. The serum is steam distilled the volatile acids percent in the latex
are determined by acidimetric titration of the distillate.
acidified with sulphuric acid. The serum is steam distilled the volatile acids percent in the latex
are determined by acidimetric titration of the distillate.
4. APPARATUS
4.1 Stream jacketed distillation apparatus (Markham still) conforming essentially to figure 1. As
an alternative to the one –piece apparatus illustrated, a ground glass joined may be inserted
between the distillation vessel and the condenser.
an alternative to the one –piece apparatus illustrated, a ground glass joined may be inserted
between the distillation vessel and the condenser.
5. REAGENTS
5.1 Ammonium sulphate solution – 30 percent (min) aqueous solution.
5.2 Sulphuric acid, Dilute – approximately 50 percent (min) aqueous solution.
5.3 Standard Barium Hydroxide Solution – 0.01 N. The solution is standardized by titration with
potassium hydrogen phthalate and stored in the absence of Carbon dioxide.
5.4 Indicator – The indicators bromo thymol blue or Phenolphthalein may be used. Bromo
thymol blue shall be used as 0.5 percent solution of the indicator in water and phenolphthalein
shall be used as 0.5 percent solution in a mixture of equal volumes of ethanol and water.
5.5 Anti foaming Agent – Silicone type.
5.2 Sulphuric acid, Dilute – approximately 50 percent (min) aqueous solution.
5.3 Standard Barium Hydroxide Solution – 0.01 N. The solution is standardized by titration with
potassium hydrogen phthalate and stored in the absence of Carbon dioxide.
5.4 Indicator – The indicators bromo thymol blue or Phenolphthalein may be used. Bromo
thymol blue shall be used as 0.5 percent solution of the indicator in water and phenolphthalein
shall be used as 0.5 percent solution in a mixture of equal volumes of ethanol and water.
5.5 Anti foaming Agent – Silicone type.
6. PROCEDURE
6.1 Weigh about 50g of latex of known dry rubber and total solid content to the nearest 0.1gm
in to a beaker. Add 50ml of ammonium sulphate solution while stirring the latex. Either place
the beaker on a steam bath or immerse the beaker in a 70°C water bath, and continue stirring
the latex until its coagulates. Cover the beaker with a watch glass and leave it in the bath for
the total period of 15min. Decant the serum which exudes through a dry filter. Transfer the
coagulum in to a mortar and press out more serum by kneading it with a pestle. Filter this
serum through the same filter. Pipette 25ml of the filtered serum in to a dry 50ml conical flask
and acidify it by accurately adding 5ml of dilute sulphuric acid. Mix well by swirling the flask.
6.2 Pass steam through the apparatus for at least 15min. With steam passing through the outer
jacket of the apparatus, introduce in to the inner tube 10ml of the acidified serum by pipette. If
foaming is a difficulty, one drop of suitable anti foaming agent may be added. Place a 100ml of
graduated cylinder under the tip of the condenser to receive the distillate. Partially close the
steam outlet to divert steam in to the inner tube. Pass steam gently at first, then fully close the
steam outlet and continue distilling at a rate of 3 to 5ml/ min until 100ml of distillate has been
collected. Transfer the distillate to a 250ml conical flask and aerate the distillate by passing
through it a stream of air free from carbon dioxide at a rate of 200 to 300ml/min for
approximately 3min. Titrate with 0.01N barium hydroxide solution using one of the indicators
specified. Run a blank test by substituting 20ml of water for the 50g of concentrated latex.
in to a beaker. Add 50ml of ammonium sulphate solution while stirring the latex. Either place
the beaker on a steam bath or immerse the beaker in a 70°C water bath, and continue stirring
the latex until its coagulates. Cover the beaker with a watch glass and leave it in the bath for
the total period of 15min. Decant the serum which exudes through a dry filter. Transfer the
coagulum in to a mortar and press out more serum by kneading it with a pestle. Filter this
serum through the same filter. Pipette 25ml of the filtered serum in to a dry 50ml conical flask
and acidify it by accurately adding 5ml of dilute sulphuric acid. Mix well by swirling the flask.
6.2 Pass steam through the apparatus for at least 15min. With steam passing through the outer
jacket of the apparatus, introduce in to the inner tube 10ml of the acidified serum by pipette. If
foaming is a difficulty, one drop of suitable anti foaming agent may be added. Place a 100ml of
graduated cylinder under the tip of the condenser to receive the distillate. Partially close the
steam outlet to divert steam in to the inner tube. Pass steam gently at first, then fully close the
steam outlet and continue distilling at a rate of 3 to 5ml/ min until 100ml of distillate has been
collected. Transfer the distillate to a 250ml conical flask and aerate the distillate by passing
through it a stream of air free from carbon dioxide at a rate of 200 to 300ml/min for
approximately 3min. Titrate with 0.01N barium hydroxide solution using one of the indicators
specified. Run a blank test by substituting 20ml of water for the 50g of concentrated latex.
7. CALCULATION
7.1 The volatile fatty acid (VFA) number is calculated as follows.
VFA = [67.32×N×V] × [50+ m (100-DRC)]
M ×TS 100 D
Where
N = normality of the barium hydroxide solution
V = final volume of barium hydroxide solution required to neutralize the distillate in
Milliliters, after correction for blank;
DRC = percentage dry rubber in the latex;
m = mass of the test portion in grams;
D = Density of the serum in gram/ ml;
= 1.02 for any latex except heat concentrated latex; and
TS = percentage total solids in the latex.
7.1.1 A difference in VFA number between duplicate determinations on the same sample is not
considered significant, if it is less than 0.01unit where the actual VFA number is 0.10 or less, or
if it is less than 10 percentage of actual VFA number where the VFA number is greater than 0.10
units.
VFA = [67.32×N×V] × [50+ m (100-DRC)]
M ×TS 100 D
Where
N = normality of the barium hydroxide solution
V = final volume of barium hydroxide solution required to neutralize the distillate in
Milliliters, after correction for blank;
DRC = percentage dry rubber in the latex;
m = mass of the test portion in grams;
D = Density of the serum in gram/ ml;
= 1.02 for any latex except heat concentrated latex; and
TS = percentage total solids in the latex.
7.1.1 A difference in VFA number between duplicate determinations on the same sample is not
considered significant, if it is less than 0.01unit where the actual VFA number is 0.10 or less, or
if it is less than 10 percentage of actual VFA number where the VFA number is greater than 0.10
units.
The rubber industry is one of the important basic industries of the national economy. It not only provides us with essential daily-use, medical and other light industrial rubber products, but also provides various rubber-made production equipment or rubber parts in heavy industry such as mining, Rubber testing
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