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Volatile Acidity Determination - Prodeedure
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Trouble shooting and Hints
1. - Set up the VA apparatus as per diagram
2. - Fill the boiling flask with distilled water and boil for a few minutes to remove any dissolved CO2.
ALERT: make sure that the steam has an outlet to prevent presure build up leading to an explosion.
3. - To wash the apparatus proceed with a dummy run using 10ml of distilled water
(a) Place 10ml of distilled water in the sample holding chamber.4. - Fill a clean 250ml conical receival flask with 50ml of distilled water to allow the receiver tube to be situated below the water level.
(b) Lift stopper to release the 10ml of distilled water into the sample receiving chamber of the still and replace stopper.
(c) Allow steam to percolate through the dummy water sample, collecting the steam distillate
(d) Remove the conical receiving flask.
(e) Remove heat from the steam supply.
The water sample will be automatically drawn from the sample receiving chamber.
This occurs because the condensation of steam in the shaded volume (see diagram), will reduce the pressure below that of the atmospheric pressure of the non-shaded area of the appartus
Note: if you do not remove the conical receiving flask in step (d), when removing the heat in step (e), the contents will be drawn into the apparatus.
This allows all of the volatile acids distilled over to be collected and entrapped within the distilled water.
5. - Add 3-4 drops of phenolphthalein indictor solution to the distilled water in the conical receiver flask
6. - Fill a burette with a standardise 0.1M NaOH solution and add this solution one drop at a time to the distilled water until the palest of pink colouration persists for at least 30 seconds.
7. - Position the conical flask under the condenser so that the end of the receiver adapter is submerged below the water level.
8. - Pipette 10ml of degassed sample into the holding chamber.
9. - Pipette approximately 0.5ml of 0.3% w/v hydrogen peroxide (H2O2) in with the 10ml sample (see H2O2 solution, more and preparation.
10. - Lift stopper to release the 10ml sample into the sample receiving chamber of the still.
11. - Use a small amount of distilled water to wash down any remaining sample residue and replace the stopper..
12. - Allow steam to percolate through the sample, until approximate 100ml of distillate (VA + Water) is collected.
13. - Remove the conical receiving flask, remove heat from boiling flask and allow spent sample to be drawn out of the sample receiving chamber.
You may want to start step 16 at this stage.
14. - Titrate the contents of the flask using the standardized 0.01M NaOH solution in the burette to a pink end point
15. - Record the amount of NaOH solution used (titre).
16. - Repeat steps 4 - 15 using a blank, (distilled water as the sample), and subtract this titre from the sample titre to compensate for any acids, steam distilled over that were not part of the sample.
17. - Calculate and record the concentration of the volatile acidity of the sample.
See Trouble shooting and Hints