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Laboratory Test Procedures arrow Titratable Acidity arrow Titratable Acidity using the pH method



Titratable Acidity using the pH method

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DETERMINATION OF TOTAL ACIDITY BY  pH MEASUREMENT 

PROCEDURE  

  1. Standardise the pH meter   
  2. Obtain a 100ml sample of wine. The wine sample should be degassed in order to remove any carbon dioxide (CO2) present. Carbon dioxide in wine produces carbonic acid which will give a higher than normal total acidity value. Therefore, place 100ml wine in a 500ml Buchner flask, fit a rubber stopper and connect flask to a water vacuum pump attached to a water tap.
  3. Shake flask for 1 minute as tap is opened.  
  4. NOTE:When using a venturi water vacuum pump ensure that the tubing attached to the flask is disconnected before the tap is closed otherwise water will be sucked back into the flask.
    A sample of wine should be clarified, if necessary,(depending on the solids content of the wine) then degassed prior to testing for pH or titratable acidity.
  5. A sample of juice can be cold settled and/or filtered prior to testing without the need to be degassed.
  6. Adjust the temperature of the degassed wine to 20C. Transfer 10ml of degassed wine or juice into a 250ml glass beaker using a 10ml volumetric pipette.  
  7. Add 150ml distilled water to the beaker, together with a magnetic stirrer bar.  
  8. Place both the pH probe and the Automatic Temperature Compensation (ATC) probe in the beaker.  Ensure the probes are sufficiently immersed in the solution, but well clear of the magnetic stirrer bar in the bottom of the beaker.
  9. Attach a 25 or 50ml burette to a retort stand via a bosshead and two-prong clamp. Position burette over the 250ml beaker.
  10. Insert a 50 mm glass funnel into the top of the burette and pour 0.1N Sodium Hydroxide (0.1N NaOH) into the funnel.  Three quarter fill the burette.
  11. Remove the funnel from the burette and 30 seconds later, record the initial volume of the 0.1N NaOH in the burette, for example
    Initial volume = 10.55 ml.
  12. Turn on the magnetic stirrer unit and the pH meter.
  13. Slowly add the 0.1N NaOH drop by drop from the burette by turning the plastic stopcock anti-clockwise.
  14. Continue to add the 0.1N NaOH drop by drop from the burette into the beaker until the pH equals approximately 7.00.
  15. Now turn the stopcock to adjust the flow of 0.1N NaOH to a drop at a time, with sufficient stirring between each addition until a pH 8.20 is obtain.
  16. Record the final volume of 0.1N NaOH used.  For example, Final volume = 18.25ml. Therefore volume of 0.1 N NaOH used = 18.25 - 10.55ml = 7.70ml (A ml).  Concentration of NaOH = 0.1N = C.
  17. Titrate the same quantity of distilled water that was added to the wine (ie. 150ml as in step 4) to a pH of 8.2 or a pale permanent pink colour using phenolphthalein indicator (3 drops) call this volume B ml.
  18. Now calculate the total acidity content of the wine using the formula below:  

             Total Acid = (A - B) x C x 7.5

                              = Volume (mls) of 0.1N NaOH x 0.75

                              = grams of  Tartaric Acid/ Litre of juice or wine.

                              = (18.25-10.55) x 0.1 x 7.5

                              = 7.70 x 0.75

                              = 5.78 g/litre

 

The total acidity can be expressed as grams/litre citric, tartaric or malic acids, i.e.:

                        Titre x 0.70 = total acidity as grams/litre citric acid

                        Titre x 0.75 = total acidity as grams/litre tartaric acid

                        Titre x 0.67 = total acidity as grams/litre malic acid

 

 

 






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