Acid Titration Procedure for Ciders,  Apple Juices and Cider Vinegar

 You will need

Keep both items closely stoppered in a cool dark place. 

 Procedure (for juices and ciders)

  1. If measuring a cider, first degas the sample by heating in a microwave oven for about 15-20 seconds until steaming hot, not boiling. Beware if the cider has a lot of gas in it it might fizz quite violently! Swirl vigorously and then cool to room temperature. 
  2. Dispense 6.7 ml of sample into a clean Sterilin pot.  The easiest way of doing this is to draw up an excess of the liquid into a 10 ml syringe, then lining up the back of the plunger with the 10 ml mark.  Dispense the contents into the pot until the back of the plunger reads 3.3 ml.  This will give an exact 6.7 ml of liquid in the pot.. A calibrated pipette can more simply be used for this, if available.  (If distilled water is available, the accurately dispensed sample can be diluted to a larger volume eg 15 - 20 ml to make handling and observation easier. But do not use tap water for dilution, since it will affect the result)
  3. Add  two or three drops of indicator solution to the pot, using the flexible bulb pipette.
  4. In a fresh clean and dry 10 ml syringe, take up 10 ml of 0.1M NaOH solution, lining up the back of the plunger with the 10 ml mark as before. A burette can be used for this stage (steps 4-6).
  5. Slowly dispense NaOH dropwise from this syringe in 0.5 ml steps into the pot, swirling after each addition. Stop when the sample has turned a permanent pink. and remains so for at least 10 seconds
  6. Read the figure shown at the back of the plunger, and subtract it from 10 to get the total volume delivered.  This volume is X ml (e.g. 10 - 4.5 = 5.5)
  7. The figure X represents the acid level in grams per litre of malic acid (e.g. 5.5). Divide X by ten to get the acid in percent (e.g. 5.5 / 10  = 0.55% malic acid)

  

For vinegars 

 
When finished...

 
Rinse out all items carefully with clean water (the syringes can be dismantled).  Blot or shake dry if possible (tap water is alkaline and may lead to inaccuracies if allowed to dry on to the equipment). Distilled water is best.

 

The Formula 

The calculation formula for cider and juice is

 
 % malic acid =  (Volume of  0.1 M NaOH) x  10 x 0.067 / (Volume of sample)

 
By using an exact 6.7 ml of sample, the calculation can be avoided because everything is in tens!  If a different volume of juice or cider is used, the formula will be needed.


*The Reagents

The alkali and indicator can be bought on eBay. You can also buy stronger sodium hydroxide solutions eg 1M and dilute them accurately with distilled or deionised water to the correct working strength. If you want to make up your own NaOH solution from solid pellets, buy the purest grade you can (laboratory or analytical, not technical). Beware that the pellets are hygroscopic and also absorb CO2 from the air, so they must be kept tightly closed in a dry place or they will lose their strength.

Dissolve exactly 4 grams of NaOH (or 5.61 g of KOH) in 500ml of distilled water and make up to exactly 1 litre to make an 0.1M solution. Beware the solution will get hot! NaOH solid and solutions are very corrosive and can cause irreparable damage to the eyes through splashes. Wear a mask and goggles when you do this!

The 0.1M solution is not stable and will absorb carbon dioxide from the air and will lose its strength, so the titration will become inaccurate. It should be replaced with fresh solution at least annually, preferably more often.

It is possible to use a calibrated pH meter to replace the phenolphthalein indicator. In this case, the end point is pH 8.2


#Note on nomenclature

0.1M is the correct chemical way to express things. M stands for 'molar' or 'one gram mole per litre'. In old fashoned literature, the term 0.1N  is often used, where N stands for 'normal'. It so happens that for monobasic alkalis like sodium hydroxide, 0.1M and 0.1N are just the same thing. So you can use them interchangeably here. But in a lot of cases you can't.  Beware.




Last modified by Andrew Lea December 2013.

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