Acid Titration Procedure for Ciders,  Apple Juices and Cider Vinegar

 You will need

• 0.1 M Sodium Hydroxide (NaOH) solution [0.1M potassium hydroxide (KOH) can also be used but is less usual]* #
• Phenolphthalein indicator in alcohol*

Keep both items closely stoppered in a cool dark place. 

• Various size syringes with volume markings (10 ml, 1 ml) and / or calibrated pipettes and a burette
• Glass flasks or Sterilin Pots of 50 - 100 ml capacity.
• Flexible bulb pipette
• Distilled water (optional). 

 Procedure (for juices and ciders)

1. If measuring a cider, first degas the sample by heating on a stove or in a microwave oven until briefly boiling. Don't allow the sample to splash or to lose volume by evaporation. And beware if the cider has a lot of gas in it since 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 

• Follow the same procedure, but using a 1 ml syringe in step 1 and measuring just 0.60 ml of vinegar. Or dilute the vinegar exactly ten times with distilled water before measurement and use 6 ml of diluted solution.
• Omit step 7. The volume dispensed will represent the acid in % acetic acid directly. Should be 5% or greater.

 
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 

 
 % 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.

#Note on nomenclature