Before any of my long standing friends wonder who I am and what I’ve done with Anne, I should point out that the chemical knowledge in this post is courtesy of the collective brains of Dr Jemima Cordle, Dr Katie Jewell and M. Ivan Larcher who have very kindly either taught me the info or have answered my questions on the subject. I hope I have interpreted it correctly. Any chemists out there, please pick me up on any mistakes so I can correct them.
The difference between titration and pH is that you use titration to calculate the concentration of Hydrogen ions by means of measuring the amount of alkali ions you have added to cause a certain colour change.
A pH measurement is the log (base10) of the concentration of Hydrogen ions.
|pH meter at Neal’s Yard Creamery recording acidity development of Finn curd|
|Titratable Acidity kit set up at Old Hall Farm|
Hydrogen Ions, Hydroxyl Ions and Acidity
In a basic solution there will be Hydrogen ions (H+) and Hydroxyl ions (OH-). The H+ ions determine the acidity of a solution and the OH- ones are in predominance in a basic solution (when you stick both ions together you get H2O which is water)
Although pure water is present largely in its molecular form rather than atomic (which means that there are bonds linking the H atoms to the O atom to form the water molecule) a portion of it is ionised and present as H+ and OH-.
Because these two ions are present in equal quantities in a pure solution of water it remains neutral with a pH of 7. (The actual pH number is derived from the concentration of H+ present).
When substances are dissolved in water they often disturb the H+ OH- balance.
When the concentration of H+ is higher than the concentration of OH- the solution is said to be acidic and the pH is less than 7.
When the concentration of H+ is less than the concentration of OH- the solution is said to be basic and the pH is more than 7.
Acids and bases can vary hugely in strength. Hydrochloric acid for instance has a lot of H+ ions present and has a pH of 1. Citric acid on the other hand has less H+ ions present in it and has a pH of around 2.5.
How the pH meter measures
A pH meter is able to directly measure the concentration of H+ ions present in a solution into which the probe is stuck. It then converts this concentration to the pH reading.
How the TA measures
Titratable acidity measurement on the other hand is an indirect technique to measure the H+ concentration. There are certain ‘indicator’ solutions available that change colour when the solution they are in gets to a certain pH.
In cheesemaking we use phenolphthalein which turns pink when the solution gets to pH8.3. So we add a basic solution (NaOH or sodium hydroxide) to the milk or whey until enough OH- have been added to cause the colour change.
The amount that has been needed will tell you how many H+ ions were present in the original solution.
To milk you need to add very little NaOH because milk contains a very small amount of H+ but to whey from cheese the morning after making, you need to add a lot more NaOH because the lactic acid bacteria have produced a lot of acid and therefore a lot of H+ ions are present.
Having said this there is a complication which means that the pH/titratable acidity measurements are not 100% correlated. The complication is an interesting acid/base phenomenon known as buffering.
Buffering happens when an acid or base is added to a solution, but the pH does not change. This happens in cheesemaking because there is a lot of protein present.
The protein is able to capture H+ ions as they are formed and therefore prevents them from ‘being seen’ in the solution. They do not add to the H+ concentration that determines the acidity.
This can also happen with OH- ions – the protein can capture them and stop them from causing a pH change.
The latter is relevant during titratable acidity measurements because sometimes even though OH- ions are being added they are not being used to neutralise the H+ present and raise the pH to 8.3. Instead the protein is capturing them. This distorts the relationship between pH and TA in a non-uniform way.
It will depend on the specific composition of the milk and the stage of acidity that has been reached.
Comparisons of pH and TA therefore on a daily basis can help you extrapolate some physical properties of the milk you are working with – see the chart that follows.