Measuring Whey Acidity by using Titration PDF Print E-mail
Written by Pav   
Monday, 21 June 2010 02:18
The premise behind acidity is that some types of molecules can release hydrogen ions (H+) into a solution of water. The ability of the molecules to release H+ influences how strong an acid is. A very strong acid, such as HCl will disassociate and release all or nearly all of its H+ into solution. In a weaker acid, such as lactic acid or acetic acid, only some of the molecules disassociate and give up H+, and the rest of the acid molecules stay in solution, but may give up their H+ later.

Two Ways to Measure Acidity

The two common ways to measure acidity are pH and titratable acidity (TA). pH measures the concentration of disassociated hydrogen ions in the solution, and titratable acidity measures the concentration of both disassociated hydrogen ions and un-disassociated hydrogen ions. In other words, TA measures all the acid in solution, both molecules that have given up their H+ and ones that have not.

Suitability of pH and TA for Cheesemaking

Because lactic acid is a weak acid, and all of it does not disassociate in water, pH does not measure the exact quantity of lactic acid present, just the dissolved H+ ions. However, this does not make TA inherently better, just different. Both indicators work for cheesemaking. Pick whichever one is easier for you. Even TA doesn’t measure the exact amount of lactic acid because milk and whey contain other acids and buffers.

Principle Underlying TA Measurement

The fundamental idea behind TA measurement is that if you add a base to an acid, the base will neutralize the acid, and if you do this slowly, you can mathematically calculate the quantity of acid. Bases also dissociate in water, but instead of releasing H+ ions, they release hydroxyl ions (OH-). Together H+ and OH- form HOH, aka H2O, aka water. An OH- ion will first combine with all the available H+ ions, and then strip the H+ ions from weak acid molecules that have not disassociated and neutralize them, too.

Requirements for TA Measurement

Practically, to measure TA, you need the following:

  • The base standard solution.   You need a dissolved (in water) base of known concentration. You need to know how many OH- ions are in the solution. The concentration of ions is measured with a unit called normality. There is a way to calculate normality or prepare a solution of known normality, covered later. If you buy a kit, it will have a NaOH solution with it.
  • Titrand.   This is a fancy name for the liquid whose acidity you’re trying to measure.
  • An acid/base indicator.   You need to know the point at which the solution has become neutral. The most common indicator is phenolphthalein. It is helpful to have a regular dropper to add the indicator.
  • Container for measured liquid.   You need to contain your sample in something, preferable with measurement marking, like a beaker or measuring cup. Regardless of the container choice, you must have a way to measure out a specific quantity of the liquid being measured. Use a nonreactive container, such as glass or porcelain.
  • Burette.   You also need a way to slowly add the base and measure how much you add. An eye dropper doesn’t work well here. You need a pipette or burette. A pipette can be cumbersome, so if you can, use the burette clamped to a stand, as shown in Figure 1.
  • White piece of paper.   Put a white piece of paper under the glass container to help you determine the exact point of color change. If you use white porcelain, you don’t need the paper, of course.

Figure 1. Basic TA setup

Making a Standard Solution

As already mentioned, it is vital that you use a solution of known normality. You can either buy this or make it yourself. Making it yourself is not difficult, but if you’re not up for it, you can buy it. The normality available commercially may vary with your location. This is because different parts of the world use different standard methods and units to measure dairy products. You can choose from the following values commonly used with TA, and prepare the normality accordingly:

  • Soxhlet Henkel degrees (°SH).   This unit is used in central Europe, except in France and the Netherlands. The commercial standard method is to use 100 ml milk and NaOH with normality of N/4 (.25 N).
  • Therner degrees (°Th).   This unit is used in Sweden and the former USSR countries. The commercial standard method is to use 100 ml milk diluted with 200 ml water and NaOH that has a normality of N/10 (.1 N)
  • Dornic degrees (°D).   This unit is used in the Netherlands and France. The commercial standard method is to use 100 ml milk and NaOH with normality of N/9 (.11 N).
  • Per cent lactic acid (% l.a.).   This unit is used in the “British” countries, that is, USA, UK, Canada, Australia, and New Zealand. It’s really the Dornic degree divided by 100.

It is critical that if you are trying to follow a cheese recipe that you know which unit is used. Table 1 shows the basic relationship between the units.

Table 1. Common acidity measurement units

°SH

°Th

°D

% L.a.

1

2.5

2.25

0.0225

.4

1

.9

0.009

4/9

10/9

1

0.01

More About Normality

If you don’t want to read about science-y stuff, feel free to skip this section. You don’t need to understand this and can just follow the procedure.

Normality, as already mentioned is the measure of the concentration of something in a solution. More specifically, it is 1 gram equivalent weight (gEW) per liter of solution. The equivalent weight is the molecular weight divided by the valence. Another way to think about it is that the equivalent weight is the volume of solute you need to have to equal one mole of ions (either OH- or H+). This gives us the following formula: Normality = Grams/(Equivalent Weight X Volume)

Procedure

Putting the above details into practice results in the following quantities shown in Table 2 that are necessary to create the various normalities. The quantities assume a pure assay of 100% NaOH, which gives it a molecular weight of almost exactly 40g per mole.

Table 2. Measurements for making common normalities

Normality

Grams NaOH

Volume distilled H20

.10 (N/10)

4

Enough to make 1 L total.

.11 (N/9)

4.4444

Enough to make 1 L total.

.25 (N/4)

10

Enough to make 1 L total.

1.0 (N)

40

Enough to make 1 L total.

The process is relatively straightforward, as follows:

  1. Measure out the appropriate grams of NaOH.
  2. Fill a flask or beaker with 500 ml water, or about half. This doesn’t have to be exact.
  3. Pour in the NaOH and stir until dissolved. Stir with a nonreactive stirrer such as stainless steel or glass.
  4. Add enough water to make exactly 1 liter.

Method to Measure TA

Once you acquire a titration kit you will have most of the things you need to measure the titratable acidity in your milk or whey. The NaOH solution should be in one of the three normalities mentioned above. The acidity in milk is measured by titrating milk that has 3-5 drops phenolphthalein added to it with the NaOH solution. Phenolphthalein changes color at a pH value of 8.2. Adding a drop at a time of NaOH slowly brings up the pH level, and then it is possible to calculate the TA by measuring the volume of NaOH solution used.

Supplies

  • A burette with clamp/stand
  • A medicine dropper for phenophthalein
  • An erlenmeyer flask or similar
  • NaOH solution
  • Phenolphthalein solution

General Procedure

  1. Fill your burette with the NaOH solution. Make sure that the bottom of the meniscus is lined up with 0. The burette should be marked in .1 ml increments.
  2. Measure out 25 grams of your sample whey or milk. If you don’t have a scale, use 25 ml. It’s very very close to 25 grams.
  3. Drop 3 drops of phenophthalein with the help of medicine dropper into the erlenmeyer and mix/swirl the erlenmeyer to distribute the phenophthalein. You can add more drops, say 4-5, if you have difficulty seeing the color change. This makes no difference chemically, it’s just an indicator.
  4. Open the cock on your burette and start dropping the NaOH solution into erlenmeyer untill you get a constant light pink color. Swirl after each drop.
  5. Once you get a light pink color, stop the NaOH dropping and measure/read how much NaOH you used on your burette’s markings. Figure 2 shows the color goal.

Figure 2. Correct color goal

Figure 3 shows the color when you have added too much NaOH.

Figure 2. Color when too much NaOH is added

Easier Procedure for .1 N NaOH

The previous procedure uses a general 25 g sample to account for the larger sample size necessary when using a more concentrated NaOH solution (like the .25 N). If you have a standard .1 N NaOH solution, you can save yourself some math calculations if you use a 9 gram sample size. With a 9 g sample and using .1 N NaOH, the % lactic acid is equal to the volume of NaOH divided by 10. So if you use 1.8 ml NaOH that is .18 % lactic acid. You can purchase a special dairy burrette that has the ml/10 marking on it directly so you can quickly read the % lactic acid. The process is as follows for .1 N NaOH:

  1. Fill your burette with the NaOH solution. Make sure that the bottom of the meniscus is lined up with 0. The burette should be marked in .1 ml increments. Buy a 10 ml burrette, it is a good size for dairy.
  2. Measure out 9 grams of your sample whey or milk. 9 ml should be very very close if you don’t have a scale.
  3. Drop 3-5 drops of phenophthalein with the help of medicine dropper into the erlenmeyer and mix/swirl the erlenmeyer to distribute the phenophthalein.
  4. Open the cock on your burette and start dropping the NaOH solution into erlenmeyer untill you get a constant light pink color. Swirl after each drop.
  5. Once you get a light pink color, stop the NaOH dropping and measure/read how much NaOH you used on your burette’s markings.
  6. Divide the volume of .1 N NaOH used by 10. That is your % lactic acid.

Measuring TA by Using a pH Meter

It is possible to measure the TA by using a pH meter instead of phenophthalein. The procedure is similar to the ones above, except instead of stopping when there is a color change, you stop when the pH meter reads 8.2.

Calculating TA

If you follow the first procedure using the 25 gram sample size, you need to calculate the TA. This is also relatively straightforward. The theory here is that you first calculate the molecular weight of the acid, and then plug it into a formula. Lactic acid is an organic acid with one carboxylic acid, CH3-CHOH-COOH, having a molecular weight of 90. The formula to calculate the % titratable acidity (this is in the industry the same as the % lactic acid) is as follows:

After you have the %TA, use the equations in table 3 to get the desired measurement unit. For more details about the relationships among the units, see Table 1.

Table3. Converting among the various units

Desired unit

Formula

Soxhlet Henkel degrees (°SH)

% TA X 400/9

Therner degrees (°Th)

% TA X 1000/9

Dornic degrees (°D)

% TA X 100

Per cent lactic acid (% l.a)

Same as % TA

Last Updated on Saturday, 26 June 2010 03:47