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Basic titration
For processing used oil, it's essential to titrate the oil to determine the Free Fatty Acid (FFA) content and calculate how much extra lye will be required to neutralise it.

An electronic pH tester is best, but you can also use phenolphthalein solution (from a chemicals supplier).

Dissolve 1 gram of pure sodium hydroxide lye (NaOH) in 1 litre of distilled or de-ionized water (0.1% w/v NaOH solution) (weight to volume).

In a smaller beaker, dissolve 1 ml of dewatered WVO in 10 ml of pure isopropyl alcohol (isopropanol).

Warm the beaker gently by standing it in some hot water, stir until all the oil dissolves in the alcohol and the mixture turns clear. If you're using phenolphthalein, add 2 drops of phenolphthalein solution.

Using a graduated syringe, add the 0.1% NaOH solution drop by drop to the oil-alcohol-phenolphthalein solution, stirring all the time. It might turn a bit cloudy, keep stirring. Keep on carefully adding the lye solution until the solution stays pink (actually magenta) for 15 seconds.

Take the number of millilitres of 0.1% lye solution you used and add 3.5 (the basic amount of lye needed for fresh oil). This is the number of grams of lye you'll need per litre of oil to process the WVO.

Potassium hydroxide (KOH) can also be used for titration, see Using KOH, below.
Better titration
Unless you have a very accurate scale, it's not easy to measure exactly 1 gram of lye. It's much easier to measure 5 g accurately than 1 g. So mix 5 g of NaOH (or KOH) with 500 millilitres of distilled or de-ionized water to make a stock solution.

Before titration measure out 5 ml of the stock solution, add 45 ml of distilled or de-ionized water. This makes a 0.1% w/v lye solution (weight/volume).

It's also not easy to measure exactly 1 millilitre of oil. Instead of the usual 1 ml of oil and 10 ml of isopropyl alcohol, mix 4 ml of oil in 40 ml of isopropyl alcohol in a glass beaker.

Warm the mixture gently by standing the beaker in hot water, stir until all the oil disperses and it becomes a clear mixture.

If you're using phenolphthalein, add 2 drops as usual -- no need to add four times as much, 8 drops gives the same result as 2 drops.

Titrate as usual, measuring millilitres of 0.1% lye solution used. When it reaches pH8.5 count up the number of millilitres used as usual and divide by 4. This will give a much more precise measurement.

To save on isopropyl alcohol, use 2 ml of oil in 20 ml of isopropyl and divide the results by two -- still twice as accurate.
Using KOH
KOH is not as strong as NaOH -- use 1.4 times as much KOH (actually 1.4025 times).

Titration is basically the same.

You can use either KOH solution or NaOH solution for titration. Unlike NaOH, your KOH probably won't be pure, but as long as you use the same strength of KOH for both the titration solution and the reaction it won't make any difference.

Mix the KOH solution the same as for NaOH: add 1 g of KOH to 1 litre of distilled water to make 0.1% KOH solution. Or do it the Better titration way. For each 1 millilitre of solution used in the titration add 1 g of KOH to the basic amount.

You can also use the usual 0.1% w/v NaOH solution and convert the end result for KOH, see below.

Instead of the basic 3.5 grams of NaOH per litre of oil, use 3.5 x 1.4 = 4.9 grams of KOH (4.90875). So, if your titration was 3 ml of 0.1% KOH solution, use 3 + 4.9 = 7.9 g KOH per litre of oil.

One more complication: KOH is generally not as pure as NaOH. KOH is usually 92%, 90% or 85% pure -- check the label. We use half-pearls assayed at 85%, with good and reliable results. KOH is available at 99% strength, but it's hard to find and it's expensive, and anyway 92% or 85% work just fine.

Adjust the basic lye quantity according to the strength of the KOH: the basic 4.9 grams of KOH at 100% strength would be:
85% KOH -- 5.8 (5.775) grams
90% KOH -- 5.5 (5.454) grams
92% KOH -- 5.3 (5.336) grams
To substitute KOH for the 3.1 grams of NaOH per litre of oil used in the acid-base process:
85% KOH -- 5.1 (5.115) grams
90% KOH -- 4.8 (4.831) grams
92% KOH -- 4.7 (4.726) grams
To convert NaOH quantities for KOH:
For 85% KOH use 165% of the NaOH amount (x 1.65).
For 90% KOH use 155.83% of the NaOH amount (x 1.56).
For 92% KOH use 152.45% of the NaOH amount (x 1.52).
Example: You're using 85% KOH and the titration needed 3 ml of KOH solution to bring the pH to 8.5.

Number of grams of KOH required for the reaction:
Basic amount: 5.8 g KOH (5.775)
Titration result: 3 ml
Add: 3 g KOH
Total: 8.8 g (8.775) of 85% KOH required per litre of WVO.
Using 0.1% NaOH solution for titration and the same WVO as above: the titration result is lower, it only needs 1.8 ml to reach pH8.5. (Actually 1.82 ml, according to the calculation.)
Basic amount: 3.5 g NaOH
Titration result: 1.8 ml (1.82)
Add: 1.8 g NaOH (1.82)
Total: 5.3 g NaOH (5.32)
Convert for 85% KOH:
5.3 (5.32) x 1.65 = 8.8 g (8.778) of 85% KOH per litre of WVO.
We usually use NaOH solution for titration and convert the result for our 85% KOH, but in practice NaOH solution and KOH solution both work equally well.
One reason for preferring NaOH solution is that it's usually the standard used in describing FFA content of different oils -- 2 ml titration WVO or 5 ml titration WVO invariably refers to ml of 0.1% NaOH solution, unless otherwise specified. In the example above, the WVO would be described as 1.8 ml titration WVO.

KOH dissolves in methanol much faster and more easily than NaOH does, and doesn't "clump" together as NaOH can do. When you use KOH the glycerine by-product is liquid and won't solidify.

KOH is easier to use than NaOH. It's more flexible and adaptible and it gives generally better results -- all round it's a better catalyst than NaOH. We seldom use NaOH (except for titration).

Note Don't mix KOH and NaOH in the same batch. If you use KOH as the catalyst, you must also use KOH if you make soap from the glycerin by-product, and again you must use KOH if you deacidify the WVO as a pre-processing step.