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Soap Calculation question

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September 10, 2009, 12:17 AM
Soap Calculation question

Your explanations nail it.

In the past chemist have explained that the amount of your sample or the strength of your acid can be varied based on what you expect the concentration of soap to be in the solution tested. However this necessitates a change in the soap calculation and doesn't allow for the simplied 304 or 320 times the ml of acid formula.

Typically, if we are testing biodiesel, even unwashed biodiesel, the amount of soap in solution is of a reasonable ppm amount, that using a 0.01N HCL solution is efficient enough and it makes the math easy.

Note though that in my earlier post regarding a soap test of GLYCERIN, I recommend using more isopropyl alcohol and Phenol Red since almost all the catalyst NOT consumed in soap production, will be in the settled glycerin by-product. This mandates the neutralization of the catalyst prior to doing the soap test.

And going along with what Reece123 said, it would be more efficient at this point to use a 0.1N HCL solution since the amount of soap in the glycerin is also much higher. Holding the sample volume to 10 ml would simply increase the multiplier by a factor of 10 (therefore instead of 304 and 320, you would use 3040 and 3200).

Incidently if you follow Rick Knicely's post in the glycerin soap making portion of the forum, you'll see this is one of steps that can be used in determining your particular glycerin profile and assist you in determining SAP values.


PS Double D that is an excellent method on Muratic Acid to Lab chemical formula - nice work!

Causing a Regenerative Economy
September 10, 2009, 07:54 AM
Double D
Thanks for the kudos Ant, GCG.
December 23, 2013, 11:33 AM
Let me start by saying im not a chemist by a long shot, but i did chemistry as n subject in school.. Having said that, I have the folllowing question. It is widely accepted that Bromothymol Blue should be used for the soap test.

Now, in my area its somewhat difficult to get hold of Bromothymol Blue... Enter Wikipedia (hehe). according to Wiki ( Bromothymol Blue changes from yellow to blue somewhere between Ph 6 and 7.6, while our trusted friend, Phenol Red that most of us use during titration changes from yellow to red between Ph 6.4 and 8...

The Ph at which these 2 changes colour, is so close, I was wondering to what extent will it affect the results of a soap test if one uses Phenol instead of Bomothymol?

Like I said earlier, getting hold of Bromothymol Blue is proving a little difficult in my area, and I'm very tempted to just use Phenol Red...

Any gurus out there who can tell me why I shouldnt..
December 24, 2013, 09:31 AM
no, not bromothymol. bromophenol blue (pH 3-4.5) is used for soap titration. you are correct in that you could use something with similar pH range.
December 24, 2013, 12:51 PM
Aaaah... I just KNEW it had to be something simple like that. You'll have to excuse me, English is not my mother tounge, so i get easily confused with something so similar in spelling
December 24, 2013, 12:54 PM
time to visit Wiki again and see if i can find something else in my area with a similar (3-4.5)Ph range