I had a customer send in this message & asked that I post it for review to see what people thought of it as a possible way to test for soap.

Due to several errors the customer found with this method, and at the request of the customer I've pulled this method down.

Refer to the following posts which give a more accurate way to perform a soap test.

Thanks,
Graydon

This message has been edited. Last edited by: Graydon Blair,

Sounds like I need a better set of scales

I would like to be able to test for soap levels in the single digits, just to satisfy my curiosity. I could see how that method could be more accurate and consistent. I wonder if they make a pipette that is capable of measuring ml to the 100th's. I guess its time to google.

Thanks for sparking my curiosity Graydon.

Graydon,

You need to ask them, "What is the difference between HLC and HCL?"

That is the part I am having a hard time with.

LOL!!!!
Thanks, good catch.
That's why this place rocks!"

I'll go correct that. I just SIC (sited it in context) with what he'd sent.

OK, corrected...
-Graydon

quote:

Originally posted by Andrew M:
You need to ask them, "What is the difference between HLC and HCL?"

You mean, HCl. :-p

Oh well, close enough...
it's an L.

So if it works we now know how much soap we have. the next question is how much soap is to much
soap are is it good enough to know that you have soap

quote:

Originally posted by Graydon Blair:
. . .
Multiply weight of HCL used in titration by 304.
This number is the ppm of soap in your sample. . . .

The instructions should probably specify which factor to use based on the catalyst.

The approximate molecular weight of sodium oleate is 304 gm/mole.

The approximate molecular weight of potassium oleate is 320 gm/mole.

Sodium oleate is formed when using sodium catalyst; i.e. NaOH or NaOCH3. Potassium oleate is formed when using KOH.

quote:

Originally posted by HARRY R:
So if it works we now know how much soap we have. the next question is how much soap is to much soap are is it good enough to know that you have soap

About 66.5 ppm for sodium soaps.

About 41.5 ppm for potassium soaps.

This message has been edited. Last edited by: producer,

quote:

Originally posted by producer:

quote:

Originally posted by HARRY R:
So if it works we now know how much soap we have. the next question is how much soap is to much soap are is it good enough to know that you have soap

About 41.5 ppm for sodium soaps.

About 66.5 ppm for potassium soaps.

I thought ASTM soap ppm pass/fail was 500 ppm, I could have misunderstood that.

The ASTM limit for Na in biodiesel is 5 ppm. The ASTM standard for K is 5ppm.

Na (mol wt = 23 gm/mol) comprises 7.57% of the weight of the sodium stearate soap molecule (mol wt = 304 gm/mol).

K (mol wt = 39 gm/mol) comprises 12.19% of the weight of the potassium stearate soap molecule (mol wt = 320 gm/mol).

Therefore, the weight of Na based soap in a biodiesel sample that measures 5 ppm of Na is a little more than 66 ppm (5/0.0757=66).

The weight of the K based soap in a biodiesel sample that measures 5 ppm of K is a little more than 41 ppm (5/0.1219=41).

All values are approximate. The more precise values in the previous post are based on more accurate weights of the various constituents. Actual biodiesel will contain other than stearate soaps, so the exact soap limit of each sample will depend on the fatty acid profile of the TG used to make the biodiesel. The estimate based on stearate soap only is accurate for the estimates needed when making biodiesel and testing soap after washing.

quote:

About 41.5 ppm for sodium soaps.

About 66.5 ppm for potassium soaps.

producer, I believe you swapped the ppm values above, also evident by your calculations.

producer,

So with that said, does that mean the conversion factors should be changed when you're testing Biodiesel made from one catalyst vs. the other when figuring soap?

ie.
if measuring soap for Na, use the 304 conversion factor?
if measuring soap for K, use the 320 conversion factor?

Or does the 41.5 ppm and 66.5 ppm already account for that?

Thanks. Great info!
-Graydon

quote:

Originally posted by jdoughy:

quote:

About 41.5 ppm for sodium soaps.

About 66.5 ppm for potassium soaps.

producer, I believe you swapped the ppm values above, also evident by your calculations.

Yes, I flipped the values. Dyslexia strikes again. Thanks for catching the error. Now I will correct the original values. It is errors like this that make me always want to show my back up data when arriving at various conclusions.

quote:

Originally posted by Graydon Blair:
producer,

So with that said, does that mean the conversion factors should be changed when you're testing Biodiesel made from one catalyst vs. the other when figuring soap?

ie.
if measuring soap for Na, use the 304 conversion factor?
if measuring soap for K, use the 320 conversion factor?

Or does the 41.5 ppm and 66.5 ppm already account for that?

Thanks. Great info!
-Graydon

Graydon, yes change the conversion factors based on the catalyst used for transesterification; i.e. use 304 if using sodium, use 320 if using potassium.

The 41.5 ppm and 66.5 ppm play NO ROLE in the calculations. Theses two values are simply reference values for soap concentrations. If your soap test shows a value for sodium based catalyst above 66.5 ppm, then there is very good chance your biodiesel will fail the ASTM test for sodium.

Perfect! Thanks!
Great info as always!
-Graydon

So, I've been using 304 in my soap titrations and I use KOH, I should be using 320?

quote:

Originally posted by fabricator:
So, I've been using 304 in my soap titrations and I use KOH, I should be using 320?

Yes, you should use 320.

Thanks Producer.

Fabricator,

Ya 320 is right for the KOH producers - sorry about that FAB - I forgot you use Potassium Hydroxide.

Hopefully the 304 to 320 didn't impact you too much. (10 x the difference between the two; is 160 ppm more or less soap).

GCG