The Blues.

[DavidA]

No, not the musical variety.

I keep noticing that the names Bromophenol Blue and Bromothymol Blue are used a lot. But are they the same  (I have both).
I am assuming that the correct one for soap testing is Bromophenol Blue.

Dave.

[WesleyB]

From a brief internet search Bromophenol Blue under goes a color change in the pH range 3.0-4.6 .  Bromothymol Blue under goes a color change in the pH range 6.0-7.6 .  Soap is basic , above pH 7 .  Titrating soap with strong dilute acid produces free fatty acids and inorganic salt , which should be acidic . Acidic pH is less than 7 .  So a pH indicator to use in testing soap content would be bromothymol blue , I think .  If I recall correctly the sample is dissolved in dry isopropyl alcohol then dilute hydrochloric acid is added with stirring till the Bromothymol blue changes color , from basic to acidic .  Then the quantity of hydrochoric acid added to acidify the alcohol is calculated .  An equivalent amount of soap expressed in moles is present . The mass of the soap present can be calculated from the average molecular weight of the soap times the number of moles of the soap in the sample .  With more calculation knowing the mass of soap present , I could get parts per thousand or parts per million in a litre of biodiesel .  It has been some time since I did this titration and calculations for it . 

[countrypaul]

Bromophenol blue is the usual indicator used I believe.

This Wiki article gives a detailed explanation: http://biopowered.co.uk/wiki/Soap_tests#Titrated_soap_test

[DavidA]

Thanks for the responses.

I was just wondering what the opinion of the folks on the ground was.

I will be using the Bromophenol Blue.  Can't recall why I acquired the other one.

[dgs]

I've used both and have both, there is very little difference. The soap test if done properly is about the quickest and most accurate of all the 'shed' tests. Make sure you blank off the ipa. With an accurate titration you can get a result of +/- 5ppm.

[DavidA]

dgs,

So you're saying that the quite different Ph switching points doesn't matter ?

Bromophenol Blue  below 3.5 yellow, above 4.6 blue

Bromothymol Blue  below 6.0 yellow, above 7.6 blue.

[dgs]

dgs,

So you're saying that the quite different Ph switching points doesn't matter ?

Bromophenol Blue  below 3.5 yellow, above 4.6 blue

Bromothymol Blue  below 6.0 yellow, above 7.6 blue.

Not quite. At low ppm levels, lets say around the K spec level of 66ppm there will be little difference as with a low titration the pH will soon alter. However at higher levels the Potassium Chloride salt formed will have the effect of buffering the solution, so the different end point pH's will have an effect on the test result.

Having said that we usually don't titrate at very high soap levels, there is no point, making the second example a little meaningless.

Hope you understand all that dribble.

[countrypaul]

Since you have both indicators, try carrying out the titrations on several different samples (with little soap though to lots of soap) using both indicators and see how different the results are. With nearly all titrations there is a difference between the endpoint (where the indicator changes) and the equivalence point (where the neutralisation in this case finishes) however the difference is normally small enough to be ignored.

The buffering is due to the mixture of a weak acid strong base (or vice versa) where the weak acid is not fully disassociated so adding a little more strong base has little effect on the pH as it results in the undisassociated acid being neutralised, but because it had no free protons it does not affect the pH. Adding a little more strong acid also has little effect as it will convert the weak acid/strong base salt to weak acid plus strong base/strong acid salt. Once the conversion is nealy complete the pH changes rapidly.

Here is a better explanation: https://en.wikipedia.org/wiki/Buffer_solution

[DavidA]

Thanks for both responses.

I intend to do a series of tests using both indicators with the same batch oil oil and the same (new) IPA.  I won't blank the IPA as that may add another variable to the test.

I'll let you know how it all turns out.

Dave.

[countrypaul]

Thanks for both responses.

I intend to do a series of tests using both indicators with the same batch oil oil and the same (new) IPA.  I won't blank the IPA as that may add another variable to the test.

I'll let you know how it all turns out.

Dave.

You should blank the IPA otherwise you are introducing another variable and none of your results will therefore be reliable. Adding an almost random amount of acid especially a weak organic acid will act as a buffer as discussed earlier.

Using the same batch of oil (do you mean bio?) will only give you one data point, ideally you want several different data points so differing titrations on each sample. You should carry out 3 titratons for each sample with each indicator - this will help check you method of tritrating and give you an idea of accuracy.

[dgs]

Just a couple of things here Dave. I agree with Paul that the ipa should be blanked, ipa tends to go acidic with age and as you are titrating from base to acidic you have to make the ipa slightly alkaline (basic) in order to blank it. I usually do this by adding a very small amount of sample (depending on variables one drop is sometimes sufficient)


Also as the result should be stated on a w/w basis it is more accurate to use an 11ml sample as this is far closer to the 10gms we should actually use.

It's good that a newcomer on this forum is doing some experimental work, a technical post, WOW its like starting all over again!

PS use a 1ml pipette to titrate with and make sure you use the same volume of ipa each time.

[DavidA]

Thanks for the comments.

I usually blank the IPA for my NaOH calculations. But assumed that as I was using the same batch of IPA for all the tests it wouldn't matter. On reflection It will be better to do so or I won't know how much of the titration solution was used on the bio and how much on the IPA.

I have been watching Graydon Blair's Video on this. Any comments on it ?

I really need to do two sets of tests. One using the old IPA and one using the new stuff I got from Dave.

And three tests for each to get an average.

I'll report back when it's done. (at 20C, naturally).

Paul, Yes, I do mean Bio. Sorry about that.


P.s.
Actually it's four sets. Two for each indicator.

[nigelb]

Always blank your IPA.  Uncle H a few years back had issues with soap testing. Turned out that all of his testing problems, following my intervention, were due to the IPA he'd bought off the interweb.

If you are having issues with testing or production...question everything.

[DavidA]

I intend to.

I reckon the best way to ensure consistency over a set of tests will be to measure out enough IPA to do the whole lot and then blank it.  Drawing off as much as I need for each test.

That way I will know that at least the IPA is all the same.

The Bromophenol blue I was waiting for has just arrived, so I can get on with the testing.

[DavidA]

Well, something happened that I wasn't expecting; or rather it didn't happen.

I put 100mL IPA (the good stuff, from dgs) into a beaker, At 20C.

Then added in ten drops of Bromophenol Blue.

And nothing happened. It didn't turn blue as expected. Just stayed clear.

I spent a long time and a lot of IPA on this problem but haven't yet come to any conclusion why it didn't go either blue or even yellow.

I'm still working on this. But am going to have to get some more IPA soon.

Life was so easy before I started on this soap thing.

p.s.  The same happened using Bromothymol Blue.

Note that I haven't got as far as blanking or adding oil  yet.