Sciencemadness Discussion Board - Bromophenol blue and soap titration

Bromophenol blue and soap titration

unfrozen - 22-10-2014 at 16:28

I am just getting started with making biodiesel and want to test a batch for soap content. The basic reaction uses triglycerides, methanol, and NaOH as the catalyst. The products are biodiesel (fatty acid methyl esters) and glycerine. Since the oil was used in a fryer, it will also have some amount of free fatty acids. So I have to add extra NaOH to turn those into soaps (otherwise it would deplete my catalyst).

After the reaction, I separate the biodiesel from the glycerine and wash lightly with water to remove MeOH and soaps.

I am using the titration instructions at this page:
http://www.dudadiesel.com/soaptitration.php

The first step is to check for any catalyst (NaOH) left in the biodiesel. I have 100ml of Isopropyl alcohol, 10ml biodiesel, and maybe 10ml water (in the ISO). I added 5 drops of phenyphtalein with no pink. Good.

Next I add 20 drops of bromophenol blue. The solution is supposed to "turn a dark blue". Nope. It turned from clear to clear with a light blue tint.

It was very hard to make out the exact titration, but with two tries I think I found the transition from faint blue to faint yellow.

Correct me if I am wrong, but I believe that the soap titration is adding 0.01N HCl until all of the soaps are converted to fatty acids and NaCl, and then the HCl brings down the PH to activate the bromophenol blue. According to Wikipedia, it goes from blue at 4.6 to yellow at 3.0

I am scratching my head on this. Did I miss something important? Did I maybe get a bad (too dilute) bottle of bromophenol blue? It was supposed to be 0.1%

Thanks!

blogfast25 - 25-10-2014 at 05:13

Quote: Originally posted by unfrozen

Correct me if I am wrong, but I believe that the soap titration is adding 0.01N HCl until all of the soaps are converted to fatty acids and NaCl, and then the HCl brings down the PH to activate the bromophenol blue. According to Wikipedia, it goes from blue at 4.6 to yellow at 3.0

I am scratching my head on this. Did I miss something important? Did I maybe get a bad (too dilute) bottle of bromophenol blue? It was supposed to be 0.1%

Thanks!

Your titrant solution of 0.01 N HCl may be a bit too weak for bromophenol blue. A pure 0.01 N HCl solution has a pH of about 2, to reach pH 3 during the titration may be hard, requiring to go past the equivalence point (thereby introducing bias in your results) to observe the colour change.

I would suggest to try bromothymol blue (0.1 %) which also changes from blue to yellow but more or less at pH = 7.

Even phenolphthalein would be better: as long as there are soaps present pH will be > 9 and the solution pink. Once all soaps have been neutralised, pH < 9 and the pink (fuchsia) disappears.

Also, is there a specific reason for using 0.01 N HCl? The stronger 0.05 N HCl would work better with bromothymol blue, I think... The 'DudaDiesel' website is rather vague: 0.1 or 0.01 N HCl, that's a wide range!

Having also downloaded their Excel spreadsheet there is no mention of standardising the HCl. That can lead to substantial errors, my friend. Almost no analyst would use HCl solutions as titrants without standardising the titrant solution first. Even more so as the spreadsheet doesn't recommend the use of calibrated volumetric flasks for instance (yes, there is more to this titration lark than meets the newcomer's eye!)

[Edited on 25-10-2014 by blogfast25]

unfrozen - 25-10-2014 at 17:29

Quote:

Okay, that sounds like a good answer. Since I posted, I learned that I probably did not get a deep blue because my bio has very little soap to begin with. Would it make sense that the bromophenol blue did not give a good blue because there were not enough alkali ions?

Quote:

I would suggest to try bromothymol blue (0.1 %) which also changes from blue to yellow but more or less at pH = 7.

Sounds good. I will look around for that. I think I really should have a good assortment of indicators. Right now I only have the bromophenol blue, phenolphtalein, and turmeric.

Is there a good vendor that sells a starter kit without going crazy on hazmat fees?
Also, it would be good to have a couple standard solutions (again without hazmat).

Quote:

Also, is there a specific reason for using 0.01 N HCl?

I can only guess it is because there is not a lot of soap to titrate. I should probably hope that it is well under 100ppm after washing.

Thanks!

gdflp - 25-10-2014 at 18:17

Try Elemental Scientific. They sell a decent selection of aqueous indicator solutions for around a dollar an ounce. Only problems are a min order of $25 and somewhat steep shipping fees, but no hazmat. A few common cheap indicators I would suggest are methyl orange, methyl red, bromothymol blue(as mentioned earlier) and congo red.

unfrozen - 25-10-2014 at 18:25

Quote: Originally posted by gdflp

Try Elemental Scientific. They sell a decent selection of aqueous indicator solutions for around a dollar an ounce. Only problems are a min order of $25 and somewhat steep shipping fees, but no hazmat. A few common cheap indicators I would suggest are methyl orange, methyl red, bromothymol blue(as mentioned earlier) and congo red.

Okay, thanks! I appreciate the information.

It turns out that I _DO_ have another indicator, phenol red for my pool PH. I suppose that could be useful for sensing the soap neutralization, since it gives a good color change between 7.2 and 7.8

unfrozen - 25-10-2014 at 19:03

Well, I give up on Elemental Scientific. Their website demands a password for ordering and they only take Paypal (NO!!!!) or some other third rate processor. I would rather deal with the devil I know (Amazon) than the devil I despise (Paypal).

blogfast25 - 26-10-2014 at 06:02

Quote: Originally posted by unfrozen

Not really. Even if there were no soaps present the indicator should still be blue, because the indicator reacts to acidity/alkalinity (H<sub>3</sub>O<sup>+</sup> ions), not to specific acids or alkalis. A perfectly neutral solution would still be blue in the presence of bromophenol blue.

I would try this. To the IPA/biodiesel mixture add also 10 ml of water. Acid base titrations (of this kind) are supposed to be carried out in water, yet in their method the only water comes from the titrant (HCl)!

I noticed also that the site calls for '20 drops of bromophenol blue'. That's a lot! Normally we'd use a few drops of indicator, even at 0.1 %, not twenty...

As indicated above: much of their procedures have not been written by professional chemists and could do with a thorough review/overhaul.

Not sure about your PayPal hang up: I use it all the time both as a seller and buyer. As a seller my only complaint is the fairly high transaction fees.

[Edited on 26-10-2014 by blogfast25]

unfrozen - 26-10-2014 at 07:57

Quote:

I would try this. To the IPA/biodiesel mixture add also 10 ml of water. Acid base titrations (of this kind) are supposed to be carried out in water, yet in their method the only water comes from the titrant (HCl)!

I used 91% IPA, so there was some water in there. I guess I am back to being confused why I only got a light blue tint.

Quote:

Not sure about your PayPal hang up: I use it all the time both as a seller and buyer. As a seller my only complaint is the fairly high transaction fees.

When I first heard about PayPal (late 1999) I smelled that they were not truthful. They claimed that there would be no transaction fees and they would make their profit on the float. Their existence is based on a lie.

blogfast25 - 26-10-2014 at 09:28

Quote: Originally posted by unfrozen

I used 91% IPA, so there was some water in there. I guess I am back to being confused why I only got a light blue tint.

Well, the indicator itself could be at fault, of course. Either yours is below the advertised concentration or it may be too old. Many of these indicators are complex molecules that may not be extremely stable over prolonged periods of time. Some (like bromophenol blue) are used as dyes and dyes almost always fade over time.

Perhaps try a fresh batch, bought from a different supplier?

I don't have any experience with bromophenol blue, so I can't vouch for its shelf life, either way.

Re. PayPal, if you apply your principle across the board there may not be many sellers you want to buy from: all marketing is deceitful to some extent. You don't really believe BP stands for 'Beyond Petroleum', do you?

unfrozen - 26-10-2014 at 10:01

Thanks again for your reply, blogfast. Since I do have some phenol red, I think I will try again with that and see how many drops of 0.01N HCl it takes to get down to around 7.0

I accept your comment about PayPal, marketing, and British Petroleum. Every time I hear "LG", the Korean semiconductor mfg, I still think "Lucky Goldstar". I apologize if I came across as a sack of bad attitude! :-)

blogfast25 - 26-10-2014 at 13:48

Quote: Originally posted by unfrozen

Thanks again for your reply, blogfast. Since I do have some phenol red, I think I will try again with that and see how many drops of 0.01N HCl it takes to get down to around 7.0

Phenol red is more of a substitute for phenolphthalein than for bromophenol blue.

Take the site's procedure, for instance.

"1.Pour 100 ml of isopropyl alcohol into your 250ml beaker, then place it onto your scale and tare/zero it.
2.Add 10 grams of your biodiesel.
3.Next add about 5 drops of 1% Phenolphthalein solution. If the liquids in the beaker stay clear, then there is no catalyst in your biodiesel and you may proceed to the soap titration.
4.If the liquids turn magenta, then you will need to neutralize the extra catalyst. Do this by adding the 0.01N HCL solution very slowly while stirring the liquid in the beaker. Stop when the liquid turns from magenta to clear."

That's correct. The 'catalyst' is of course NaOH or KOH, a strong alkali, traces of which will take the pH of the biodiesel to above the turning point of phenolphthalein (so fuchsia).

Substituting phenolphthalein with phenol red will allow you to detect traces of catalyst, but not of soaps, because once the catalyst has been titrated away the solution will be yellow, not colourless.

The choice of the two indicators in the procedure above, phenolphthalein and bromophenol blue, is correct if you want to detect catalyst AND soaps.

Methyl orange (red 3.1 to yellow 4.4) is probably the closest substitute for bromophenol blue and it's very common (I sell it on Amazon UK). The colour change requires a bit of practice though. MO is widely used for the titration of weak bases, like soaps.

The procedure would then be: add phenolphthalein and if fuchsia, titrate to colourless (catalyst titration). Then add MO and if red, titrate to yellow (soaps titration).

[Edited on 26-10-2014 by blogfast25]

aga - 26-10-2014 at 14:45

Titration of one compound in a complex mix of OC and IC is rather difficult.

At a guess (i am certainly no expert) the Real question is What do you Want in the end product ?

i.e. How Much of each component is Acceptable.

Once that is known, perhaps there are tests other than pH that can be applied.

unfrozen - 26-10-2014 at 16:21

Quote: Originally posted by aga

At a guess (i am certainly no expert) the Real question is What do you Want in the end product ?

Okay, sorry if I didn't mention that. Ideally, I want 100% fatty acid methyl esters. Possible impurities include water, partially converted glycerides, methanol, and soap. I have tests for water and conversion, and if the water is gone, so is the methanol. If possible, I would like to see the soaps under 41ppm ("within ASTM standards"), but under 200 should be okay for my own use.

I believe that what I am doing is titrating by converting the soap in a test sample back into the fatty acid it started out as.
FA-Na + HCl -> FA-H + NaCl.
Well, that's what I think I am doing anyway.

Thanks again for all the replies!

blogfast25 - 27-10-2014 at 05:47

Quote: Originally posted by unfrozen

I believe that what I am doing is titrating by converting the soap in a test sample back into the fatty acid it started out as.
FA-Na + HCl -> FA-H + NaCl.
Well, that's what I think I am doing anyway.

Thanks again for all the replies!

Yes, that's what you're doing (after any residual catalyst has been titrated). FAs are weak acids, which explains why the soaps need to be titrated against an indicators with a turning point pH < 7.

gdflp - 27-10-2014 at 09:28

If you're going to use methyl orange, I would practice first with known amounts of acid and base. I can tell you first hand that the transition point is rather difficult to see. If you practice and know exactly when to look for the transition point, it can very helpful. Another thing I found useful is to put a backlit piece of blank white paper behind the beaker. It helps to make the colors more visible.

aga - 27-10-2014 at 11:43

Wholeheartedly agree with gdflp that a White background is essential, as is great lighting.

Generally i do titrations on a sheet of clean white copier paper with a halogen bulb close by.

Better Eyes aren't an option, sadly.

Another thing to know is that your End Point is where the entire solution has changed colour, and stays that way, rather than just when the first spots appear for a while.

I started out with red cabbage water as an indicator for acid/base titrations.
Very cheap, and changes colours nicely.

Practice with that a couple of times using vinegar and a weak NaOH solution and you'll see.

(you may end up finding the Sodium Acetate that is created in that process is worth investigating too)

http://www.google.es/url?sa=t&rct=j&q=&esrc=s&am...

tomholm - 27-10-2014 at 13:06

Quote: Originally posted by unfrozen

Well, I give up on Elemental Scientific. Their website demands a password for ordering and they only take Paypal (NO!!!!) or some other third rate processor. I would rather deal with the devil I know (Amazon) than the devil I despise (Paypal).

Elemental Scientific does take VISA and MasterCard. On page 1 of the checkout process, just below name and password, you can choose between Paypal and Authorize.net. Authorize.net is our processor for VISA and Mastercard, so just choose that option and you should be good to go. Unfortunately, our website requires that you set up an account to process an order. I can't do anything about that. Let us know if you have any problems.

Thx.

Tom

unfrozen - 27-10-2014 at 14:07

Quote:

Looks like I got that wrong, then. Sorry about that.

aga - 28-10-2014 at 15:34

Quote: Originally posted by tomholm

just below name and password, you can choose between Paypal and Authorize.net. Authorize.net is our processor for VISA and Mastercard, so just choose that option and you should be good to go. Unfortunately, our website requires that you set up an account to process an order.

The fact that Data *can* be collected on a customer should not make it a Mandatory Requirement/Barrier to buying.

I have no idea if there has ever been a Study on this, as to whether the Requirement for an online customer to Register actually increases or decreases sales.

Personally i use one site in particular by preference, as i can just fill up my cart and Buy the stuff i want, then pay with PayPal.

No illusions exist as to my anonymity, as PayPal is a bank, and would happily divulge all my details to the authorities if ever asked.

Asking the person who wants to buy chemicals online which Payment Processor they prefer is crazy - 90% wouldn't care, or understand the question.

Simplify, and sell more.

Obviously at least 1 sale fell through because of the current system.

blogfast25 - 29-10-2014 at 05:59

Quote: Originally posted by aga

Simplify, and sell more.

Hmmm... people like choice, no matter how irrational that desire may actually be.

PayPal has one disadvantage: through its ads it creates the illusion that all buyers need to be PayPal members (but that is not so, you can pay a PayPal using vendor with any major credit card). I'm sure I've lost sales over that and am in the process of setting up WorldPay as an alternative payment method.

And some people really don't like PayPal anyway. Like the OP here...