I haven't done much with biodiesel yet, but I can shed some light on pH meters vs. phenolphthalien(PHTH) from a chemist's perspective.
As acids become weaker, it is important to become more careful in the selection of a chemical indicator (such as PHTH), as the change in pH is much slower than it is for a strong acid titration. PHTH is the most commonly used indicator for strong acid titrations because it has such a clear color change. Fortunately it appears to change color over the same range as the equivalence pt. for FFAs.
The difficulty in either method is knowing what the equivalence point is going to be. This will vary based on the type of oil and the degree of hydrogenation. I don't suspect it will vary much, but it will vary. If your goal is a pH of 8 or 9, then using PHTH is going to be as good as a pH meter. If you have a better idea of what the equivalence point is going to be, then a pH meter works better. And if you haven't a clue, there is a rather tedious method of determining what it should be using a pH meter. I'll probalby use this method myself at least once so I have a better understanding of the behavior, but I wouldn't necessarily recommend it for everyone, and I certainly am not going to do it for each batch.
The point is there are a number of reasons why PHTH may not work as well as a pH meter in some cases yet works fine in other cases. This is something I think everyone will have to decide for themselves.
Keith Addison <[EMAIL PROTECTED]> wrote:
Howdy Bob
>Howdy Kieth and Willem, the issue of the use of a pH meter as
>opposed to other methods for determination of the end point of a
>titration comes up from time to time. It seems that many believe
>one really needs a pH meter. I think not and here is why. Although
>a pH meter may be more accurate if all the variables were tightly
>controlled, they're not. How accurately can one measure the 1 ml of
>oil? One maybe two significant digits?
You can measure 4ml four times as accurately, so use 4ml and divide by 4. See:
Better titration
http://journeytoforever.org/biodiesel_make2.html#bettertitrate
Accurate measurement
http://journeytoforever.org/biodiesel_make2.html#measure
This is one of the reasons, among the more obvious ones, that we
always recommend starting with one-litre test batches - you learn
precision that way. Or as much precision as you're capable of.
>If different oils have different densities, (they do but small) then
>one should really adjust the volume of oil used in the titration
>accordingly.
That will start to matter when you get to high-FFA oils titrating at
say 7 or 8ml or above, very marginal at less than that IMHO. We've
processed higher titration oils than 7 or 8 accurately and reliably
(single-stage base) using pH meters, but phenolphthalein wasn't
reliable at that level.
>Because density is a function of temperature, how accurately does
>one know the temperature?
At processing temperature.
>How accurately can one prepare the titrant solution? How accurately
>can one measure the titrant? Finally the titration is done in a
>nonaqueous heterogeneous solution, hence pH isn't even strictly
>defined; pH being defined as minus the log of the hydrogen ion
>concentration in water.
Bob, how can it be a non-aqueous solution when you're adding 0.1% v/w
NaOH (or KOH) solution which is 99.9% water?
>If you heat the oil/isopropyl alcohol mix to get homogeneity, the
>hydrogen ion concentration changes. (It is actually worse, it is the
>hydrogen ion activity, but that is another matter.)
But it will be the same as what's encountered in the subsequent processing.
>Hence use of a pH meter to me is overkill. What you get is a very
>precise endpoint for a titration which is really fairly inaccurate,
>due to the aforementioned difficulties in the volumetric
>measurements, temperature, etc.
We've had three different pH meters, well-calibrated and maintained,
in consistent agreement and differing from phenolphthalein results,
and the pH meter results were consistently borne out by the
subsequent processing results, whereas phenolphthalein were off, this
with high-FFA oils where you can really see the difference.
The phenolphthalein was obtained from two different lab supply houses
and was fresh. We use phenolphthalein for demos and at our seminars
because of the pretty pink colour and it's a cheap entry, but we
seldom use it otherwise. At our workshops we use pH meters.
>I use phenolphthalein to determine the endpoint. It is quick,
>requires no calibration, costs a heck of a lot less than a pH meter,
>and provides sufficient precision for the measurement at hand.
To each his own Bob.
Best wishes
Keith
>
>>Hi Willem
>>
>>>Hi all,
>>>
>>>I am slowly getting the parts together to construct a small biodiesel set-up
>>>(approx. 30 litres).
>>>When I told a friend recently avout my plans he gave me an electronic PH
>>>meter, pen type. Accuracy is 1 decimal. He said this would replace the
>>>titration, but could give me no more info.
>>
>>
>>It won't replace titration, you use it for titration. You can use
>>phenolphthalein, or pH test strips, or a pH meter - whichever you
>>use, titration involves measuring the pH. See:
>>
>>Basic titration
>>Better titration
>>pH meters
>>Phenolphthalein
>>http://journeytoforever.org/biodiesel_make2.html#titrate
>>
>>pH meters are best, IMO. You need to calibrate them often (get
>>calibration fluid from a lab supply) and you have to look after
>>them properly. There's a "Technical tips" link at the url above.
>>
>>Best wishes
>>
>>Keith
>>
>>
>>>Can anyone tell me if this is so and how I convert the reading to the amount
>>>of lye to be used?
>>>
>>>Best regards,
>>>Willem
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