So how does this thermometric titration stuff work?

So how does this thermometric titration thingy work? Well, labrat old son, I'm glad you asked that question. You can see how desparate things get around here when I have to answer my own questions
The answer is: exactly like any other titration, except that the heat of the reaction between the analyte and the titrant is used to detect the endpoint. Let's take an example: you want to determine the sulfate content of a particular sample. Your titrant will be 1 mol/L barium chloride. The reaction of barium ions with sulfate ions to form insoluble barium sulfate is exothermic (heat is given out by the reaction). In a thermometric titration, the burette delivers titrant at a constant dose rate, so the reaction between the barium and sulfate ions is occurring at a constant rate. In an exothermic reaction, you'd expect that the temperature would increase at a constant rate too. There are a few other things around like heats of mixing and stuff, but by and large, we see an increase in temperature. Check the titration plot from a Metrohm 859 Titrotherm instrument below. The red line is the temperature trace of the reaction. The volume of titrant added is plotted on the "x"-axis, the solution temperature is plotted on the "y"-axis. When all the sulfate has reacted, there is nothing to increase the solution temperature at the same rate, and we see a break in the curve. That's the "endpoint" or "breakpoint" of the titration. We measure the amount of titrant added to this point, and calculate the equivalent amount of sulfate that this represents.

Finding the exact endpoint used to be a tedious business until cheap PC's came along to run the powerful algorithms needed to get accurate results. In thermometric titration, the second derivative of the temperature plot is used to locate the endpoint, as you can see in the next titration plot.The yellow curve represents the second derivative. The endpoint is at the peak shown on this curve. Of course, there's some pretty flash maths going on the background, but you and I don't have to worry about that. The results can be sent automatically to a dedicated spreadsheet for calulation, which all makes it pretty easy.

A Tale of Old El Paso (Lite)

There is a mineral called "elpasolite", because it was first identified in a deposit near El Paso, Texas. Sorry about the pun in the title, BTW. It's certainly nothing remarkable to look at. It's a sort of dull grey, not much lustre, and quite soft. You're not about to rush into your local jeweller and ask him to mount it into a ring for your significant other. So why is it my favourite mineral at this time? Well, it has the molecular formula NaK2AlF6. OK, that's confirmed your suspicion that I may be certifiably strange. But wait, there's more.

Elpasolite is the key to a neat series of thermometric titrations where you can determine sodium, potassium, aluminium and fluoride. The reaction is simply:

The reaction is exothermic, and proceeds at room temperature. This makes it a good candidate for thermometric titrimetry. By suitable arrangement of the experimental conditions, you can employ the same chemistry to analyze for Na, K, Al and F.

Let's take aluminium as our first example. Aluminium-based chemicals have important uses in water treatment and as anti-perspirants, among other applications. Aluminium in such materials can be tedious to analyze by titration. The traditional method has been to add an excess of standard EDTA, boil to ensure full complexation with the EDTA, then titrate the excess EDTA with Zn solution to a visually-indicated endpoint. It's not an easy endpoint to pick.

By contrast, the elpasolite aluminium method is easy. Just measure the Al containing sample directly into the titration vessel, add a buffer, and titrate with standard sodium fluoride. The buffer comprises sodium and potassium acetate and acetic acid, and brings the solution to ~pH4.5. The buffer is designed to supply the excess Na and K ions required to drive the reaction strongly to the right. The only proviso is that the Al has to be present as Al+++ . This means that for aluminium chlorohydrate, this material needs to be hydrolyzed with some HCl prior to the buffering step. Similarly, sodium aluminate needs to be strongly acidified.

The elpasolite aluminium titration is very precise, and analytical precisions <0.1%.

I first came across it in one of the texts that I have on thermometric titration, G.A. Vaughan, Thermometric and Enthalpimetric Titrimetry, Van Nostrand Reinhold Company (1973) The original method was for the determination of sodium by direct-injection enthalpimetry(!) and the reference given is: Sajo, I., Magy. Kem. Folyoirat 75 1-3 (1969). Since I don't know Hungarian, and since I don't have the gear for that type of determination, I jiggered around until I got it to work as a thermometric titration. It works a treat, too. Sharp endpoint, great precision, and fast as a whippet. Generally, you're looking at under 2 minutes for a titration.

Interesting, you say, but why would you use it when there's AAS and ICP around. Well....

1. The method is intended for Na in the g/L or % region, so with AAS and ICP you have to dilute and dilute and dilute and dilute and dilute until you get it in range with the instrument. With the elpasolite method, you just weigh your sample directly into the titration vessel.

2. You don't have to filter your sample, either, because with thermometric titration, you can titrate with solids in the sample solution.

3. In common with all titration techniques, it's a linear method. By that I mean that instrument response (measured in mL of titrant) is directly proportional to the amount of analyte present, whereas with spectroscopic procedures such as AAS and ICP, the signal is proportional to the logarithm of the analyte concentration, which is not so great when you're going after maximum precision.

Introducing me.

This is me. The labrat in the white coat. I've been a lab rat for a long time. Ever since I was small, I wanted to wear a white coat, and stuff the top pocket with spatulas, indicator paper, and things which you use to prod and poke. I can't seem to get out of the lab. Every time I try to get out, it keeps dragging me back (where have I heard that before?). Actually, I don't want to get out of the lab. It's where I belong.

So about this blog. This is going to be about what I like to do, and what I do for a living, which is pretty much the same thing. You can shout "get a life!", but all I can say is that I have a life, and this is it. I'm a professional analytical chemist specializing in the development of analytical methods for process and quality control. My company consults to the global market leader in titration technology, Metrohm. We are assisting them introduce the technique of thermometric titrimetry to the marketplace in the shape of their new thermometric titration system Titrotherm. So yes, this blog does have a commercial aspect to it, but I am a professional chemist and I do take the responsibilities that go with that seriously. That means that I will only make claims which can be backed up by published literature; either in the shape of application notes published by Metrohm or published elsewhere in scientific literature.