2013: Advent Chemistry: Titration

by on December 16, 2013
This is an indicator called, no word of a lie, phenolphthalein.

This is an indicator called, no word of a lie, phenolphthalein.

Titration is, like most of the concepts I’ve covered this year, basically quite a simple trick. You take an unknown amount of something, and you add somethig it reacts with in small, known amounts, until the reaction stops. Then you know how much of the second thing you’ve added, so you can calculate how much of the first thing must have been there at the start.

Also like most of the concepts I’ve covered this year, the reality is somewhat more fiddly.

You start with a burette, which is a glass tube about a centimetre across by seventy long, marked with millilitres and tenths of millilitres along the side. The exact size and precision vary, but you get the idea. The top of the tube is open, and the bottom tapers to a narrow funnel and has a tap.

You fill the burette with a solution of known concentration – if you’re lucky, you can get away with using a standard laboratory solution that the lovely technicians have made up and left around the lab for you, but you are just as likely to start by making up your own known solution. This is a simple procedure of weighing out a material and dissolving it in a fairly precise amount of solvent, and then doing the sums to work out the concentration. It’s not important that the concentration should be round number. It only matters that it’s likely to result in you needed something between, say, fifty and three hundred millilitres to completely react your unknown, and that you know what it is. Oh, and that you can see the numbers on the burette through it – relevant with some very strongly coloured compounds.

Then, having found or made your solution of known concentration, you fill the burette, ideally without pouring stuff over your hands, your bench or your labmates, and note down the volume reading once the burette is clamped tidily upright and level. It is important not to miss this step. You will feel very silly if you don’t have this number later, and also you will have to start over. Like the concentration, it’s less important exactly what the number is than that you know it.

Then you take your unknown, dissolve it if necessary (in as little solvent as you can get away with, to keep the total volume down), and either add a squirt of an indicator or rejoice that your reaction will produce a colour change all by itself.

Open up the burette to a medium-fast flow of droplets, and watch like a hawk for the first signs of a colour change. As soon as you see anything, slow the drops right down and start swirling the flask to keep the reaction mixture, um, mixed. You should start seeing flashes of colour as each drop lands, which fade away quite rapidly. Stop the flow completely, and start putting in one drop at a time, mixing between each drop. At some point, usually quite unpredictably, the whole solution will change from one colour to the other.

Stop. Right there. Don’t add another drop.

Note down the level the burette now contains and subtract it from what it held at the beginning. This is the volume of your known solution it took to fully react your unknown. Do some sums, and you know how much unknown you started with. Congratulations!

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