Basically, in titrations, you have one solution with a known concentration, and another solution of an unknown concentration that you want to find the concentration of. To do so, you basically get a known quantity of one solution, like 20mL, and then you add gradually increasing quantities of the other solution until you get some kind of indication that the reaction has gone to completion. Then you use all the stoichiometry stuff learned from before to work out the concentration of the unknown substance.
Now here comes the kicker. Titration lab work. Ugh.
As I mentioned, you need to have a solution with a known concentration. And the best way to know what the exact concentration of your solution is is to make it yourself so that you know exactly what goes inside it.
The problem? A lot of caution is needed to make sure nothing gets contaminated. Also, you need to decide on a good solution to make up. Not all solutions are equal for this purpose. The best substances are those known as primary standards. Primary standards have characteristics that make them good for making up into solutions of known concentrations: they are pure and aren't deliquescent (i.e. don't absorb water). They are anhydrous and don't react readily with stuff in the atmosphere. It's also good if the substance has a relatively high molar mass, which will make it easier to measure out one mole (or however much you need) with a greater accuracy.
Making a Solution of a Primary Standard
The first step in making a primary standard solution is working out how much solid needs to be dissolved. To do this, take a look at my post on simple calculations involving moles. Measure this amount out in a petri dish (remember to put the petri dish on the scales and tare before putting any solid on) and then tip the solid into a beaker (make to rinse the beaker with distilled water first to avoid contamination). Dissolve it with a little bit of distilled water in here. Remember, you must use distilled water, otherwise you'll have other random ions floating around in your solution which you don't want. After dissolving, first rinse a volumetric flask of whatever size you need with distilled water, then pour in the solution and make it up to the mark with some more distilled water. Remember that the bottom of the meniscus must be level with the mark (when read from eye level). Then put a stopper on and swirl the solution around a bit so that everything's all nice and mixed around. Finally, get a storage bottle, and rinse a couple of times with distilled water, and then with a little bit of the solution. Now that's done, pour in the rest of the solution. You have now made up a solution with a known concentration. Yay.
The Titration Part: Acid/Base Titrations
Now that you have a solution of a known concentration, it's time to titrate it with something to make all that hard work worth it. There are different types of titrations- I'm just going to start off with acid/base ones because we've been doing acid/base titrations in class.
What you'll need: a pipette (a reasonably large one that can hold 20-25mL, not one of those puny ones), a burette (kind of like a long plastic pipe with measurements and a little tap thing on the end so you can control how much liquid comes out), some conical flasks, a retort stand and burette holder. You'll probably want one or two small beakers so that you can pour stuff into there before transferring it to the burette or conical flask. Oh, and you'll also need some indicator for acid/base titrations. The type of indicator depends on what you're titrating- more on that later.
I really should draw a picture of something of how to set it up. Better yet, I could take a photo so that you can see all the equipment properly. It would probably help if I could be bothered to sync my phone up to my computer though. Ah well.
Anyway... basically what you do is first you rinse the pipette with whatever you're going to put into the conical flasks, the burette with whatever you're going to put in the burette, and the conical flasks with distilled water. Also you should rinse the little beakers with whatever you're planning to put in there. (I'll explain the reasons behind all the rinsing later.) Then you use the pipette to put a certain amount of one solution into the conical flask, and a small funnel to put the other solution into the burette (you don't have to fill all the way to the 0mL mark, but you can't go over). Write down the initial reading. Put a few drops of indicator into the conical flask.
Now you're ready for your rough titration. Turn on the tap of the burette and swirl the flask around. Occasionally, stop pouring stuff in and stop swirling the flask to see if there's a permanent colour change throughout the whole liquid. Stop titrating when you get a permanent change. Write down the reading on the burette and work out how much of the solution was consumed. Now, do it again, except stop titrating a few mL before you use up the amount that you used before (e.g. if you needed 20mL last time, stop titrating once you've used up 17 or 18mL). Now go more slowly, drop by drop, until you work out the exact drop needed to get a permanent colour change. Do this a few times. When you have more or less the same amount consumed for multiple titrations, choose this amount for your calculations.
Now I've explained what a titration is, I'm going to explain the finer points in a bit more detail!
Rinsing
What do you rinse with what, and why? Here's a guide:
When preparing a solution
- Before making a solution up to the mark, rinse everything with distilled water. The petri dish should be dried before weighing so that you don't have water stuffing up your calculations, but otherwise it doesn't matter so much for everything else as you're going to be dissolving the solid and diluting the solution anyway.
- When you're storing the solution, you'll first have to rinse the storage bottle a couple of times with distilled water to get rid of any other contaminants already in the bottle before rinsing with a little bit of the solution. Rinsing with the solution gets rid of the last little bits of distilled water and ensures that the concentration stays more or less the same. After all, you wouldn't want to do all that hard precise work and then have it all crushed by something stupid like not rinsing the storage bottle correctly, would you?
When doing a titration
- The pipette and burette need to be rinsed with the solutions that are about to go in them so the concentration stays constant and thus allows you to calculate the number of moles of each substance in whatever amount of solution that you end up using. Ditto with the little beakers if you're using them to store solutions before putting them in the pipette or burette. (Make sure you know which beaker's which!)
- The conical flasks need to be rinsed with distilled water to keep the number of moles of the substance constant. If you rinse with solution, you increase the number of moles of the substance in the conical flask, which will stuff up your calculations.
Indicator Choice
Okay, firstly I have to talk about the "end point" and "equivalence point" of a titration.
The "end point" is the point at which you stop titrating because of a colour change, and the "equivalence point" is the point where the reaction has actually stopped. Naturally, you want to choose an indicator that will change colour close to the equivalence point.
For example, let's look at the reaction between NaOH and HCl. This reaction is complete when the pH is around about 7. One indicator that you might want to look at is phenolphthalein, an indicator that is colourless for low pH and pink at high pH. The colour change occurs at around about pH 8, which is relatively close to pH 7. You might ask why not use an indicator that changes colour at pH 7, but the thing is, with strong acids and bases, one small drop can cause a massive change in pH when you're close to the equivalence point, so it's generally better to have an indicator that changes slightly early. (I think.)
There's different kinds of indicators which are useful for different titrations but you have to pick your indicator carefully, otherwise you'll stop titrating too early or too late and your calculations will be stuffed up. Don't get too hung up on this, though, as there are lots of times where several different indicators might be appropriate. For example, I think that it's also acceptable to use methyl orange for the above experiment, which changes colour at around pH 4.
And that's all for now on titrations! I have a practical test on titrations when school starts again... argh
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