Step By Step Eye Make Up-What is the step by step way to correctly carry out a titration practical experiment?

by Chris Devers

Question by Sweeps: What is the step by step way to correctly carry out a titration practical experiment?
Im having a little trouble at college getting it right. Im also getting marked during the practical and need the correct step by step method of how to carry it out in order to get full marks.

Best answer:

Answer by Mike S
You should already have been given a list of steps, or better, a list of steps + a live demo by your instructor. That said, these are the steps that I would look for. (I assume that the unknown sample and the titrant are already prepared, and that you’re titrating an acid with a base using phenolphthalein indicator.)

1. Make sure the burette is thoroughly clean. Rinse it several times with DI water, and then once again with the titrant.

2. Close the stopcock on the burette. Fill it with titrant to well above the top mark. Open the stopcock and let it drain to a little below the top mark. (Discard the titrant used here and in rinsing.) Wait a minute for all the solution to drain down, then record the volume on the burette as “initial volume”.

3. Move a few feet down the bench and prepare the flask with the sample. Pipette a known volume of the sample into an Erlenmeyer flask. Use a flask big enough to hold at least 4x the sample volume.

How much? Depends on how much acid is present. More is usually better. There should be enough acid present to react with about 25-40 mL of base (assuming a 50 mL burette – maybe 15 to 20 if it’s a 25 mL). You should also have enough sample to do at least 4 titrations with this volume – 3 replicates plus a spare – so don’t use more than a quarter of what you’ve got. Add a drop of indicator – one is enough, maybe two. (Moving away reduces accidental contamination from the titrant.)

4. Now lift the burette up to allow the flask to slide in place underneath. (Do move the burette, it’s too easy to spill the sample or touch the tip of the burette to the inside of the flask if you try to tilt the flask into place.)

A piece of white paper – paper towel, Chemwipe, even a page of writing paper – is good under the flask. That lets you see a faint color more clearly. Don’t tear a page out of your notebook.

5. This first time through, you probably don’t know how much final volume is going to be required. Going drop-by-drop takes forever! Slowly open the stopcock enough to give a decent trickle into the flask. You don’t want it going too fast; it may splash, or you’ll overshoot the end point. You don’t want it going too slow; that wastes time. I usually go about 5-8 mL/min.

SWIRL ONCE, without picking up the flask. Watch the area where the solution falls. It’ll turn bright pink, because the pH in that area goes up very fast. The pink color will fade as the base mixes in and is consumed by “excess” acid. Keep going as long as it’s only a single pink cloud in the middle of the flask. Keep one hand on the flask, the other on the stopcock, and your eye glued to the flask!

When that pink color starts to spread, or stops fading, slow down, or stop entirely. Give it one more swirl. All the pink color should go away at this point. (If it doesn’t, we’ve past the endpoint on this first fast one. Record the volume.)

Add a little more base, if the pink has gone away; go slowly, we’re close to the finish. Swirl again. The color should fade, more slowly this time. Add a few drops and swirl. Write down the volume. Color gone? Add a few more drops and swirl. Write down the volume. Color gone? Add a few more drops, recording the volume after each swirl. (This makes sure you’re never more than “a few drops past” the endpoint for whatever the last volume is that you write down.)

Now you’re done with the first sample. Calculate the amount of acid present in the sample. (If all the samples are the same, like replicate volumes of the same solution, then this is less important.) Predict, for the second sample, how much base will be required to reach the end point, assuming that the acid has the same concentration.

Now fill up the burette again, same way as before. Set aside the flask and get a clean one for the second titration. (You can rinse it out, if you have to reuse the same flask.) Pipette in the second sample and get ready to titrate.

This time, you already have a good idea how much you’re going to need. Let’s say it will take 30 mL. It’s okay to drain in most of that – say, 20 mL – all at once, then swirl it up to make sure it stays clear. Now add in most of the rest – say up to 26 mL total. This time, you’re going for high precision and accuracy: Let the base into the flask one drop at a time, slowly swirling the flask while you keep an eye on the color. As it gets closer and closer, slow down the drops. One every 5-10 seconds is about right. (Too slow and you’ll actually cause error by CO2 getting absorbed from the air as carbonic acid, so your answer goes high.)

When the color just barely persists, stop the burette and write down the volume. Add 1-2 more drops; it should be definitely pink. Write down this volume too.

(Your instructor may have his/her own idea about how “pink” the endpoint should be. Follow those if you know them, for the sake of your grade, but you really don’t want it

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