Science Buddies: "Ask an Expert"

For a chemistry lab, I have to determine the concentration of acetic acid in different types of Vinegars by titration. I understand what to do for this lab because I have watched a lot of videos on how to do it, however, this would be a very simple lab if the 0.1 M NaOH was provided but it was not, I have to make it myself and I need about 250 mL of it... How exactly do you do this? I've been looking everywhere, some websites said you need KPH while others say... other things I don't understand. Can someone please help me, I am in desperate need. I believe my classroom has all the materials/chemicals needed to standardize (NaOH, KPH etc...) but I don't know how to do the calculations and stuff.

Well, I did this often 50 years ago. Let's hope someone more up-to-date will weigh in. If not, here goes.

You say 0.1M NaOH. I will speak in equivalents, not molarity. Don't despair, for NaOH it's the same. For other things it's not, and equivalents is far more general, and is usually used. Acetic acid also has equivalent weight and molar mass the same. "N" stands for normal.

Potassium hydrogen phthalate is considered a stable, reliable chemical for calibrating other acids and bases. It has a molar mass of 204.2 (check my result yourself, please) and has an equivalent weight equal to its molar mass. That means there will be one reactable hydrogen ion in every molecule. Sodium hydroxide, whose molar mass is about 40, also has an equivalent weight equal to its molar mass. It has one reactable hydroxyl ion per molecule.

The equipment I'd use if I were doing this would be some volumetric flasks, a pipette, and a burette. Lets take the simplest example, and I will leave to you to figure out what will work for you. Weigh exactly 0.1 equivalent of KHP and carefully put it into a 1 litre volumetric flask. Fill the flask exactly to the neck mark with distilled water. Now you have a solution of 0.1N KHP. This is your calibration solution. Now prepare your NaOH solution the same way--about 4 g/litre in another volumetric flask. Careful, NaOH will dissolve lots of stuff you don't want to dissolve, such as your eyes and skin and anything wool. You need to know what you're doing in this step. Now, you have a flask of exactly 0.1N acid (the KHP) and approximately 0.1N NaOH. Even if you weigh it precisely, the NaOH picks up so much stuff from the atmosphere that your solutions always need to be calibrated.

Still with me? Now you need a pipette. Put a precise amount of your KHP solution in a flask. Put some of your NaOH solution into a burette. Here you need to know how to titrate. It isn't rocket science, but it does require good technique. Practice! You will need to be pretty good when you get to acetic acid.

You need an indicator--something that changes color when the pH changes. The pH change for these two materials is very fast, and at the "end point" one drop will make the difference in color. Proceed with appropriate caution. So, if you put 10 ml of KHP in your flask, you should expect to see the end point when ABOUT 10 ml of NaOH has been added. If you add precisely 10 ml, your NaOH solution is 0.1N. If you add 11 ml, the solution is obviously weaker than 0.1 N, and would be in fact 0.1 x (10/11) N. Not to belabor the obvious, but your KHP solution has 0.1 equavilents/litre. So, the equivalents in your flask will be 0.1 x ml added/1000. The equivalents of NaOH added will be equal at the end point, so you will know how many equivalents of NaOH you added from your burette. Divide that by the volume, and you have the normality of the NaOH. That is defined as equivalents/litre. Most of work with milliequivalents/millilitre, which is numerically the same.

You won't hit 0.100N. That's fine. You may go through life without ever hitting exactly 0.1000 N NaOH. All you need to know is exactly what it is, and you can calculate everything else.

I hope my ancient memory was reliable and helpful.