ACID-BASE TITRATION

In this experiment you will determine the amount of acetic acid in a vinegar sample and the amount of ascorbic acid in a Vitamin C tablet.

To do this you will obtain some approximately 0.2 M sodium hydroxide solution from the stockroom and determine its exact concentration.
This process is called
standardization.
It involves weighing out a known amount of very pure acid called a standard. You will use Potassium Hydrogen Phthalate, abbreviated KHP. Remember KHP is an abbreviation, not a formula!
You will measure the volume of NaOH solution needed to neutralize this acid. Knowing the weight and molecular weight of the acid and the volume of NaOH solution used to neutralize the acid, you can calculate the exact Molarity of the NaOH solution.
Since this is a very important number you will repeat the standardization with another weighed sample of KHP. If the molarity for your two standardizations agree to within 1 %, you can go on. Otherwise try a third standardization or fourth, fifth......or maybe you need HELP!

Once you have standardized the NaOH solution, you will use it to titrate known amounts of vinegar and a vitamin C tablet. From the amounts of these used and the concentration and volume of the NaOH used, you can determine the weight/volume pre cent of acetic acid in the vinegar and the veight of ascorbic acid in the vitamin C tablet.

TITRATION

You will measure out the amount of Sodium Hydroxide you use in the standardization and titrating the unknowns using a buret.

In this experiment, the buret always contains the NaOH.

Fill the buret past the 0.00 mark. The 0.00 mL mark is toward the top of the buret. The stopcock at the bottom of the buret allows you to start and stop the liquid flow.

Drain to the 0.00 mL mark, making sure there are no air bubbles in the buret and no drop hanging out of the tip.

The reaction flask contains your acid dissolved in water and a couple drops of a dilute phenolphthalein solution. It should sit on a white paper under the buret, with the tip of the buret just inside the flask's neck.

Add the NaOH from the buret into the reaction flask. Swirl the flask continuously to make sure the reaction is thoroughly mixed as you add the NaOH. Stop the titration and read the buret when the phenolphthalein indicator just turns pink with one drop of NaOH. This means that although you can add the NaOH rapidly at first, at the end point you must be adding it slowly, dropwise.

READING the BURET

The MENISCUS

There exists an attractive force between the molecules of the glass buret and the water filling it. Since the water molecules are strongly attracted to the glass silicate molecules, the top of the water column displays a downwardly curved (concave) surface.

This is called a meniscus.

Where there is little or no glass to liquid attraction relative to the molecular attraction of the liquid to itself, the miniscus becomes convex. (The middle of the miniscus is higher than the edges.) Mercury is such a liquid.

READING a BURET

  • Since the smallest graduation on the buret is 0.1 mL, you must try to read "between the lines", that is estimate the buret reading to the next smaller unit which is .01 mL.
  • The level of a transparent liquid in the buret is measured by "reading" the "bottom" of the meniscus. Please verify that the buret above reads 15.46 mL.
  • In reading the buret the eye must be positioned at exactly the same level as the miniscus.
    • In the image above, the camera is positioned correctly. By looking at the full circles at the 14, 15, 16 and 17 mL marks, you can see exactly where the camera was positioned. Use the same technique when reading your buret.
    • An error in reading a buret due to incorrect positioning of your eye is call a parallax error.
  • Read the buret from the top (0.00 mL) to the bottom (25.00 mL).
    • Never start a titration with the liquid level above 0.00 mL.
    • Never let the liquid level go below the 25.00 mL mark. If you need more titrant, stop at the 25.00 mL mark and refill the buret.
  • Measuring a volume delivered by a buret requires two readings. Always read the buret the same way. That way, inadvertent systematic reading errors may cancel out.

RWK 1997