Statistical Comparison of Titration Techniques

READING (IN HARRIS TEXT):

• • 153-157 (Titrations and Titration Calculations)

OBJECTIVES:

• • To determine the amount of potassium hydrogen phthalate (KHP) in an unknown mixture using both volumetric and gravimetric titrations

  • To make statistical comparisons between the two titration techniques

You’ve all performed countless titrations over the course of your chemistry education, and in each case you’ve probably used a volumetric buret to measure the amount of titrant you delivered.  A standard 50-mL buret that meets Class A requirements (±0.05 mL tolerance) costs over $300, and we’ve all been programmed to believe this is the most accurate and precise instrument for performing titrations. But this belief may be based more in tradition than in fact. Beyond the high cost of the buret, there are several other disadvantages to the traditional volumetric titration:

• • Even a Class A buret is relatively inaccurate with a ±0.05 mL tolerance. Burets used for the most precise work must be painstakingly calibrated.

  • Volumetric glassware must be extremely clean. Titrant will not drain properly from a dirty buret.

  • The rate at which the titrant is delivered must be slow enough to ensure that it is no longer draining down the buret walls when a volume reading is made.

  • Errors in reading buret volumes (e.g., parallax error, meniscus error) are commonplace.

  • “Leaky” and plugged stopcocks are also common.

  • Air bubbles in the buret can lead to errors in measurements.

  • Liquid volumes are dependent on temperature, so the volume of titrant delivered and the calculated concentration are also dependent on temperature.

Consider the gravimetric titration as an alternative. In a gravimetric titration, we simply fill a plastic bottle with titrant, then weigh the bottle both before and after the titration. Most, if not all, the disadvantages of a volumetric buret can be overcome:

• • Plastic bottles are much cheaper than burets.

  • Most top-loading electronic balances are accurate to ±0.01 g, which is approximately ±0.01 mL assuming most titrants have a density of around 1 g/mL.

  • There are no volumes to read so any effects of dirty glassware, slow drainage, volume reading problems, and/or air bubbles are eliminated.

  • The titrant is calibrated in moles/kg instead of moles/L, so the effect of temperature on volume and concentration is eliminated.

In this experiment we’ll be performing a series of titrations using both techniques to determine the amount of potassium hydrogen phthalate in an unknown mixture, then we’ll compare the results to see which technique is truly better.  Before we can analyze the unknown we’ll need to standardize an NaOH solution, i.e., determine its exact concentration. The storage bottle says 0.05 M, but this is only approximate and not nearly accurate enough for our purposes.  Reagent sodium hydroxide solutions always contain significant amounts of carbonate due the reaction of hydroxide with carbon dioxide from the atmosphere (OH–  +  CO2  →  HCO3–), and this reaction affects the OH-concentration of any solutions we prepare.  Sodium hydroxide solutions are usually standardized by using them to titrate known amounts of primary-standard grade potassium hydrogen phthalate (KHC8H4O4 or KHP).  Since the KHP is a primary standard, we can be reasonably sure it is essentially pure once we dry it in oven.  KHP is a monoprotic acid that reacts with NaOH according to the following reaction:

KHC8H4O4(aq)  +  NaOH(aq)  →  KNaC8H4O4(aq)  +  H2O(l)

The important thing to note is that the stoichiometry is 1:1, i.e., one mole of KHP reacts with one mole of NaOH.

Once the NaOH is standardized we’ll then be using it to titrate the KHP in an unknown mixture.  So you’ll actually be performing four different types of titration: 

• 1. Volumetric standardization of NaOH solution.

  2. Gravimetric standardization of NaOH solution.

  3. Volumetric determination of KHP in unknown mixture.

  4. Gravimetric determination of KHP in unknown mixture.

For each I’ll ask that you do two GOOD titrations.  (A good titration is one where you nail the color change to a fraction of drop.)  This means you need to do a total of eight good titrations.

REFERENCES:

Hawkes, S.J., J. Chem Ed., 2004, 81, p. 1715.

Ramette, R.W., J. Chem Ed., 2004, 81, p. 1715.

Skoog, D.A.; West, D.M.; Holler, F.J.; and Crouch, S.R.; Fundamentals of Analytical Chemistry (8th ed.) Belmont, CA: Brooks/Cole, 2004, pp 1066-1071 (web).