Titrations

September 20, 2002

You should work in a group of three students. Each of you will perform a different analysis.

• determination of oxalate (in the green salt you made)
• determination of potassium (in the green salt, in vitamin pills)
• determination of calcium (in tap water, in milk)

The basic features of a titration are

• 1. the sample is reacted with a second chemical species, called the titrant
  • The term analyte refers to the species we wish to measure.
• 2. the titrant is a solution of carefully known concentration
• 3. the titration continues until the analyte in the sample has completely reacted.
  • Only a very slight excess of the titrant is added
  • This is called the endpoint of the titration
  • A critical feature is a method to tell when the endpoint is reached.
• 4. measuring the volume of titrant allows one to calculate moles of titrant used.
  • It is quite easy to accurately measure the volume of titrant (with a buret)
• 5. in a simple reaction, the number of moles of analyte is either the same or differs by a
  • simple multiple fixed by the stoichiometry of the reaction.

• instead, we determine the volume of titrant that reacts with the sample
• this is an indirect (but effective) solution to the original problem

In titrating an acid we might perform the following steps

• 1. measure out a sample of our acid
  • (a fixed volume if it is a solution or a weighed sample for a solid acid sample.)
• 2. if necessary, add water and dissolve the sample
• 3. fill a buret with a solution of a base, such as sodium hydroxide
• 4. add an acid base indicator like phenolphthalein to the sample
• 5. record the initial volume of base in the buret to the nearest 0.01 ml.
• 6. gradually add base from the buret while stirring the solution
• 7. stop adding base when the indicator changes color, signifying the end point
  • proceed slowly at the endpoint; titrate to nearest tiny drop of base
• 8. record the volume of base remaining in the buret.
• 9. calculate
  • subtract initial volume from final volume to find volume of base added
  • # moles of base = (conc. of base) x (volume of base added)
  • #moles of acid = # moles of base (if a monoprotic acid)
  • concentration of acid = (#moles of acid) / (volume of acid sample)

• 1. Acid / base titrations
  • titrate an acid with a base (or vise versa)
  • rely on the abrupt change in pH to indicate when acid (base) is completely reacted
  • rely on an acid base indicator to make that endpoint easily observed.
    • used last week to determine the distribution of Benzoic acid between two liquid layers.
    • we will use acid-base titrations to indirectly determine Potassium.
• 2. Oxidation /reduction titrations
  • many samples are easily oxidized
  • there are several good oxidizing titrants
  • it's a little harder, but there are ways to determine when the sample is completely reacted
  • (We will titrate oxalate ion with potassium permanganate and use the color of permanganate ion as our visual indicator.)
• 3. Precipitation reactions
  • the titrant reacts with the solid to form a precipitate
  • solid forms until the sample is completely reacted
  • again, we need a practical way to determine when that occurs
• 4. Chelation reactions
  • calcium and magnesium will react with a titrant called EDTA
  • a specialized indicator reacts with the EDTA to signal a color change at the endpoint

• 1. use of a transfer pipet
• 2. use of a buret
  • filling a buret
  • measuring volume in a buret and parallax errors
  • controlling the stopcock and the flow of titrant
• 3. stirring during a titration
  • by hand
  • with the use of a magnetic stirrer

• The volume in a 50 ml. buret is typically measured to +0.01 or +0.02 ml.
• For a titration volume of 40.00 this is about a 0.025-0.05 % uncertainty
• If the titration volume were only 4.00 ml, the error is still + 0.02 ml.
  • that is now a 0.25-0.5% uncertainty
  • An alternative would be to use a 10 ml buret and read to + 0.002 ml.
• We also want to avoid titration volumes > 50 ml
• we'd need to refill the buret and this would add additional errors as well as slowing the process considerably..

• Permanganate titration of Oxalate Ion
• Determination of Calcium by EDTA titration
• Determination fo Potassium by Ion Exchange
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