Titrations: Introduction

Volumetric AnalysisMeasure volume of a reagent needed to react with an analyte

Volumetric TitrationAdd Increments of a reagent to a solution of an analyte until the reaction is complete

Ideal: Rxn Complete; Rxn Rapid

Common TitrationsA. Acid-BaseB. Oxidation-ReductionC. Complex FormationD. Precipitation Reactions

Definitions

Indicator - Compd whose color changes abruptly near the equiv pt

Equivalence Point - pt in the titration where the quantity of titrant added is stoich. equiv to amt. of analyte present.

End Point - Typically what is measured; marked by sudden change in prop. of soln

Titration Error - diff btwn end pt & equiv pt

Back Titration - Add excess of titrant then determine the amount excess

Example

5H2C2O4 + 2 MnO4- + 6H+ 10 CO2 + 2 Mn+2 + 8H2O

analyte titrant

colorless purple

Equiv pointpoint when 2 mmol of MnO4

- has been added to 5 mmoles of H2C2O4

End pointdetect by monitoring color of solutionbetter eyes, closer endpoint will be to equiv point

Back titrationadd xs MnO4

-, back titrate with another standard to determine excess slow reactions, standard solutions not stable.

What’s Needed

Solution of known compositionPrimary standardStandard SolutionAnalytical BalanceVolumetric Flask

Accurately measure volumeBuretVolumetric Pipet

Mercuric Nitrate was standardized by titrating a solution containing 147.6 mg of NaCl. 28.06 mL of the Mercuric Nitrate solution was required to reach the end point. This solution was then used to titrate 2.000 mL of urine. It took 22.83 mL to reach the endpoint. Find the concentration of Cl- in Urine as mg/mL

Hg2+ + 2 Cl- = HgCl2

Titration Calculations

The CO in a 20.3 L sample of gas was converted to CO2 by passage over iodine pentoxide heat to 150 C.

I2O5 + 5CO = 5 CO2 + I2

The iodine distilled at this temperature was collected in an absorber containing 8.25 mL of 0.01101 M Na2S2O3

I2 + 2S2O32- = 2I- + S4O6

2-

The excess Na2S2O3 was back titrated with 2.16 mL of a 0.00947 M I2 solution.

Calculate the number of mgs of CO per liter of sample

Titration Calculations

Applications of Acid-Base Titrations

1. Determination of Nitrogen

Kjeldahl Analysis:

Important method to accuratelydetermine nitrogen in proteins andother nitrogen containingcompounds

Steps

Digest the material in sulfuric acid convert to NH4HSO4

Cool the solution and make it alkali to convert to NH3 (gaseous)

Volatile ammonia distilled into known volume of acid

Excess acid back titrated

A protein contains 16.2 wt% nitrogen. A 0.500 mL sample of the protein solution was digested and the liberated NH3 distilled into 10.00 mL of a 0.02140 M HCl solution. The unreacted HCl required 3.26 mL of a 0.0198 M NaOH. Calculate the concentration (mg/mL) of protein

in the original sample.

Example