Experiment 3 Standardisation of hydrochloric acid with borax

Goal: This experiment is designed to enhance your skills in the precise use of volumetric glassware and the analytical balance. In addition, you will be introduced to the procedure for preparation of a solution of a primary standard and the importance of selecting the correct indicator for an acid-base titration.

Objectives: On completion of this lab you should be able to:
  1. Weigh accurately using an analytical balance and assess random errors in balance readings and masses weighed.
  2. Prepare a standard solution.
  3. Select an appropriate indicator for an acid base titration
  4. Determine the concentration of acid in an unknown solution using a primary standard
  5. Determine the error in calculated concentration.
Theory
Borax (Na2B4O7 .10H2O) can be used as a primary standard since it does not decompose under normal storage, it is readily obtained in a very pure form (99.999% purity), it reacts with a known stoichiometry and can be weighed and used directly.

In water, the salt dissociates:
Na2B4O7.10H2O → 2Na+ + B4O72- + 10H2O
and the anion is hydrolysed:

B4O72- + 7H2O → 4H3BO3 + 2OH-

The liberated hydroxide ions can be titrated with a strong acid and at the point of their neutralization, the solution in the flask will contain the very weak acid H3BO3, which will dissociate according to:

H3BO3 + H2O → H2BO3- + H3O+. Ka = 5.8 x 10-10.

Thus at the neutralization point (or equivalence point: the point when all the liberated OH- have been converted to H2O) the solution will be slightly acidic (pH ~ 4.8; the actual pH depending on volumes and amounts used). The indicator used to identify the strong base-strong acid equivalence point must therefore change colour over the pH range 4.8 ± 1.

Safety Notes
During this experiment you will be working with a weak base and a strong acid. You must wear eye protection at all times. In the event that any reagent used in this investigation comes in contact with your skin or eyes, wash the affected area immediately with lots of water. Notify your instructor.

Method
Record all observations directly onto your worksheet.

Weigh accurately in triplicate (on an analytical balance) about 0.15 g of borax into 150 cm3 conical flasks. Dissolve each sample in about 25 cm3 of water, warming gently if necessary, and titrate with the supplied hydrochloric acid (approximately 0.05 mol dm-3) using Screened Methyl Orange (SMO) indicator. For accurate results, only one drop of indicator should be used, in which case, the end point will be almost colourless against a white background.

As indicated on your worksheet
a)        Calculate the concentration of your hydrochloric acid.
b)        Calculate the random error in your hydrochloric acid concentration by the propagation of errors method.

Apparatus per student:
One 25 cm3 (or 50 cm3) burette, one funnel to fit the burette, three 150 cm3 conical flasks, one wash bottle, one white tile, one burette stand and one burette clamp, access to an analytical balance, one weighing bottle or 50 cm3 beaker.

Chemicals per student:
0.5 g Na2B4O7.10H2O, 100 cm3 0.05 mol dm-3 HCl, 1cm3 SMO indicator.

Questions

1. What would you expect to observe if phenolphthalein was used as the indicator in the above titration?
2. Name one other indicator suitable for the above titration.
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Created and maintained by Prof. Robert J. Lancashire,
The Department of Chemistry, University of the West Indies,
Mona Campus, Kingston 7, Jamaica.
Created March 1997. Links checked and/or last modified 14th September 2012.
URL http://wwwchem.uwimona.edu.jm/lab_manuals/c10expt3.html