Chem 3250 Experiment #4
Volumetric Soda Ash Determination
I. Introduction
Reference Method 27C-3, 27C-6, 27C-10, in Skoog et. al. 7th Ed. (Note- typically
the method you are to use follows and the method in the book should be similar and
might help add some insight into the procedure. If you deviate from the method given
here be sure to mention this in your lab book).
The classic titration of soda ash (sodium carbonate) involves the titration of the sodium
carbonate as a base with standardized HCl. The HCl can be standardized against
previously standardized NaOH or more appropriately, against primary standard sodium
carbonate. This offers the analyst an opportunity of analyzing the sample under exactly
the same conditions as in the standardization and realistically all the procedure can be run
sequentially before any calculations performed affording the opportunity of completing
the experiment with only one equipment preparation. That is, sample of standards and
unknowns can be weighed out into individual flasks and titrated one after the other with
calculations of HCl and analyte concentration left for post experimental analysis. Until
you are fairly secure in your titration abilities it may prove advantageous to do the
calculations for the standardization first to confirm the precision of your technique.
The reaction of HCl with sodium carbonate is two fold as the neutralization of the
Na2CO3 involves two steps:
H+ + CO32- Ù HCO3- ( HCl + Na2CO3 Ù NaHCO3 )
H+ + HCO3- Ù H2CO3 ( HCl + NaHCO3 Ù H2CO3 )
Since the carbonic acid produced in the reaction is a weak acid it also contributes to the
pH around the second equivalence point. In order to minimize this and produce a more
