Hanna Instruments HI 901 - Manual (Page 129)

A2-3
APPENDIX 2
A2 BACK TITRATION (HI 902 only)
A2.1 Applicability Domain
Back titrations are generally used for one of these reasons:
• Reaction kinetics are too slow for the direct titration of the analyte.
• The metal precipitates at the desired pH (complexometric titrations).
• The reaction between titrant and analyte produces some auxiliary compounds
(e.g. CO
2
) that can affect the mV signal and also the equivalence point detection.
• Heavily soluble analyte in the solvents used.
• Titrations with more flat first derivative curves.
A2.2 Method Principles
In a back titration, an excess of a reagent is added to the sample solution, helping to lead the
reaction to its completion. The unreacted excess reagent is then titrated. The difference
between the total reagent first added and that determined from the final titration is the
quantity of reagent required to complete the reaction with the analyzed compound from the
sample. With this reagent quantity will be calculated the sample concentration.
The back titration can be an acid – base, complexometric, redox etc.
A2.3 Back Titration Example
An example of back titration might be the determination of an antacid neutralizing capacity.
A2.3.1 Introduction
An excess of stomach acid (primarily HCl) causes heartburn and acid indigestion. Commercial
antacids consist of a number of ingredients, such as binders and flavorings, but the active
ingredient is simply a basic salt such as Mg(OH)
2
(milk of magnesia), NaHCO
3
(sodium
bicarbonate), CaCO
3
(calcium carbonate) or Al(OH)
3
(aluminum hydroxide), to neutralize
the acid. In this experiment we will analyze a typical antacid containing CaCO
3
, which reacts
with an acid to form a salt, water and a gas:
CaCO
3
+ 2HCl —> CaCl
2
+ H
2
O + CO
2
A conventional acid/base titration is very difficult in this case as the active ingredient
is less soluble in water, and the CO
2
gas formed has an important influence in pH
measurements.
In order to overcome this limitation we will completely dissolve the tablets in excess acid:
CaCO
3
+ 4HCl —> CaCl
2
+ H
2
O + CO
2 +2HCl
excess