Titrations

The Titrations Model is a JavaScript simulation that complements experimental work. A typical student's training in a chemistry laboratory is the titration of an acidic solution by a padronized base in order to determine the acid concentration in the sample. If the titration is followed by a visual method, a suitable indicator has to be previously chosen to detect the end point of the titration (the so-called equivalence point) by the sudden change of the indicator color. This is a common practice (alternative to using a pH meter) and does not imply the errors that may be thought on the subjective aspect of a color change, providing that the right indicator is chosen. This application is restricted to monoprotic acids and monofunctional strong bases.

Take a flask with a known volume (VA) of an aqueous solution of a monoprotic acid whose concentration is unknown. In a buret there is a padronized (well-known molar concentration) aqueous solution of a monofunctional strong base (sodium hydroxide, for example). Add to the flask a few drops of a suitable visual indicator and start pouring, slowly, the base over the acid.

(source: "Mark Bishop's Chemistry site")

The titration end point (equivalence point) is detected by a sudden change of the indicator color. Then, the buret scale indicates the volume of the base (VB) used in the neutralization reaction, and as its molar concentration, [Base], is well-known the corresponding number of moles is NB=VB*[Base]/1000. This number is equal to the number of acid moles, NA=NB, that have reacted and as the sample volume is VA, the acid molar concentration is worked out: [Acid] = NA*1000/VA. Therefore, the titration coordinate (f=NB/NA) is equal to 1 at the equivalence point.

The indicator is chosen accordingly to its effectiveness color range, that is, the equivalence point pH should be approximately at the middle of that range.

The simulations determine the pH at the equivalence point and choose the proper indicator from a table included in the program. The acid is characterized by the ionization constant (Ka); the default value is for acetic acid. As the base is assumed strong (fully ionized) its constant (Kb) is not a program input. The pH at equivalence point depends on Ka, [Base] and also on [Acid]. But the acid concentration is precisely what we want to know! Of course, the indicator choice requires a previous trial and error study to screen possible concentration bands of the acidic sample and assess whether or not the same indicator is appropriate for each band. To this end, different simulations should be done by changing the [Acid] and [Base] fields for a given acid. The program can probe wide ranges of Ka, from 10^{- 10} (very weak acids like hydrocyanic acid and phenol) to 10⁹ (very strong acids like hydrochloric and sulphuric acids), and of [Acid] and [Base].