Are titration and neutralization the same process



 

The goal of one Acid-base titration is to determine the molar concentration c in mol / l of an acid or base of unknown concentration. One uses the idea that an acidic and a basic solution completely neutralize each other at a certain proportion. The pH of the solution approaches 7.

If an acid is titrated with a base of known concentration (standard solution), this process is also called Alkalimetry. In the opposite case, titration of a base with an acid, one speaks of Acidimetry.

Example: Sodium hydroxide solution with a known concentration is added dropwise from a burette to hydrochloric acid of unknown concentration. Based on the amount of hydrochloric acid solution used and the amount of sodium hydroxide solution used, the concentration of hydrochloric acid can be determined with the aid of the reaction equation.

There are various options for determining the equivalence point (complete neutralization of acid and base) of the titration.

Titration with an indicator

Titration of a monoprotonic acid, using the example of hydrochloric acid with sodium hydroxide solution and methyl orange as an indicator

25 ml of the hydrochloric acid solution are poured into a 100 ml Erlenmeyer flask (usually a wide-necked Erlenmeyer flask) with a pipette. The solution is diluted to 100 ml with distilled water. 2-3 drops of the indicator methyl orange are added. A burette is filled with caustic soda with a concentration of c = 1 mol / l to just above the top mark. Burettes often have a blue stripe on the back (Schellbach stripe)[1][2]. The fluid level can be read at the point where it tapers. The burette must be set exactly to 0 ml (allow the excess to drain into a beaker). A stir bar can be used to ensure that the solution is mixed.

The Erlenmeyer flask with the solution of unknown concentration is placed under the burette on a white sheet in order to notice a color change in good time. Now the sodium hydroxide solution is allowed to run slowly, drop by drop, into the acid solution. After some time, the indicator initially only changes color to the point where the drops are immersed.

Towards the end of the titration, the color change of the methyl orange lasts longer and longer. If this is the case, the sodium hydroxide solution is only added in individual drops. The pH value is reached when the indicator shows its full color change. This means that the equivalence point and thus the end of the titration has been reached.

You can now read the consumption of the sodium hydroxide solution on the burette and determine the concentration of the hydrochloric acid solution using the following calculation.

The amount of substance n of the sodium hydroxide solution used results from the molarity c and the volume V.

The reaction equation for neutralization is:

It follows from this: Hydrochloric acid reacts with sodium hydroxide solution in a molar ratio of one to one:

The concentration of hydrochloric acid can now be calculated from the known amount of substance and the volume used:

Choice of indicator

The choice of indicator depends on the acid and base used. The color change of the indicator should always be as close as possible to the equivalence point. For the example of the titration of hydrochloric acid with sodium hydroxide solution (strong acid with strong base) the indicator bromothymol blue is suitable, as its color changes at a pH value of 7, which corresponds exactly to the equivalence point of the neutralization reaction of hydrochloric acid with sodium hydroxide solution.

If, on the other hand, the concentration of weak acids such as acetic acid and formic acid is to be determined with the aid of sodium hydroxide solution, then the indicator phenolphthalein, for example, whose transition point from colorless to red-purple is around the pH value of 10, is used. This is necessary because a solution of equal amounts of acetic acid and caustic soda has a slightly alkaline reaction.

Accuracy of titration with indicator

There are some uncertainties in determining the endpoint. In particular, the determination of the changeover point of the indicator results in great inaccuracy, depending on the color change (abrupt or very creeping, clearly different colors or similar colors ...) of the indicator.

By cleverly choosing the appropriate indicator or a combination of several indicators, however, the concentration can usually be determined very precisely.

Experience also plays a major role. There are / were definitely methods of determining a "personal factor" with which the analysis value was corrected.

Titration with a pH electrode

The end point of the titration can also be determined with the aid of a pH electrode, i.e. with an electrical measuring device. This measuring method gives a clear result that does not depend on the experience of the person carrying out the work. There is of course no need to use an indicator.

Since the polyvalent acids in particular have so-called buffer capacities, in which the pH value remains constant for a relatively long time during the titration and the neutral point can be reached quite suddenly, this behavior can be better observed with a pH device.

Automatic titration

A further development of the titration with the pH meter means that not only the pH value is recorded electronically by a computer, but also the addition of the titration liquid can be regulated automatically. Furthermore, the computer connected to the titration apparatus can immediately process the results and B. convert into a concentration value.

This means that the titration can be fully automated.

See also:Laboratory automation

swell

  1. Schellbach burette after the German mathematician and physicist K. H. Schellbach: Duden, The dictionary of chemical technical expressions: The key to chemical technical language; for school, studies and work / Otto-Albrecht Neumüller. Edited by the editorial office of Studium und Beruf. Mannheim; Leipzig; Vienna; Zurich: Dudenverlag c 2003,ISBN 3-411-04171-4 P. 608.
  2. Schellbach, Karl Heinrich: born December 25, 1804, Eisleben, died May 29, 1892, Berlin: Lexicon of important mathematicians / ed. by Siegfried Gottwald ... (authors Günter Asser ...) Thun et al .: German, 1991, references, ISBN 3-8171-1164-9, P. 412.

Categories: Chemical Analysis Process | Acid-base reaction