Acid–base titration
Acid -base titration is a titrimetric method for determining the concentration of acids or bases in a solution. In place of the preamble acid -base titration to determine the concentration of an acid using a base is also called alkalimetry. Similarly, the determination of the concentration of a base, using an acid is also referred to as acidimetry. [Note 1]
The determination is made by titration with a standard solution suitable. For alkalimetry a basic (alkaline ) solution ( 0.1 M sodium hydroxide solution often ), for acidimetry an acidic solution ( 0.1 M hydrochloric acid often ) is used as a standard solution. During the titration, the pH of the sample solution moves by neutralization toward a neutral solution because H3O or be converted to H2O OH. The endpoint of the titration, the equivalence point and is indicated by appropriate indicators or a pH electrode. The equivalence point depends on the anions ( and cations ) and is characterized by a significant change in pH at low acid or base addition. Lying strong acids (eg, HCl, HNO3, H2SO4 ) and bases (eg, NaOH, KOH) are present, the equivalence point is at pH = 7, when other anions are present ( eg, phosphate, carbonate, acetate) as the equivalence points can also be in other pH ranges. If, instead of the pH electrode, a color indicator is used, the dye must be selected appropriate for the equivalence point concerned.
Course of the titration curves
Titration curves of aqueous solutions of strong acids and very very strong bases all have a similar course. In the reaction of oxonium and hydroxide are reacted quantitatively to water:
They are the only proton donors or proton acceptors in such aqueous solutions. Cause is the leveling of very strong acids and bases. In the case of hydrochloric acid hydrogen chloride is the very strong acid which has been hydrolyzed (formal or real):
In the case of sodium hydroxide is the very strong base sodium hydroxide, which was completely hydrolyzed upon reaction with water:
Measured variables are the volume of the sample solution, each of which added volume of standard solution and the respective pH value of the solution. In the acidic range of the pH of the sample solution is prepared by
In the basic region of the pH - value is
The autoionization of water
Is negligible in almost all areas, however, the pH at the equivalence point determined at pH 7 at 25 ° C.
Titration curves of aqueous solutions of strong acids and medium medium strong bases show up to the equivalence point a different course, since the dissolved acids or bases are not completely hydrolyzed. In addition to implementing
Takes place at the alkalimetry
Or in the acidimetry
The particles referred to in the last two reactions as an acid and base are each the conjugate acid -base pairs in Figure 1, there are acetic acid and the acetate ions, in Figure 2, the ammonium ions and ammonia. The profiles of the titrations can be estimated mathematically with known concentration and volume of the sample solution and the titrant. In the titration of moderately strong acids or bases can (apart from starting point), the proteolysis of acetic acid or ammonia can be neglected with water and a quantitative conversion of the investigated acid or base with OH - or H3O will be accepted. The pH of each solution is determined by the present concentrations of the acid -base pairs and is determined by the Henderson -Hasselbalch equation
Described. Proton in the case of the determination of acetic acid, the acetic acid itself, with a pKa of 4.75, and in the case of determination of the ammonia, the ammonium ion, with a pKa value of 9.25. In a 50 - % conversion the respective proton donors and acceptors are in the same concentration and the pH is equal to the respective pKa:
This point is sometimes called half-equivalence point. Around that point, the change in pH during the titration proceeds extremely flat, since buffers are present. From the equivalence point the pH curve is determined only by the further addition of the titrant.
Choice of the indicator
The color change of a suitable indicator should be in the range of the equivalence point (vertical course of a titration curve ).
The envelope range of pH indicators has the width of two pH units in general. Also in the indicators is an acid -base reaction steps:
The indicators follow the Henderson-Hasselbalch equation, and an indicator having a pK a value. Because of their low concentration, however, the course of the titration curves by indicators remains largely unaffected.
Since it is used to produce a very accurate primary standard solution for Säuremaßlösungen often sodium carbonate ( anhydrous, by drying in an oven at 200 ° C), methyl orange is a very important color indicator for precise adjustment of acids.
For the titration of a strong acid with a strong base, such as hydrochloric acid and sodium hydroxide to the indicator is bromothymol blue as its color turns approximately at a pH value of 6.0 to 7.6, which is within the range of the equivalence point. In contrast, if the concentration of a moderately strong acid, such as acetic acid by means of sodium hydroxide solution to be determined, is used as the indicator phenolphthalein, the cover of which range from colorless to red - purple in the pH range of 8.2 through 10. Methyl, with a transition range of pH 4.4 to 6.2 is suitable for a provision of moderate base such as ammonia.
One can thus determine the end point of titration with the aid of a pH meter, with an electrical measuring device. This method of measurement yields a clear result, which does not depend on the experience of the performer. The gradually added standard solution and the respective pH of the solution can be represented and analyzed in a titration curve.
Especially as the polybasic acids have a so-called buffer capacity, in which the pH during the titration is a relatively long time constant, and the neutral point can be achieved quite suddenly, this behavior can be observed with a pH meter better. There is no need then of course the use of an indicator.
Automatic titration
A development of the titration using the pH meter will cause not only the pH - value is recorded electronically by a computer, but also the addition of the titration liquid can be controlled automatically. Furthermore, the device connected to the computer titration apparatus further process the results of the same and convert, for example, in a concentration value. Thus, the titration can be fully automated.