Buffer solution
A buffer is a substance mixture, the pH (concentration of oxonium ions ) is much less pronounced changes upon addition of an acid or a base, as would be the case in an unbuffered system. The effect of the buffer is based on the conversion of supplied by the acid or base hydronium ( H3O ) and hydroxide ions (OH - ) to weak acids and bases, which itself is little formation of H3O or OH - ions tend.
Buffers are in the chemistry specifically prepared aqueous buffer solutions. More complex buffer systems are in body fluids such as blood or in ground waters that are, for example, with humus interacted before.
Chemical basis of a buffer solution
Buffer solutions containing a mixture of a weak acid and its conjugate base, or the corresponding (or of the respective salt ) or a weak base and its conjugate acid or corresponding. Also ampholytes ( bifunctional molecules ) can serve as a buffer. The pH -determining factor is the ratio or the protolysis equilibrium of the buffer pair.
For the acid - base equilibrium of an acid HA is true:
According to the law of mass action would be in the denominator in addition, the concentration of the water. However, this can be because it is very large compared to the ion concentrations by 55.6 mol / l are considered to be constant and is defined as the dissociation constant Ks ( the acid constant ) involved.
Rearranging we obtain:
If one forms thereof the negative logarithm, one obtains:
This corresponds to:
Respectively:
With this equation - it is valid under the approximation that the activities of the substances corresponding to their concentrations in solution - can be given a known pKa value for a given pH, the concentration ratio of acid and base determined. The higher the concentration, the lower to additions of acids or bases affect. The amount of the strong base (or acid ), can be absorbed by a buffer solution with no substantial change of the pH value, is expressed by the buffer capacity.
Examples of buffer solutions of the acetic acid / ammonium acetate buffer, or buffer containing ammonium ions and ammonia.
Types of buffer systems
The carbonate buffer ( a mixture of carbonic acid and bicarbonates ) regulates the concentration of CO2 between the atmosphere, oceans and the biosphere. It is also the main part of the blood buffer. This keeps the pH of the blood pH between 7.35 and 7.45 and balances the metabolism caused by the fluctuations. At a pH - value below 7.35 is called acidosis, about 7.45 of an alkalosis. Death occurs at pH values below 6.8 or above 8.0.
Considering a buffer system, it must be distinguished between closed and open buffer systems. In a closed system, buffer ( e.g., acetic acid / acetate - buffer), the resulting chemical reaction at a proton (H ) or hydroxide ions (OH - ) can be intercepted by the buffer substance. They react to form the corresponding and the corresponding conjugate acid or base of the buffer and thus remain in the solution. With an open buffer system (eg the Hydrogencarbonat-/CO2-Puffersystem in the lungs), the system is in exchange with the environment. It is to maintain the proper pH in the situation by issuing a component of the environment, eg, by exhaling CO2.
Importance of buffer systems
Buffer systems have an important role in industrial chemistry, such as in electroplating or in photography, as well as in analysis.
Also play buffer systems in soil science an important role; refer buffer area ( soil science ).
Importance in the life sciences: buffers are essential for many animals and not least for the human organism. Thus, the human blood cells and many enzymes are dependent on a constant pH. Without buffer already would smallest amounts of acid - eg lactic acid from the energy metabolism - enough to paralyze the organism, because various proteins denature and therefore would be useless.
Some examples of buffer
- Carbonic acid -bicarbonate system (pH 6.2 to 8.6; neutral)
- Carbonate -silicate buffer ( pH 5.0 to 6:2; mildly acidic )
- Acetic acid -acetate buffer (pH 3.7 to 5.7 )
- Citric acid or citrate
- HEPES: 4 - (2-hydroxyethyl )-1- piperazinethanesulfonsäure (pH 6.8 to 8.2 )
- HEPPS: 4 - (2-hydroxyethyl ) piperazine - 1-propanesulfonic acid (pH 7.3 to 8.7 )
- MES: 2 - (N- morpholino) ethanesulfonic acid (pH 5.2 to 6.7 )