- characteristics
- Composition
- They neutralize both acids and bases
- Efficiency
- Preparation
- Examples
- References
The buffer solutions or buffers are those that can decrease changes in pH due to ions H 3 O + and OH -. In the absence of these, some systems (such as physiological ones) are harmed, as their components are very sensitive to sudden changes in pH.
Just as shock absorbers in automobiles reduce the impact caused by their movement, buffers do the same but with the acidity or basicity of the solution. Furthermore, buffers establish a specific pH range within which they are efficient.
Otherwise, the H 3 O + ions will acidify the solution (the pH drops to values below 6), resulting in a possible alteration in the reaction performance. The same example can be applied for basic pH values, that is, greater than 7.
characteristics
Composition
They are essentially composed of an acid (HA) or a weak base (B), and salts of their conjugated base or acid. Consequently, there are two types: acid buffers and alkaline buffers.
Acid buffers correspond to the HA / A - pair, where A - is the conjugate base of the weak acid HA and interacts with ions - such as Na + - to form sodium salts. Being this way, the pair remains as HA / NaA, although they can also be potassium or calcium salts.
Derived from the weak acid HA, it buffers acidic pH ranges (less than 7) according to the following equation:
HA + OH - => A - + H 2 O
However, being a weak acid, its conjugate base is partially hydrolyzed to regenerate part of the HA consumed:
A - + H 2 O <=> HA + OH -
On the other hand, alkaline buffers consist of the B / HB + pair, where HB + is the conjugated acid of the weak base. Generally, HB + forms salts with chloride ions, leaving the pair as B / HBCl. These buffers buffer basic pH ranges (greater than 7):
B + H 3 O + => HB + + H 2 O
And again, HB + can be partially hydrolyzed to regenerate some of the B consumed:
HB + + H 2 O <=> B + H 3 O +
They neutralize both acids and bases
While acidic buffers buffer acidic pH and alkaline buffer pH basic, both can react with H 3 O + and OH - ions through these series of chemical equations:
A - + H 3 O + => HA + H 2 O
HB + + OH - => B + H 2 O
Thus, in the case of the HA / A - pair, HA reacts with the OH - ions, while A - -its conjugated base- reacts with the H 3 O +. As for the B / HB + pair, B reacts with the H 3 O + ions, while HB + - its conjugated acid - with the OH -.
This allows both buffers to neutralize both acidic and basic species. The result of the above compared to, for example, the constant addition of moles of OH -, is the decrease in the variation of pH (ΔpH):
The image above shows the pH buffer against a strong base (OH - donor).
Initially the pH is acidic due to the presence of HA. When the strong base is added, the first moles of A are formed - and the buffer begins to take effect.
However, there is an area of the curve where the slope is less steep; that is, where the damping is more efficient (bluish box).
Efficiency
There are several ways to understand the concept of damping efficiency. One of these is to determine the second derivative of the curve pH versus the volume of base, solving for V for the minimum value, which is Veq / 2.
Veq is the volume at the equivalence point; This is the volume of base needed to neutralize all the acid.
Another way to understand it is through the famous Henderson-Hasselbalch equation:
pH = pK a + log (/)
Here B denotes the base, A the acid, and pK a is the smallest logarithm of the acid constant. This equation applies both for the acidic species HA, and for the conjugated acid HB +.
If it is very large with respect to, the log () takes a very negative value, which is subtracted from the pK a. If, on the other hand, it is very small with respect to, the value of log () takes a very positive value, which is added to pK a. However, when =, the log () is 0 and the pH = pK a.
What does all of the above mean? That the ΔpH will be greater in the extremes considered for the equation, while it will be minimum with a pH equal to the pK a; and as the pK a is characteristic of each acid, this value determines the range pK a ± 1.
The pH values within this range are those in which the buffer is most efficient.
Preparation
To prepare a buffer solution, the following steps should be kept in mind:
- Know the required pH and, therefore, the one you want to keep as constant as possible during the reaction or process.
- Knowing the pH, one looks for among all the weak acids, those whose pK a is closer to this value.
- Once the HA species has been chosen and the concentration of the buffer calculated (depending on how much base or acid needs to be neutralized), the necessary amount of its sodium salt is weighed.
Examples
Acetic acid has a pK a of 4.75, CH 3 COOH; therefore, a mixture of determined amounts of this acid and sodium acetate, CH 3 COONa, form a buffer that efficiently buffers in the pH range (3.75-5.75).
Other examples of monoprotic acids are benzoic (C 6 H 5 COOH) and formic (HCOOH) acids. For each of these values of pK a are 4.18 and 3.68; therefore, its pH ranges with the highest buffering are (3.18-5.18) and (2.68-4.68).
Furthermore, the polyprotic acids such as phosphoric acid (H 3 PO 4) and carbon (H 2 CO 3) have many pK values to as proton can be released. Thus, H 3 PO 4 has three pK a (2.12, 7.21 and 12.67) and H 2 CO 3 has two (6.352 and 10.329).
If you want to maintain a pH of 3 in a solution, you can choose between the buffers HCOONa / HCOOH (pK a = 3.68) and NaH 2 PO 4 / H 3 PO 4 (pK a = 2.12).
The first buffer, that of formic acid, is closer to pH 3 than the phosphoric acid buffer; therefore, HCOONa / HCOOH buffers better at pH 3 than NaH 2 PO 4 / H 3 PO 4.
References
- Day, R., & Underwood, A. Quantitative Analytical Chemistry (5th ed.). PEARSON Prentice Hall, p 188-194.
- Avsar Aras. (April 20, 2013). Mini Shocks. Retrieved on May 9, 2018, from: commons.wikimedia.org
- Wikipedia. (2018). Buffer solution. Retrieved on May 9, 2018, from: en.wikipedia.org
- Assoc. Prof. Lubomir Makedonski, PhD.. Buffer solutions. Medical University of Varna.
- Chem Collective. Buffer tutorials. Retrieved on May 9, 2018, from: chemcollective.org
- askIITians. (2018). Buffer Solution. Retrieved on May 9, 2018, from: askiitians.com
- Quimicas.net (2018). Examples of Buffer, Buffer or Buffer Solutions. Retrieved on May 9, 2018, from: quimicas.net