Buffer solutions resist changes in pH. When a small amount of base or acid is added to a solution containing a buffer, it tries to minimize the change in pH as much as possible. For example, the pH of pure water is 7.0. When an acid or base gets added to water, it turns into an acidic or a basic solution. However, water + buffer solutions have very little change in their pH upon the addition of acid or base.
Buffers come into play when due to a change in the acidity of a solution, a reaction stops or ceases to occur. Many of the biochemical processes taking place in living organisms use buffers to maintain their internal conditions and environment for life processes to go on.
Types of Buffer Solutions
There are two main types of buffer solutions:
- Acidic buffers are prepared from a weak acid and its salt with a strong base.
- Basic / Alkaline buffers are prepared from a weak base and its salt with a strong acid.
Acidic Buffers
A buffer solution containing a large amount of weak acid and its salt with a strong base is termed acidic buffer. It has acidic pH (less than 7 at 298 K). However, the range of pH value of the acidic buffers varies to some extent with the change in the ratio of the two compounds.
The pH of an acid buffer is given by Henderson-Hasselbalch equation.
pH = pKa + log ([salt] / [acid])
Base and acid are conjugated with each other, where pKa is the negative log of the acid ionization constant of the weak acid.
Basic/Alkaline Buffers
A buffer solution containing a relatively large amount of weak base and its salt with a strong acid is known as a basic buffer. Alkaline buffers have a pH greater than 7 at 298 K. The pH of the basic buffer varies over the shrink range by changing the ratio of the two compounds.
This type of buffer is also worked on by the Henderson-Hasselbalch equation.
pH = pKa + log ([salt] / [acid])
Working Principle of Buffers
Buffers follow the Le-Chatelier principle. It states that “the position of equilibrium will move in such a way as to counteract the change”. This means that, upon adding either a base or an acid to a buffer solution, equilibrium will behave in such a way, to minimize the effect of added acid or base as much as possible.
For example
If sodium acetate (CH3COONa) is added to acetic acid (CH3COOH) solution, it does not react directly with acetic acid or with water. However, the increase of acetate ions from (CH3COONa) suppresses the ionization of acetic acid.
The following reactions sum up to show a common ion, that is acetate ion (CH3COO–);
The mixture of CH3COOH and CH3COONa, in specific molar ratios in water, makes a buffer solution. The acidity of this solution is less than an acetic acid solution because the acetate ions hinder the ionization of acetic acid.
Now, consider a buffered solution containing a relatively large amount of weak acid HA and its conjugate base A–. The base is obtained from its salt i.e. Na-A. When hydroxide (OH–) ions are added to the solution, they react with acid (HA) readily to form water, a neutral molecule. This is how a change in pH of the solution is resisted by the buffer solutions, upon the addition of a base.
The following reaction summarizes the whole above paragraph;
where,
The equilibrium expression for the dissociation of acid HA is:
From the concentration of H+ ions, we can calculate pH as;
pH = – log10 [H+]
Additional topics
Buffer Equilibrium examples
As a matter of fact, acids, especially weak organic acids are buffers themselves. This is because they self ionize into conjugate bases that live in equilibrium with parent acids.
- Hydrofluoric acid exists in equilibrium between HF and its conjugate base (F–). It becomes a buffer of pH 3.20.
- Acetic acid has an equilibrium established between acetic acid and acetate ion (its conjugate base). This buffer has a pH of 4.76.
- Carbonic acid exists in an equilibrium state between carbonic acid (H2CO3) and bicarbonate ion (HCO3–). This buffer has a pH of 6.35.
- Phosphoric acid (ion), in an equilibrium of H2PO4– / HPO42- has a pH of 7.2.
- Ammonia, when dissolved in water, forms a buffer situation. This is because of the NH4+/ OH– system. It has a pH of 9.4.
- (C2H5NH3+ / C2H5NH2) buffer equilibrium pH is about 10.70.
Calculation of pH of a buffer solution
A buffered solution contains 0.25M NH3 (Kb = 1.8 x 10-5) and 0.40M NH4Cl. Calculate the pH of this solution.
The major species in the given solution are:
- Ammonia (NH3)
- Ammonium cation (NH4+)
- Chloride ion (Cl–)
- Water (H2O)
Here the Cl– ion is a weak base and water can behave as acid as well as base. The calculations are as follows:
Kb = [NH4+] [OH–] / [NH3]
Rearranging
[OH–] = Kb [NH3] / [NH4+] —> (i)
As, Kb = 1.8 x 10-5
-log Kb = -log (1.85x 10-5)
pKb = 4.75
By taking negative of log on both sides of equation (i)
-log [OH–] = -log Kb – log [NH3] / [NH4+]
pOH = pKb [NH3] / [NH4+]
pOH = pKb + log [salt] / [base] —> (ii)
Putting values in equation (ii) from the given data;
pOH = 4.75 + log 0.4M/0.25M
pOH = 4.75 + 0.25
pOH = 4.95
pH = 14 – pOH
= 14 – 4.95
pH = 9.05
Therefore, the pH of the solution is 9.05.
Henderson-hasselbalch equation
The Henderson/Hasselbalch equation is used to work out, pH, pOH, pKa, pKb, etc.
pH = pKa + log ([salt] / [acid])
The form of acid dissociation equilibrium is,
[H+] = Ka [HA]/[A–]
A– is the conjugate base of acid HA. However, a major part of A– in the buffer solution is from Na+and A–. The concentration of A– can be written as the concentration of salt.
By taking a negative log;
-log [H+] = -log Ka – log [HA]/[A–]
Hence,
-log [H+] = pH -log Ka = pka
pH = pka – log [HA]/[A–]
Hence, inverting the long term, reverses the sign.
pH = pka + log [A–]/[HA]
pH = pKa + log ([salt] / [acid])
The above equation is the Henderson-Hasselbalch equation.
Applications of Buffer Solutions
Some of the most useful applications of buffer solutions are given below:
- Buffers are used in chemical analysis and calibration of measurement instruments.
- The pH resistance by buffers is useful in chemical manufacturing and essential for many biochemical processes.
- Naturally, buffers are present in living organisms to maintain the pH in a specific range. For example, bicarbonate buffers maintain the pH of our blood between 7.35 to 7.45.
- In industry, fermentation is a common process that uses buffers.
- Cultural media for the growth of bacteria requires maintaining pH by buffer solutions.
- In the medicinal industry, buffers solutions are useful not only to maintain the specific pH values but also to ensure the stability of drug components.
- Buffers are important in dyeing processes. Hence, it is required by the textile industry.
- The flavors and appearances of the processed foods are maintained by buffers i.e. by keeping a specific pH level.
Concepts Berg
What are buffer grades?
Buffers are graded on the basis of their quality and purity. While selecting a buffer, one must check its grade mentioned on it. For example, when we are using a buffer for culture in the lab, we need a lab-grade buffer. Similarly, buffers are being used in medicines that are pharmaceutical grade.
What is a buffer composed of?
Buffers are solutions composed of a weak acid and a salt of that acid with a strong base or a weak base and a salt of that base with a strong acid.
What are the types of buffer solutions?
There are mainly two types of buffer solutions:
- Acidic buffer solutions
- Basic/Alkaline buffer solutions
How do buffer solutions work?
Buffer solutions work on the Le Chatelier principle.
“When a small amount of an acid or a base is added to a solution, the chemical reaction moves in such a way, as to minimize the effect as far as possible. Hence, pH remains the same as the reaction re-establishes its equilibrium”.
How to differ between buffer solutions?
pH value can help distinguish between an acidic and basic buffer solution. As pH 0-7 is acidic, whereas 7-14 is basic in nature.
Why are buffers used?
Buffers are used to maintain the pH of solutions. The pH is a very important factor that may affect reaction kinetics and the yield of chemical reactions.
For example, the total hardness of water is determined by the EDTA method. EDTA binds with Ca2+ and Mg2+ only at pH 12.
What makes an effective buffer?
An effective buffer contains an equivalent amount of both acid and salt. The salt used should be highly soluble (high solubility product) so that it can readily ionize to give a conjugate base in the solution.
What is buffer capacity?
Buffer capacity is a quantitative measure of the resistance to changing the pH of the buffer solution.
What is the general use of a buffer solution?
The most general use of buffer solutions is to maintain the pH of a solution. For example, a buffer solution is used in packed milk to maintain its nutritional value or (pH).
What are some real-life applications of buffer solutions?
- Photographic materials
- Leathers
- Dyes
- Food preservation chemicals
- Calibration of pH meters
- Maintaining the pH of culture media for the growth of bacteria
- The pH of soil, etc
What are some buffer solutions in daily life?
Bicarbonates and phosphate buffer solutions are mostly used in daily life. Although, if we talk about living beings, hemoglobin and proteins are buffers too.
What is a buffer action?
When an acid or base is added to the buffer solution, as a result, it affects the equilibrium of the solution. The system behaves in such a way to minimize this effect. This is known as buffer action.
Why are buffers important to cells?
The phosphate buffers maintain the pH in cells, as a change in pH can result in ruptured cells. There are many more examples.
What are the biological functions of a buffer?
Buffers have an important role in maintaining pH in human blood, protein, hemoglobin, etc.
How are neutral buffers made?
Neutral buffers can be made by taking their pH value near 7.
Which set of compounds would form a buffer in an aqueous solution?
Chemical species that can make a buffer solution in an aqueous medium are;
- HCN and NaCN
- Acetic acid / Acetate ion
- Phosphoric acid / Phosphate
- Carbonic acid / Carbonate, etc
What is the role of a buffer in a pharmacy?
In pharmaceutical fields, buffers can be used to;
- Enhance the quality and stability of specific components.
- Improve the biological activity and solubility of the drugs.
- The purity of medicines, etc.
What is a nontoxic buffer I can use to keep sugar solution around neutral pH?
We can use buffers for this purpose whose pH value is near 7. These buffer solutions are basically nontoxic. For example, carbonic acid, etc.
What is the purpose of lysis buffer in DNA extraction?
In order to extract DNA, cells are first broken down. There are two types of lysis buffers used for this purpose. Cell and nuclear lysis buffers. Cell lysis buffer breaks the membrane of the cell. Nuclear lysis buffer is used to break the nucleus of the cell.
What’s the role of salt in a buffer solution?
Salt acts as a source of the base (conjugate base) in the buffer solutions.
Why does temperature decrease buffer capacity?
Temperature is directly proportional to the dissociation of ions. So, the higher the temperature, the more the dissociation of ions. Hence, the numbers of undissociated ions become less resulting in decreased buffer capacity.
Can an acidic buffer solution have a pH of more than 7?
No. Acidic buffers have a pH of less than 7 and basic buffers have more than 7.
Can you make a buffer solution by adding NaOH to an ammonia solution?
Yes. A buffer can be made by adding NaOH to an ammonia solution.
Can you store a buffer solution for extended periods?
The buffer solution can be stored for a long time provided that such conditions are given in which the pH and temperature of the solution do not change.
References
- Chemistry | Fifth edition, by Steven S. Zumdhal and Susan A. Zumdhal (University of Illinois, Urbana Champaign, IL, USA)
- Buffer Solutions (chemguide.co.uk)
- Buffers (opentextbc.ca)
- Introduction to buffers (chem.libretext.org)
