Buffer solution
Overview
 
For an individual weak acid or weak base component, see Buffering agent
Buffering agent
A buffering agent is a weak acid or base used to maintain the acidity of a solution at a chosen value. The function of a buffering agent is to prevent a rapid change in pH when acids or bases are added to the solution. Buffering agents have variable properties—some are more soluble than others;...

. For uses not related to acid-base chemistry, see Buffer (disambiguation).


A buffer solution is an aqueous solution
Aqueous solution
An aqueous solution is a solution in which the solvent is water. It is usually shown in chemical equations by appending aq to the relevant formula, such as NaCl. The word aqueous means pertaining to, related to, similar to, or dissolved in water...

 consisting of a mixture of a weak acid
Weak acid
A weak acid is an acid that dissociates incompletely. It does not release all of its hydrogens in a solution, donating only a partial amount of its protons to the solution...

 and its conjugate base or a weak base
Weak base
In chemistry, a weak base is a chemical base that does not ionize fully in an aqueous solution. As Brønsted–Lowry bases are proton acceptors, a weak base may also be defined as a chemical base in which protonation is incomplete. This results in a relatively low pH compared to strong bases...

 and its conjugate acid
Conjugate acid
Within the Brønsted–Lowry acid-base theory , a conjugate acid is the acid member, HX, of a pair of two compounds that transform into each other by gain or loss of a proton. A conjugate acid can also be seen as the chemical substance that releases, or donates, a proton in the forward chemical...

. It has the property that the pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...

 of the solution changes very little when a small amount of strong acid
Strong acid
A strong acid is an acid that ionizes completely in an aqueous solution by losing one proton, according to the equationFor sulfuric acid which is diprotic, the "strong acid" designation refers only to dissociation of the first protonMore precisely, the acid must be stronger in aqueous solution than...

 or base is added to it.
Unanswered Questions
Encyclopedia
For an individual weak acid or weak base component, see Buffering agent
Buffering agent
A buffering agent is a weak acid or base used to maintain the acidity of a solution at a chosen value. The function of a buffering agent is to prevent a rapid change in pH when acids or bases are added to the solution. Buffering agents have variable properties—some are more soluble than others;...

. For uses not related to acid-base chemistry, see Buffer (disambiguation).


A buffer solution is an aqueous solution
Aqueous solution
An aqueous solution is a solution in which the solvent is water. It is usually shown in chemical equations by appending aq to the relevant formula, such as NaCl. The word aqueous means pertaining to, related to, similar to, or dissolved in water...

 consisting of a mixture of a weak acid
Weak acid
A weak acid is an acid that dissociates incompletely. It does not release all of its hydrogens in a solution, donating only a partial amount of its protons to the solution...

 and its conjugate base or a weak base
Weak base
In chemistry, a weak base is a chemical base that does not ionize fully in an aqueous solution. As Brønsted–Lowry bases are proton acceptors, a weak base may also be defined as a chemical base in which protonation is incomplete. This results in a relatively low pH compared to strong bases...

 and its conjugate acid
Conjugate acid
Within the Brønsted–Lowry acid-base theory , a conjugate acid is the acid member, HX, of a pair of two compounds that transform into each other by gain or loss of a proton. A conjugate acid can also be seen as the chemical substance that releases, or donates, a proton in the forward chemical...

. It has the property that the pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...

 of the solution changes very little when a small amount of strong acid
Strong acid
A strong acid is an acid that ionizes completely in an aqueous solution by losing one proton, according to the equationFor sulfuric acid which is diprotic, the "strong acid" designation refers only to dissociation of the first protonMore precisely, the acid must be stronger in aqueous solution than...

 or base is added to it. Buffer solutions are used as a means of keeping pH at a nearly constant value in a wide variety of chemical applications. Many life forms thrive only in a relatively small pH range; an example of a buffer solution is blood
Blood
Blood is a specialized bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells....

.

Principles of Buffering

Buffer solutions achieve their resistance to pH change because of the presence of an equilibrium between the acid HA and its conjugate base A-.
HA H+ + A-

When some strong acid
Strong acid
A strong acid is an acid that ionizes completely in an aqueous solution by losing one proton, according to the equationFor sulfuric acid which is diprotic, the "strong acid" designation refers only to dissociation of the first protonMore precisely, the acid must be stronger in aqueous solution than...

 is added to an equilibrium mixture of the weak acid
Weak acid
A weak acid is an acid that dissociates incompletely. It does not release all of its hydrogens in a solution, donating only a partial amount of its protons to the solution...

 and its conjugate base, the equilibrium is shifted to the left, in accordance with Le Chatelier's principle
Le Châtelier's principle
In chemistry, Le Chatelier's principle, also called the Chatelier's principle, can be used to predict the effect of a change in conditions on a chemical equilibrium. The principle is named after Henry Louis Le Chatelier and sometimes Karl Ferdinand Braun who discovered it independently...

. Because of this, the hydrogen ion concentration increases by less than the amount expected for the quantity of strong acid added. Similarly, if strong alkali is added to the mixture the hydrogen ion concentration decreases by less than the amount expected for the quantity of alkali added.

The effect is illustrated by the simulated titration of a weak acid with pKa = 4.7. The relative concentration of undissociated acid is shown in blue and of its conjugate base in red. The pH changes relatively slowly in the buffer region, pH = pKa ± 1, centered at pH = 4.7 where [HA] = [A-], but once the acid is more than 95% deprotonated the pH rises much more rapidly.

Buffer capacity

Buffer capacity, β, is a quantitative measure of the resistance of a buffer solution to pH change on addition of hydroxide ions. It can be defined as follows.


where dn is an infinitesimal amount of added base and d(p[H+]) is the resulting infinitesimal change in the cologarithm
Cologarithm
In mathematics, the base-b cologarithm, sometimes shortened to colog, of a number is the base-b logarithm of the reciprocal of the number...

 of the hydrogen ion concentration. With this definition the buffer capacity of a weak acid, with a dissociation constant Ka, can be expressed as


where CA is the analytical concentration of the acid. pH is approximately equal to -log10[H+].

There are three regions of high buffer capacity.
  • At very low p[H+] the first term predominates and β increases in proportion to the hydrogen ion concentration. This is independent of the presence or absence of buffering agents and applies to all solvents.
  • In the region p[H+] = pKa ± 2 the second term becomes important and β rises to a maximum at p[H+] = pKa. Buffer capacity is proportional to the concentration of the buffering agent, CA, so dilute solutions have little buffer capacity. It is also proportional to the acid dissociation constant, Ka (not pKa); the weaker the acid the greater its buffering capacity.
  • At very high p[H+] the third term predominates and β increases in proportion to the hydroxide ion concentration. This is due to the self-ionization of water
    Self-ionization of water
    The self-ionization of water is the chemical reaction in which a proton is transferred from one water molecule to another, in pure water or an aqueous solution, to create the two ions, hydronium, H3O+ and hydroxide, OH−...

     and is independent of the presence or absence of buffering agents.

Calculating buffer pH (monoprotic acid)

First write down the equilibrium expression.
HA A- + H+

The initial, change and equilibrium concentrations of these three components can be organized in an ICE table
ICE table
An ICE table, ICE chart, or ICE box is a tabular system of keeping track of changing concentrations in an equilibrium reaction. ICE stands for "Initial, Change, Equilibrium". It is used in chemistry to keep track of the changes in amount of substance of the reactants and also organize a set of...

.
ICE table for a monoprotic acid
[HA] [A-] [H+]
I C0 0 0
C
x x
E C0-x x x

The first row, labelled I, lists the initial conditions: the concentration of acid is C0, initially undissociated, so the concentrations of A- and H+ are zero. The second row, labelled C for change, specifies the changes that occur when the acid dissociates. The acid concentration decreases by an amount -x and the concentrations of A- and H+ both increase by an amount +x. This follows from the equilibrium expression. The third row, labelled E for equilibrium concentrations, adds together the first two rows and shows the concentrations at equilibrium.

To find x, use the formula for the equilibrium constant:

Substitute the concentrations with the values found in the last row of the ICE table:

Simplify to:

With specific values for C0 and Ka this equation can be solved for x. Assuming that pH = -log10[H+] the pH can be calculated as pH = -log10x.

Note. When a pH meter is calibrated using known buffers, the reading gives the hydrogen ion activity
Activity (chemistry)
In chemical thermodynamics, activity is a measure of the “effective concentration” of a species in a mixture, meaning that the species' chemical potential depends on the activity of a real solution in the same way that it would depend on concentration for an ideal solution.By convention, activity...

 rather than its concentration. In this case the meter reading may differ from the value calculated as above. For example, calculation of pH of phosphate-buffered saline would give the value of 7.96, whereas the meter reading would be 7.4. The discrepancy arises when the acid dissociation constant
Acid dissociation constant
An acid dissociation constant, Ka, is a quantitative measure of the strength of an acid in solution. It is the equilibrium constant for a chemical reaction known as dissociation in the context of acid-base reactions...

 value is specified as a concentration quotient and would not occur if Ka were specified as a quotient of activities.

Calculating buffer pH (polyprotic acid)

Polyprotic acids are acids that can lose more than one proton. The constant for dissociation of the first proton may be denoted as Ka1 and the constants for dissociation of successive protons as Ka2, etc. Citric acid
Citric acid
Citric acid is a weak organic acid. It is a natural preservative/conservative and is also used to add an acidic, or sour, taste to foods and soft drinks...

, H3A, is an example of a polyprotic acid as it can lose three protons.
equilibriumpKa value
H3A H2A + H+ pKa1 = 3.13
H2A HA2− + H+ pKa2 = 4.76
HA2− A3− + H+ pKa3 = 6.40


When the difference between successive pK values is less than about three there is overlap between the pH range of existence of the species in equilibrium. The smaller the difference, the more the overlap. In the case of citric acid, the overlap is extensive and solutions of citric acid are buffered over the whole range of pH 2.5 to 7.5. Calculation of the pH of a particular mixture requires a speciation calculation to be performed.

Applications

Buffer solutions are necessary to keep the correct pH for enzyme
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...

s in many organisms to work. Many enzymes work only under very precise conditions; if the pH moves outside of a narrow range, the enzymes slow or stop working and can denature
Denaturation (biochemistry)
Denaturation is a process in which proteins or nucleic acids lose their tertiary structure and secondary structure by application of some external stress or compound, such as a strong acid or base, a concentrated inorganic salt, an organic solvent , or heat...

. In many cases denaturation can permanently disable their catalytic activity.
A buffer of carbonic acid
Carbonic acid
Carbonic acid is the inorganic compound with the formula H2CO3 . It is also a name sometimes given to solutions of carbon dioxide in water, because such solutions contain small amounts of H2CO3. Carbonic acid forms two kinds of salts, the carbonates and the bicarbonates...

 (H2CO3) and bicarbonate
Bicarbonate
In inorganic chemistry, bicarbonate is an intermediate form in the deprotonation of carbonic acid...

 (HCO3) is present in blood plasma
Blood plasma
Blood plasma is the straw-colored liquid component of blood in which the blood cells in whole blood are normally suspended. It makes up about 55% of the total blood volume. It is the intravascular fluid part of extracellular fluid...

, to maintain a pH between 7.35 and 7.45.

Industrially, buffer solutions are used in fermentation
Fermentation (biochemistry)
Fermentation is the process of extracting energy from the oxidation of organic compounds, such as carbohydrates, using an endogenous electron acceptor, which is usually an organic compound. In contrast, respiration is where electrons are donated to an exogenous electron acceptor, such as oxygen,...

 processes and in setting the correct conditions for dyes used in colouring fabrics. They are also used in chemical analysis and calibration of pH meters.

The majority of biological samples that are used in research are made in buffers, especially phosphate buffered saline
Phosphate buffered saline
Phosphate buffered saline is a buffer solution commonly used in biological research. It is a water-based salt solution containing sodium chloride, sodium phosphate, and, in some formulations, potassium chloride and potassium phosphate. The buffer's phosphate groups help to maintain a constant pH...

 (PBS) at pH 7.4.

Useful buffer mixtures

ComponentspH range
HCl
Hydrochloric acid
Hydrochloric acid is a solution of hydrogen chloride in water, that is a highly corrosive, strong mineral acid with many industrial uses. It is found naturally in gastric acid....

, Sodium citrate
Sodium citrate
Trisodium citrate has the chemical formula of Na3C6H5O7. It is sometimes referred to simply as sodium citrate, though sodium citrate can refer to any of the three sodium salts of citric acid. It possesses a saline, mildly tart flavor. For this reason, citrates of certain alkaline and alkaline earth...

1 - 5
Citric acid
Citric acid
Citric acid is a weak organic acid. It is a natural preservative/conservative and is also used to add an acidic, or sour, taste to foods and soft drinks...

, Sodium citrate
Sodium citrate
Trisodium citrate has the chemical formula of Na3C6H5O7. It is sometimes referred to simply as sodium citrate, though sodium citrate can refer to any of the three sodium salts of citric acid. It possesses a saline, mildly tart flavor. For this reason, citrates of certain alkaline and alkaline earth...

2.5 - 5.6
Acetic acid
Acetic acid
Acetic acid is an organic compound with the chemical formula CH3CO2H . It is a colourless liquid that when undiluted is also called glacial acetic acid. Acetic acid is the main component of vinegar , and has a distinctive sour taste and pungent smell...

, Sodium acetate
Sodium acetate
Sodium acetate, CH3COONa, also abbreviated NaOAc, also sodium ethanoate, is the sodium salt of acetic acid. This colourless salt has a wide range of uses.-Industrial:...

3.7 - 5.6
K2HPO4, KH2PO4
Monopotassium phosphate
Monopotassium phosphate -- 24 -- is a soluble salt which is used as a fertilizer, a food additive and a fungicide. It is a source of phosphorus and potassium. It is also a buffering agent...

5.8 - 8
Na2HPO4
Disodium phosphate
Disodium hydrogen phosphate is a sodium salt of phosphoric acid. It is a white powder that is highly hygroscopic and water soluble. It is therefore used commercially as an anti-caking additive in powdered products. It is also known as disodium hydrogen orthophosphate, sodium hydrogen phosphate...

, NaH2PO4
Monosodium phosphate
Monosodium phosphate , also known as anhydrous monobasic sodium phosphate is a chemical compound of sodium with a phosphate counterion. It is used as a laxative and, in combination with other sodium phosphates, as a pH buffer....

6 - 7.5
CHES
N-Cyclohexyl-2-aminoethanesulfonic acid
N-Cyclohexyl-2-aminoethanesulfonic acid, also known as CHES, is a buffering agent.Typically appears as a white crystalline powder....

8.6–10
Borax
Borax
Borax, also known as sodium borate, sodium tetraborate, or disodium tetraborate, is an important boron compound, a mineral, and a salt of boric acid. It is usually a white powder consisting of soft colorless crystals that dissolve easily in water.Borax has a wide variety of uses...

, Sodium hydroxide
9.2 - 11

"Universal" buffer mixtures

By combining substances with pKa values differing by only two or less and adjusting the pH a wide-range of buffers can be obtained. Citric acid
Citric acid
Citric acid is a weak organic acid. It is a natural preservative/conservative and is also used to add an acidic, or sour, taste to foods and soft drinks...

 is a useful component of a buffer mixture because it has three pKa values, separated by less than two. The buffer range can be extended by adding other buffering agents.
The following two-component mixtures (McIlvaine's buffer solutions have a buffer range of pH 3 to 8.
0.2M Na2HPO4 /mL 0.1M Citric Acid /mL pH...
20.55 79.45 3.0
38.55 61.45 4.0
51.50 48.50 5.0
63.15 36.85 6.0
82.35 17.65 7.0
97.25 2.75 8.0


A mixture containing citric acid
Citric acid
Citric acid is a weak organic acid. It is a natural preservative/conservative and is also used to add an acidic, or sour, taste to foods and soft drinks...

, potassium dihydrogen phosphate, boric acid
Boric acid
Boric acid, also called hydrogen borate or boracic acid or orthoboric acid or acidum boricum, is a weak acid of boron often used as an antiseptic, insecticide, flame retardant, as a neutron absorber, and as a precursor of other chemical compounds. It exists in the form of colorless crystals or a...

, and diethyl barbituric acid
Barbital
Barbital , also called barbitone, was the first commercially marketed barbiturate. It was used as a sleeping aid from 1903 until the mid-1950s. The chemical names for barbital are diethylmalonyl urea or diethylbarbituric acid...

 can be made to cover the pH range 2.6 to 12.

Other universal buffers are Carmody buffer and Britton-Robinson buffer
Britton-Robinson buffer
Britton–Robinson buffer is a "universal" pH buffer used for the range pH 2 to pH 12. Universal buffers consist of mixtures of acids of diminishing strength so that the change in pH is approximately proportional to the amount of alkali added. It consists of a mixture of 0.04 M H3BO3, 0.04 M H3PO4...

, developed in 1931.

Common buffer compounds used in biology

Common Name pKa
Acid dissociation constant
An acid dissociation constant, Ka, is a quantitative measure of the strength of an acid in solution. It is the equilibrium constant for a chemical reaction known as dissociation in the context of acid-base reactions...


at 25°C
Buffer Range Temp Effect
dpH/dT in (1/K) **
Mol.
Weight
Full Compound Name
TAPS
TAPS (buffer)
TAPS is used to make buffer solutions. It has a pKa value of 8.44 . It can be used to make buffer solutions in the pH range 7.7–9.1....

 
8.43 7.7–9.1 −0.018 243.3 3-{[tris(hydroxymethyl)methyl]amino}propanesulfonic acid
Bicine
Bicine
Bicine is an organic compound used as a buffering agent. It is one of Good's buffers and has a pKa of 8.35 at 20 °C....

 
8.35 7.6–9.0 −0.018 163.2 N,N-bis(2-hydroxyethyl)glycine
Tris
Tris
Tris is an abbreviation of the organic compound known as trisaminomethane, with the formula 3CNH2. Tris is extensively used in biochemistry and molecular biology. In biochemistry, tris is widely used as a component of buffer solutions, such as in TAE and TBE buffer, especially for solutions of...

 
8.06 7.5–9.0 −0.028 121.14 tris(hydroxymethyl)methylamine
Tricine
Tricine
Tricine is an organic compound that is used in buffer solutions. The name tricine comes from tris and glycine from which it was derived. It is a zwitterionic amino acid with a useful buffering range of pH 7.4-8.8. It is a white crystalline powder that is moderately soluble in water. It has a pH...

 
8.05 7.4–8.8 −0.021 179.2 N-tris(hydroxymethyl)methylglycine
TAPSO
TAPSO (buffer)
TAPSO is used to make buffer solutions. It has a pKa value of 7.635 . It can be used to make buffer solutions in the pH range 7.0-8.2....

 
7.635 7.0-8.2 259.3 3-[N-Tris(hydroxymethyl)methylamino]-2-hydroxypropanesulfonic Acid
HEPES
HEPES
HEPES is a zwitterionic organic chemical buffering agent; one of the twelve Good's buffers...

 
7.48 6.8–8.2 −0.014 238.3 4-2-hydroxyethyl-1-piperazineethanesulfonic acid
TES
TES (buffer)
TES is used to make buffer solutions. It has a pKa value of 7.550 . It is one of the Good's buffers and can be used to make buffer solutions in the pH range 6.8–8.2....

 
7.40 6.8–8.2 −0.020 229.20 2-{[tris(hydroxymethyl)methyl]amino}ethanesulfonic acid
MOPS
MOPS
MOPS is the common name for the compound 3-propanesulfonic acid, a buffer introduced by Good et al. in the 1960s. It is a structural analog to MES. Its chemical structure contains a morpholine ring. HEPES is a similar pH buffering compound that contains a piperazine ring...

 
7.20 6.5–7.9 −0.015 209.3 3-(N-morpholino)propanesulfonic acid
PIPES
PIPES
PIPES is the common name for piperazine-N,N′-bis, and frequently used buffering agent in biochemistry. It is an ethanesulfonic acid buffer developed by Good et al. in the 1960s.-Applications:...

 
6.76 6.1–7.5 −0.008 302.4 piperazine-N,N′-bis(2-ethanesulfonic acid)
Cacodylate  6.27 5.0–7.4 138.0 dimethylarsinic acid
SSC
SSC buffer
In biochemistry and molecular biology, the saline-sodium citrate buffer is used as a hybridization buffer, to control stringency for washing steps in protocols for Southern blotting, in situ hybridization, DNA Microarray or Northern blotting...

 
7.0 6.5-7.5 189.1 saline sodium citrate
MES
MES (buffer)
MES is the common name for the compound 2-ethanesulfonic acid. Its chemical structure contains a morpholine ring. It has a molecular weight of 195.2 and the chemical formula is C6H13NO4S...

 
6.15 5.5–6.7 −0.011 195.2 2-(N-morpholino)ethanesulfonic acid


** Values are approximate.

See also

  • Henderson–Hasselbalch equation
  • Buffering agent
    Buffering agent
    A buffering agent is a weak acid or base used to maintain the acidity of a solution at a chosen value. The function of a buffering agent is to prevent a rapid change in pH when acids or bases are added to the solution. Buffering agents have variable properties—some are more soluble than others;...

  • Good's buffers
    Good's buffers
    Good's buffers are twelve buffering agents selected and described by Norman Good and colleagues in 1966. Good selected the buffers based on a number of criteria which make them candidates for use in biochemistry and biological research...

  • Common-ion effect
    Common-ion effect
    The common ion effect is an effect which results when two substances, which both ionize to give the same ion, are involved in a chemical equilibrium.-Solubility effects:...


External links

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