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a solution containing a phosphate buffer (同)PBS
a homogeneous mixture of two or more substances; frequently (but not necessarily) a liquid solution; "he used a solution of peroxide and water"
the successful action of solving a problem; "the solution took three hours"
a statement that solves a problem or explains how to solve the problem; "they were trying to find a peaceful solution"; "the answers were in the back of the book"; "he computed the result to four decimal places" (同)answer, result, resolution, solvent
the set of values that give a true statement when substituted into an equation (同)root
a method for solving a problem; "the easy solution is to look it up in the handbook"
add a buffer (a solution); "buffered saline solution for the eyes"
(chemistry) an ionic compound that resists changes in its pH
(computer science) a part of RAM used for temporary storage of data that is waiting to be sent to a device; used to compensate for differences in the rate of flow of data between components of a computer system (同)buffer storage, buffer store
a cushion-like device that reduces shock due to an impact (同)fender
a power tool used to buff surfaces (同)polisher
of the yellowish-beige color of buff leather
a soft thick undyed leather from the skins of e.g. buffalo or oxen
an implement consisting of soft material mounted on a block; used for polishing (as in manicuring) (同)buffer
bare skin; naked; "swimming in the buff"
polish and make shiny; "buff the wooden floors"; "buff my shoes" (同)burnish, furbish

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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2014/06/06 19:30:30」(JST)

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For an individual weak acid or weak base component, see Buffering agent. For uses not related to acid-base chemistry, see Buffer (disambiguation).

A buffer (more precisely, pH buffer or hydrogen ion buffer) is an aqueous solution consisting of a mixture of a weak acid and its conjugate base, or vice versa. Its pH changes very little when a small amount of strong acid or base is added to it and thus it is used to prevent changes in the pH of a solution. 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 so they utilize a buffer solution to maintain a constant pH. One example of a buffer solution found in nature is blood.

Principles of buffering

Simulated titration of an acidified solution of a weak acid (pK_a = 4.7) with alkali.

Addition of hydroxide to a mixture of a weak acid and its conjugate base

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 is added to an equilibrium mixture of the weak acid and its conjugate base, the equilibrium is shifted to the left, in accordance with Le Chatelier's principle. 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 pK_a = 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 = pK_a ± 1, centered at pH = 4.7 where [HA] = [A^-]. The hydrogen ion concentration decreases by less than the amount expected because most of the added hydroxide ion is consumed in the reaction

OH^- + HA → H₂O + A^-

and only a little is consumed in the neutralization reaction which results in an increase in pH.

OH^- + H⁺ → H₂O

Once the acid is more than 95% deprotonated the pH rises rapidly because most of the added alkali is consumed in the neutralization reaction.

Applications

Buffer solutions are necessary to keep the correct pH for enzymes 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. In many cases denaturation can permanently disable their catalytic activity.^[1] A buffer of carbonic acid (H₂CO₃) and bicarbonate (HCO₃⁻) is present in blood plasma, to maintain a pH between 7.35 and 7.45.

Industrially, buffer solutions are used in fermentation processes and in setting the correct conditions for dyes used in colouring fabrics. They are also used in chemical analysis^[2] and calibration of pH meters.

The majority of biological samples that are used in research are made in buffers, especially phosphate buffered saline (PBS) at pH 7.4.

Simple buffering agents

Buffering agent	pK_a	useful pH range
Citric acid	3.13, 4.76, 6.40	2.1 - 7.4
Acetic acid	4.8	3.8 - 5.8
KH₂PO₄,	7.2	6.2 - 8.2
CHES	9.3	8.3–10.3
Borate	9.24	8.25 - 10.25

For buffers in acid regions, the pH may be adjusted to a desired value by adding a strong acid such as hydrochloric acid to the buffering agent. For alkaline buffers, a strong base such as sodium hydroxide may be added. Alternatively, a buffer mixture can be made from a mixure of an acid and its conjugate base. For example, an acetate buffer can be made from a mixture of acetic acid and sodium acetate. Similarly an alkaline buffer can be made from a mixture of the base and its conjugate acid.

"Universal" buffer mixtures

By combining substances with pK_a values differing by only two or less and adjusting the pH, a wide range of buffers can be obtained. Citric acid is a useful component of a buffer mixture because it has three pK_a 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.^[3]

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, potassium dihydrogen phosphate, boric acid, and diethyl barbituric acid can be made to cover the pH range 2.6 to 12.^[4]

Other universal buffers are Carmody buffer^[5] and Britton-Robinson buffer, developed in 1931.

Common buffer compounds used in biology

Common Name	pK_a at 25 °C	Buffer Range	Temp Effect dpH/dT in (1/K)^[6]	Mol. Weight	Full Compound Name
TAPS	8.43	7.7–9.1	−0.018	243.3	3-{[tris(hydroxymethyl)methyl]amino}propanesulfonic acid
Bicine	8.35	7.6–9.0	−0.018	163.2	N,N-bis(2-hydroxyethyl)glycine
Tris	8.06	7.5–9.0	−0.028	121.14	tris(hydroxymethyl)methylamine
Tricine	8.05	7.4–8.8	−0.021	179.2	N-tris(hydroxymethyl)methylglycine
TAPSO	7.635	7.0-8.2		259.3	3-[N-Tris(hydroxymethyl)methylamino]-2-hydroxypropanesulfonic Acid
HEPES	7.48	6.8–8.2	−0.014	238.3	4-2-hydroxyethyl-1-piperazineethanesulfonic acid
TES	7.40	6.8–8.2	−0.020	229.20	2-{[tris(hydroxymethyl)methyl]amino}ethanesulfonic acid
MOPS	7.20	6.5–7.9	−0.015	209.3	3-(N-morpholino)propanesulfonic acid
PIPES	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	7.0	6.5-7.5		189.1	saline sodium citrate
MES	6.15	5.5–6.7	−0.011	195.2	2-(N-morpholino)ethanesulfonic acid
Succinic acid	7.4(?)	7.4-7.5	?	118.1	2(R)-2-(methylamino)succinic acid

Biological buffers cover 1.9 to 11 pH range. ^[7]

Buffer capacity

Buffer capacity for a 0.1 M solution of an acid with pK_a of 7

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

where C_A is the analytical concentration of the acid.^[2] pH is defined as -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⁺] = pK_a ± 2 the second term becomes important. Buffer capacity is proportional to the concentration of the buffering agent, C_A, so dilute solutions have little buffer 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 and is independent of the presence or absence of buffering agents.

The buffer capacity of a buffering agent is at a maximum when p[H⁺] = pK_a. It falls to 33% of the maximum value at p[H⁺] = pK_a ± 1 and to 10% at p[H⁺] = pK_a ± 1.5. For this reason the useful range is approximately pK_a ± 1.

Calculating buffer pH

Monoprotic acids

First write down the equilibrium expression.

HA A^- + H⁺

This shows that when the acid dissociates equal amounts of hydrogen ion and anion are produced. The equilibrium concentrations of these three components can be calculated in an ICE table.

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

The first row, labelled I, lists the initial conditions: the concentration of acid is C₀, initially undissociated, so the concentrations of A^- and H⁺ would be zero; y is the initial concentration of added strong acid, such as hydrochloric acid. If strong alkali, such as sodium hydroxide, is added y will have a negative sign because alkali removes hydrogen ions from the solution. 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 in terms of concentrations:

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

Simplify to:

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

Polyprotic acids

% species formation calculated for a 10 millimolar solution of citric acid.

Polyprotic acids are acids that can lose more than one proton. The constant for dissociation of the first proton may be denoted as K_a1 and the constants for dissociation of successive protons as K_a2, etc. Citric acid, H₃A, is an example of a polyprotic acid as it can lose three protons.

equilibrium	pK_a value
H₃A H₂A⁻ + H⁺	pK_a1 = 3.13
H₂A⁻ HA²⁻ + H⁺	pK_a2 = 4.76
HA²⁻ A³⁻ + H⁺	pK_a3 = 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 with a polyprotic acid requires a speciation calculation to be performed. In the case of citric acid, this entails the solution of the two equations of mass balance

C_A is the analytical concentration of the acid, C_H is the analytical concentration of added hydrogen ions, β_q are the cumulative association constants

K_w is the constant for Self-ionization of water. There are two non-linear simultaneous equations in two unknown quantities [A^3-] and [H⁺]. Many computer programs are available to do this calculation. The speciation diagram for citric acid was produced with the program HySS.^[8]

In general the two mass-balance equations can be written as

\begin{align} C_A & = [A]+\sum p\beta_q [A]^p[H^+]^q \\ C_H & = [H^+]+\sum q\beta_q [A]^p[H^+]^q -K_w[H]^{-1} \end{align}

In this general expression [A] stands for the concentration of the fully deprotonated acid and the electrical charge on this species is not specified.

References

^ Scorpio, R. (2000). Fundamentals of Acids, Bases, Buffers & Their Application to Biochemical Systems. ISBN 0-7872-7374-0.
^ ^a ^b Hulanicki, A. (1987). Reactions of acids and bases in analytical chemistry. Horwood. ISBN 0-85312-330-6. (translation editor: Mary R. Masson)
^ McIlvaine, T.C. (1921). "A buffer solution for colorimetric comparaison". J. Biol. Chem. 49 (1): 183–186.
^ Medham, J.; Denny, R.C.; Barnes, J.D.; Thomas, M (2000). Vogel's textbook of quantitative chemical analysis (5th. Ed. ed.). Harlow: Pearson Education. ISBN 0-582-22628-7. Cite uses deprecated parameters (help) Appendix 5
^ Carmody, Walter R. (1961). "Easily prepared wide range buffer series". J. Chem. Educ. 38 (11): 559–560. Bibcode:1961JChEd..38..559C. doi:10.1021/ed038p559.
^ "Buffer Reference Center". Sigma-Aldrich. Retrieved 2009-04-17.
^ "Biological buffers". REACH Devices.
^ Alderighi, L.; Gans, P.; Ienco, A.; Sabatini, A.; Vacca (1999). "Hyperquad simulation and speciation (HySS): a utility program for the investigation of equilibria involving soluble and partially soluble species". Coordination Chemistry Reviews 184 (1): 311–318. doi:10.1016/S0010-8545(98)00260-4.

External links

Online pH buffer calculator
phosphate buffer

UpToDate Contents

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English Journal

A novel tyrosinase biosensor based on chitosan-carbon-coated nickel nanocomposite film.

Yang L, Xiong H, Zhang X, Wang S.AbstractA novel nanocomposite film of tyrosinase-chitosan-carbon-coated nickel nanoparticles (CNi) had been constructed for the detection of catechol. The tyrosinase-chitosan-CNi bionanocomposite film was characterized with scanning electron microscopic (SEM) and electrochemical impedance spectroscopy (EIS). In pH 6.5 phosphate buffer solutions (PBS), the biosensor was applied to detect catechol with a broad linear range from 0.25nM to 27μM, the detection limit was brought down to 0.083nM (S/N=3). The proposed biosensor demonstrated rapid response, as well as good reproducibility and stability. The chitosan-CNi film was propitious to the immobilization of tyrosinase and to the retention of its bioactivity to a large extent. Therefore, the film has potential applications in the immobilization of other enzyme-based biosensors.
Bioelectrochemistry (Amsterdam, Netherlands).Bioelectrochemistry.2012 Apr;84:44-8. Epub 2011 Nov 20.
A novel nanocomposite film of tyrosinase-chitosan-carbon-coated nickel nanoparticles (CNi) had been constructed for the detection of catechol. The tyrosinase-chitosan-CNi bionanocomposite film was characterized with scanning electron microscopic (SEM) and electrochemical impedance spectroscopy (EIS)
PMID 22172649

Metabolism of amino acids, dipeptides and tetrapeptides by Lactobacillus sakei.

Sinz Q, Schwab W.SourceTechnische Universität München, Biotechnology of Natural Products, Liesel-Beckmann-Straße 1, D- 85354 Freising, Germany.
Food microbiology.Food Microbiol.2012 Apr;29(2):215-23. Epub 2011 Jul 22.
The microbial degradation of proteins, peptides and amino acids generates volatiles involved in the typical flavor of dry fermented sausage. The ability of three Lactobacillus sakei strains to form aroma compounds was investigated. Whole resting cells were fermented in phosphate buffer with equimola
PMID 22202875

Buffer standards for the physiological pH of the zwitterionic compound of 3-(N-morpholino)propanesulfonic acid (MOPS) from T = (278.15 to 328.15) K.

Roy LN, Roy RN, Allen KA, Mehrhoff CJ, Henson IB, Stegner JM.SourceHoffman Department of Chemistry, Drury University, 900 N. Benton Ave., Springfield, MO 65802, USA.
The Journal of chemical thermodynamics.J Chem Thermodyn.2012 Apr 1;47:21-27.
This paper reports the pH values of five NaCl-free buffer solutions and eleven buffer compositions containing NaCl at I = 0.16 mol·kg(-1). Conventional pa(H) values are reported for sixteen buffer solutions with and without NaCl salt. The operational pH values have been calculated for five buffer s
PMID 22247568

Japanese Journal

Controlling the stoichiometry and strand polarity of a tetramolecular G-quadruplex structure by using a DNA origami frame.

Rajendran Arivazhagan,Endo Masayuki,Hidaka Kumi,Lan Thao Tran Phong,Mergny Jean-Louis,Sugiyama Hiroshi
Nucleic acids research 41(18), 8738-8747, 2013-10
… Here, we describe the direct visualization and single-molecule analysis of the formation of a tetramolecular G-quadruplex in KCl solution. … Further, the G-quadruplex formation in KCl solution and its disruption in KCl-free buffer were analyzed in real-time. …
NAID 120005347132

カキ殻を用いた土の微生物固化処理に関する基礎的研究

金山素平,赤司将高,中野晶子,大坪政美,東孝寛
粘土科学 52(1), 1-8, 2013-08-19
… The soil samples were mixed with buffer solution and the oyster shell in test tubes, and were shaken for 24 hours, and then commercial yeast and glucose were added to the soil samples. … The calcium concentration in the solution decreased rapidly and reached a constant value after 12 hours, which indicates that the calcium in the liquid phase of the soil samples was precipitated as carbonate. …
NAID 110009635538

pH Sensor Based on Chemical-Vapor-Deposition-Synthesized Graphene Transistor Array

Zaifuddin Nursakinah Mohd,Okamoto Shogo,Ikuta Takashi,Ohno Yasuhide,Maehashi Kenzo,Miyake Masato,Greenwood Paul,Teo Kenneth B. K.,Matsumoto Kazuhiko
Jpn J Appl Phys 52(6), 06GK04-06GK04-4, 2013-06-25
… The sensing characteristics of the CVD-synthesized graphene-based device were investigated using three buffer solutions with different pH values (pHs 4.0, 6.8, and 9.3). … The electrical measurements reveal that for most of the graphene FETs in the array, a similar stepwise increment in drain current was observed upon the introduction of each buffer solution with increasing pH value sequence. …
NAID 150000107287

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[Scorpio_2000-1] Scorpio, R. (2000). Fundamentals of Acids, Bases, Buffers & Their Application to Biochemical Systems. ISBN 0-7872-7374-0.

[Hulanicki-2] Hulanicki, A. (1987). Reactions of acids and bases in analytical chemistry. Horwood. ISBN 0-85312-330-6. (translation editor: Mary R. Masson)

[3] McIlvaine, T.C. (1921). "A buffer solution for colorimetric comparaison". J. Biol. Chem. 49 (1): 183–186.

[4] Medham, J.; Denny, R.C.; Barnes, J.D.; Thomas, M (2000). Vogel's textbook of quantitative chemical analysis (5th. Ed. ed.). Harlow: Pearson Education. ISBN 0-582-22628-7. Cite uses deprecated parameters (help) Appendix 5

[carmody-5] Carmody, Walter R. (1961). "Easily prepared wide range buffer series". J. Chem. Educ. 38 (11): 559–560. Bibcode:1961JChEd..38..559C. doi:10.1021/ed038p559.

[6] "Buffer Reference Center". Sigma-Aldrich. Retrieved 2009-04-17.

[7] "Biological buffers". REACH Devices.

[8] Alderighi, L.; Gans, P.; Ienco, A.; Sabatini, A.; Vacca (1999). "Hyperquad simulation and speciation (HySS): a utility program for the investigation of equilibria involving soluble and partially soluble species". Coordination Chemistry Reviews 184 (1): 311–318. doi:10.1016/S0010-8545(98)00260-4.

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