Test tubes containing solutions of pH 1–10 colored with an indicator
Relation between pH and pOH. Red represents the acidic region. Blue represents the basic region.
Nutritional elements availability within soil varies with pH. Light blue color represents the ideal range for most plants.
Lemon juice tastes sour because it contains 5% to 6% citric acid and has a pH of 2.2 (high acidity).

Scale used to specify the acidity or basicity of an aqueous solution.

- PH

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Molecule or ion capable of either donating a proton , known as a Brønsted–Lowry acid, or forming a covalent bond with an electron pair, known as a Lewis acid.

Zinc, a typical metal, reacting with hydrochloric acid, a typical acid
Svante Arrhenius
Acetic acid, a weak acid, donates a proton (hydrogen ion, highlighted in green) to water in an equilibrium reaction to give the acetate ion and the hydronium ion. Red: oxygen, black: carbon, white: hydrogen.
Hydrochloric acid (in beaker) reacting with ammonia fumes to produce ammonium chloride (white smoke).
This is an ideal titration curve for alanine, a diprotic amino acid. Point 2 is the first equivalent point where the amount of NaOH added equals the amount of alanine in the original solution.
Carbonated water (H2CO3 aqueous solution) is commonly added to soft drinks to make them effervesce.
Basic structure of an amino acid.
Aspirin (acetylsalicylic acid) is a carboxylic acid

An aqueous solution of an acid has a pH less than 8 and is colloquially also referred to as "acid" (as in "dissolved in acid"), while the strict definition refers only to the solute.

Base (chemistry)

In chemistry, there are three definitions in common use of the word base, known as Arrhenius bases, Brønsted bases, and Lewis bases.

Soaps are weak bases formed by the reaction of fatty acids with sodium hydroxide or potassium hydroxide.
Ammonia fumes from aqueous ammonium hydroxide (in test tube) reacting with hydrochloric acid (in beaker) to produce ammonium chloride (white smoke).
Sodium hydroxide
Barium hydroxide

In water, by altering the autoionization equilibrium, bases yield solutions in which the hydrogen ion activity is lower than it is in pure water, i.e., the water has a pH higher than 7.0 at standard conditions.


Common name for the aqueous cation, the type of oxonium ion produced by protonation of water.

Zundel cation
Primary reaction pathways of in the interstellar medium (specifically, dense clouds).

The concentration of hydronium or H+ ions determines a solution's pH according to

PH indicator

pH indicators: a graphic view
pH measurement with indicator paper
Absorption spectra of bromocresol green at different stages of protonation
Hydrangea in acid soil
Hydrangea in alkaline soil
A gradient of red cabbage extract pH indicator from acidic solution on the left to basic on the right
Purple cauliflower soaked in baking soda (left) and vinegar (right). Anthocyanin acts as an pH indicator.
Turmeric dissolved in water is yellow under acidic and reddish brown under alkaline conditions

A pH indicator is a halochromic chemical compound added in small amounts to a solution so the pH (acidity or basicity) of the solution can be determined visually.

Buffer solution

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

A buffer solution (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.

Logarithmic scale

Way of displaying numerical data over a very wide range of values in a compact way—typically the largest numbers in the data are hundreds or even thousands of times larger than the smallest numbers.

A logarithmic scale from 0.1 to 100
Semi-log plot of the Internet host count over time shown on a logarithmic scale
The two logarithmic scales of a slide rule
Map of the solar system and distance to Alpha Centauri using a logarithmic scale.
Various scales: lin–lin, lin–log, log–lin, and log–log. Plotted graphs are: y = 10x ( red ), y = x ( green ), y = loge(x) ( blue ).
Plot on log–log scale of equation of a line

pH for acidity

Acid dissociation constant

Quantitative measure of the strength of an acid in solution.

Acetic acid, a weak acid, donates a proton (hydrogen ion, highlighted in green) to water in an equilibrium reaction to give the acetate ion and the hydronium ion. Red: oxygen, black: carbon, white: hydrogen.
Variation of the % formation of a monoprotic acid, AH, and its conjugate base, A−, with the difference between the pH and the pKa of the acid.
Phosphoric acid speciation
Dimerization of a carboxylic acid.
pKa of acetic acid in dioxane/water mixtures. Data at 25 °C from Pine et al.
Fumaric acid
Maleic acid
Proton sponge
A calculated titration curve of oxalic acid titrated with a solution of sodium hydroxide

In particular, the pH of a solution can be predicted when the analytical concentration and pKa values of all acids and bases are known; conversely, it is possible to calculate the equilibrium concentration of the acids and bases in solution when the pH is known.


[[File:Logarithm plots.png|right|thumb|upright=1.35|Plots of logarithm functions, with three commonly used bases. The special points

Plots of logarithm for bases 0.5, 2, and e
The 1797 Encyclopædia Britannica explanation of logarithms
Schematic depiction of a slide rule. Starting from 2 on the lower scale, add the distance to 3 on the upper scale to reach the product 6. The slide rule works because it is marked such that the distance from 1 to x is proportional to the logarithm of x.
The logarithm keys (LOG for base 10 and LN for base e) on a TI-83 Plus graphing calculator
A nautilus displaying a logarithmic spiral
A logarithmic chart depicting the value of one Goldmark in Papiermarks during the German hyperinflation in the 1920s
Distribution of first digits (in %, red bars) in the population of the 237 countries of the world. Black dots indicate the distribution predicted by Benford's law.
Billiards on an oval billiard table. Two particles, starting at the center with an angle differing by one degree, take paths that diverge chaotically because of reflections at the boundary.

In chemistry, pH is a logarithmic measure for the acidity of an aqueous solution.

Purified water

Water that has been mechanically filtered or processed to remove impurities and make it suitable for use.

Bottle for distilled water in the Royal Academy of Pharmacy (Spain)
Large cation/anion ion exchangers used in demineralization of boiler feedwater.

Softening consists in preventing the possible precipitation of poorly soluble minerals from natural water due to changes occurring in the physico-chemical conditions (such as pCO2, pH, and Eh).


Common laboratory method of quantitative chemical analysis to determine the concentration of an identified analyte (a substance to be analyzed).

A burette and Erlenmeyer flask (conical flask) being used for an acid–base titration.
Analysis of soil samples by titration.
A typical titration curve of a diprotic acid titrated with a strong base. Shown here is oxalic acid titrated with sodium hydroxide. Both equivalence points are visible.
Methyl orange
Phenolphthalein, a commonly used indicator in acid and base titration.
Color of iodometric titration mixture before (left) and after (right) the end point.
An elementary pH meter that can be used to monitor titration reactions.
A titration is demonstrated to secondary school students.

Many non-acid–base titrations require a constant pH during the reaction.