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 HA ↽ − − ⇀ A − + H + known as dissociation in the context of acid–base reactions; the chemical species HA is an acid that dissociates into A−, the conjugate base of the acid and a hydrogen ion, H+. The system is said to be in equilibrium when the concentrations of its components will not change with the passing of time, because both forward and backward reactions are occurring at the same rate; the dissociation constant is defined by K a = or p K a = − log 10 ⁡ K a = log 10 ⁡ where quantities in square brackets represent the concentrations of the species at equilibrium. The acid dissociation constant for an acid is a direct consequence of the underlying thermodynamics of the dissociation reaction; the value of the pKa changes with temperature and can be understood qualitatively based on Le Châtelier's principle: when the reaction is endothermic, Ka increases and pKa decreases with increasing temperature.

The value of pKa depends on molecular structure of the acid in many ways. For example, Pauling proposed two rules: one for successive pKa of polyprotic acids, one to estimate the pKa of oxyacids based on the number of =O and −OH groups. Other structural factors that influence the magnitude of the acid dissociation constant include inductive effects, mesomeric effects, hydrogen bonding. Hammett type equations have been applied to the estimation of pKa; the quantitative behaviour of acids and bases in solution can be understood only if their pKa values are known. In particular, the pH of a solution can be predicted when the analytical concentration and pKa values of all acids and bases are known; these calculations find application in many different areas of chemistry, biology and geology. For example, many compounds used for medication are weak acids or bases, a knowledge of the pKa values, together with the water–octanol partition coefficient, can be used for estimating the extent to which the compound enters the blood stream.

Acid dissociation constants are essential in aquatic chemistry and chemical oceanography, where the acidity of water plays a fundamental role. In living organisms, acid–base homeostasis and enzyme kinetics are dependent on the pKa values of the many acids and bases present in the cell and in the body. In chemistry, a knowledge of pKa values is necessary for the preparation of buffer solutions and is a prerequisite for a quantitative understanding of the interaction between acids or bases and metal ions to form complexes. Experimentally, pKa values can be determined by potentiometric titration, but for values of pKa less than about 2 or more than about 11, spectrophotometric or NMR measurements may be required due to practical difficulties with pH measurements. According to Arrhenius's original definition, an acid is a substance that dissociates in aqueous solution, releasing the hydrogen ion H+: HA ⇌ A− + H+; the equilibrium constant for this dissociation reaction is known as a dissociation constant.

The liberated proton combines with a water molecule to give a hydronium ion H3O+, so Arrhenius proposed that the dissociation should be written as an acid–base reaction: HA + H2O ⇌ A− + H3O+. Brønsted and Lowry generalised this further to a proton exchange reaction: acid + base ⇌ conjugate base + conjugate acid; the acid loses a proton. For aqueous solutions of an acid HA, the base is water; the Brønsted–Lowry definition applies to other solvents, such as dimethyl sulfoxide: the solvent S acts as a base, accepting a proton and forming the conjugate acid SH+. HA + S ⇌ A− + SH+. In solution chemistry, it is common to use H+ as an abbreviation for the solvated hydrogen ion, regardless of the solvent. In aqueous solution H+ denotes a solvated hydronium ion rather than a proton; the designation of an acid or base as "conjugate" depends on the context. The conjugate acid BH+ of a base B dissociates according to BH+ + OH− ⇌ B + H2Owhich is the reverse of the equilibrium H2O + B ⇌ OH− + BH+; the hydroxide ion OH−, a well known base, is here acting as the conjugate base of the acid water.

Acids and bases are thus regarded as donors a

Brigadier-General Sir Henry Percy Maybury was a British civil engineer. He began his career as a railway engineer, working on many railways in England and Wales before becoming the county surveyor for Kent. At the start of the First World War he was appointed to supervise roads used by the Allies in France, holding the British Army rank of Brigadier-General. In recognition of his services in this theatre he was appointed a Companion of the Order of the Bath and a Knight Commander of the Order of St Michael and St George by the British government and an officer of the Legion of Honour by the French. After the war he held various civil service positions within the Ministry of Transport, was elected president of the Institution of Civil Engineers in 1933. Maybury was born in Uffington in Shropshire, fourth son of Charles Maybury, a farmer, his wife Jane, was educated at nearby Upton Magna. Upon finishing his studies Maybury began work at Shrewsbury railway station in the office of RE Johnston, the chief engineer of the Shrewsbury and Hereford Railway, a joint venture between the Great Western and London and North Western railway companies, where he worked for five years.

In 1884 he joined the Wrexham contracting firm of Johnson Brothers and Slay, where he became manager, worked on the Glyn Valley Tramway during the rebuilding in 1886. From 1892 until 1895 he served as the engineer and surveyor of the Ffestiniog Local Highways Board, followed by a similar appointment to the Malvern Urban District Council. From 1904 until 1913 Maybury served as the county surveyor for Kent during which time he was responsible for constructing new roads and classifying the existing network in light of the rapid increase in motor traffic following the revised speed limits of the Motor Car Act 1903, he carried out experiments on road surfaces at Sidcup to try to improve durability and reduce dust produced by the increased speed of traffic. In 1904 he developed sewerage and drinking water schemes for Ludlow County Borough and Worcestershire County Council. In 1910 he was invited to become a member of the Advisory Engineering Committee to the Road Board, impressed by his road surface trials, served as their chief engineering officer upon leaving his position in Kent.

He became manager and secretary of the board. One of his innovations was to divide the road network intro three categories on the basis of which road maintenance grants would be distributed and he appointed a large staff of engineers to carry out this categorisation. From the start of the First World War he was appointed by the War Office to build and maintain roads at military camps in the United Kingdom. In 1916 he visited France to advise the British Army's Engineer-in-Chief on matters to do with road transportation and was asked to form a highway engineering service in France; this organisation was absorbed into the British Army that year and Maybury was placed at the head of the Roads Directorate and commissioned as a Brigadier-General. Maybury had charge of the roads used by the Allied forces in France, under Eric Geddes, director-general of transportation; the directorate was responsible for 40,000 men and 4,000 miles of roads plus associated works such as quarries. Maybury was mentioned in dispatches four times for his work during the war and was appointed a Companion of the Order of the Bath and an Officer of the Legion of Honour in 1917.

He retired from the army in 1919. In 1919 he was created a commander and knight commander of the Order of St Michael and St George and was appointed Director General of the Roads Department of the Ministry of Transport, a position he would hold for the next nine years. In this role Maybury developed new arterial highways and modernized existing roads, providing a considerable means of employment during a period of depressed economic output, he served as president of the newly formed Institute of Transport in 1921 and as a Justice of the Peace for Kent in 1922. Upon retiring from the ministry in 1928 he was made a Knight Grand Cross of the Order of the British Empire and given the freedom of the borough of Shrewsbury. Maybury was elected as president of the Institution of Civil Engineers in 1933, an annual accolade awarded to the profession's most regarded engineers, he had been a member of the institution since 1910 and served as president of the Institute of Quarrying. He was appointed to the twelve member London and Home Counties Traffic Advisory Committee when it was formed in 1928 and had responsibility to advise the Minister of Transport on the London Traffic Area.

He served as the committee's representative on the London Passenger Transport Board from its formation in 1933 to his death. Maybury was chairman of the Lights on Vehicles. In 1936 he chaired the committee for the Development of Civil Aviation in the United Kingdom. In 1927 he opened Maybury Road in Edinburgh, named in his honour, in his capacity as Director General of the Ministry of Transport. In 1928 the Paviors' Company, to which he had been elected in 1918, founded a professorship in highways engineering at the University of London named after himself. For 1936 he was appointed President of the Shropshire Horticultural Society and he had been President of the Shropshire Society in London. Maybury held a number of commercial directorships up to his death, including chairmanship of the British Quarrying Company and trustee of the West Midlands Savings Bank. Maybury ran a private engineering consultancy based in Aldwych, he was president of the Smeatonian Society of Civil Engineers at the time of his death.

Maybury was twice married. He first married, in

Poteat House known as Forest Home, is a historic plantation house located near Yanceyville, Caswell County, North Carolina. It was built in 1855–1856, consists of a two-story main block, three bays wide, with flanking one-story wings in the Greek Revival style, it has a center hall plan and was restored in 1928–1929 by Helen Poteat and her husband and playwright Laurence Stallings. It features a reconstructed double pedimented portico supported by four plain Roman Doric order columns. On the property is a contributing small slave cabin; the house was the birthplace of painter Ida Isabella Poteat. It was added to the National Register of Historic Places in 1979