The French chemist Rouelle (1703–1770) created the modern idea of a base as the solid material that was the basis for the action of an acid to form a salt. At that time, many acids were volatile liquids and salts were crystalline. Hence it was thought that there should be a solid base for the salt to grow out of, giving rise to the term. Many bases are insoluble but the salts they form with acids are soluble, making it appear that the base dissolves in the acid.

Some bases are soluble in water Some bases are soluble in water. The solutions are then described as alkaline. There is an issue with some materials such as ammonia gas. Ammonia is soluble in water making a solution with pH > 7, i.e. alkaline. Traditionally this has been rationalised using the label ‘ammonium hydroxide solution’. However, the solution is mainly neutral ammonia molecules in water, with only a small proportion reacting to give ammonium and hydroxide ions, depending on the concentration.

SALTS A characteristic of acids is that they form salts with bases, carbonates and metals. The term ‘salt’, which appears in a similar form in many languages, is derived from an Old English word for the sea, ‘sealt’. Many salts can be extracted from sea water, including sodium chloride, or common salt, widely used in the past as a food preservative. ‘Salary’ is also related to this use, with Roman soldiers thought to be paid part of their wages in salt, a valuable commodity that could be exchanged readily in markets.

BASES Sodium hydroxide history can be traced as far back as Ancient Egypt when it was used in early soap making. In Babylonian times it is believed that soap-like substances were in use for general bathing. Indeed, evidence has been found on a clay tablet dating back to around 2800bc that a soap-like material made from water, oil and lye was used for bathing, ‘lye’ being another name for sodium hydroxide. An interesting discovery about sodium hydroxide history concerns the Ebers Papyrus dating from around 1550bc which indicates that animal or vegetable fats were mixed with lye to produce a type of early soap. References in the papyrus indicate that this substance was used for bathing but that it was also used for washing wool, probably before it was woven into cloth

ACIDS Working in the late 1800s, Arrhenius stumbled on a new principle of acidity, while investigating a quite different problem. Arrhenius was interested in physics, not chemistry, and chose to work on the conductivity of electrolytes. In 1840, Kohlrausch, a German physicist, had invented a method of measuring very low conductivities using a kind of alternating current to detect the position of breaks in underground telephone wires. In the 1880s, Arrhenius adopted this instrument for his investigation. From his data, he came up with the idea that some compounds existed as charged bodies when dissolved in water. These bodies eventually became known as ions. Arrhenius’ professors were surprised, but unimpressed, by this and he was graded at a very low level for his doctorate. Later, he was to win the Nobel Prize for this work.

ARRHENIUS THEORY In the part of his study relating to acids, Arrhenius proposed that acids form H+ ions in solution and that it is this which then reacts. It is no longer part of the acid, but is produced in solution, and then itself reacts with bases and alkalis. The converse of this idea is that the alkaline component is the hydroxide ion, OH–. The neutralisation reaction is, essentially: H+ + OH– → H2O

Brønsted and Lowry Theory In 1923 Brønsted and Lowry were working to extend the notion that H+ is the agent of acidity. They noted the similarity of the reaction of ammonia and hydrogen chloride gases to make ammonium chloride with the parallel reaction in aqueous solution. They took the idea of an acid one stage further to include reactions where a hydrogen ion is transferred from one chemical species to another: HCl + NH3 → NH4+Cl– In this equation, the focus is on the reaction that defines the acid as a proton donor. Conversely, a base is a proton acceptor.

LIMITI DELLA TEORIA DI ARRHENIUS La teoria di Arrhenius presenta dei limiti: Le definizioni di acido e base sono valide solo in acqua Non spiega il comportamento acido basico di composti che non hanno gruppi OH- o H+ come CaO, CO2, NH3

TEORIA DI LEWIS Rappresenta un’estensione della teoria di Bronsted-Lowry. Acido: qualsiasi sostanza in grado di accettare una coppia di elettroni Base: qualsiasi sostanza in grado di fornire una coppia di elettroni non condivisi B: +A B:A

L’atomo centrale di una molecola con un ottetto incompleto può completare il suo ottetto accettando una coppia di elettroni: BF3 (acido di Lewis) + :NH3 (base di Lewis)
F3B:NH3 (addotto)

Questa definizione include la definizione di Brønsted-Lowry come un caso speciale dato che il protone può essere considerato un accettore di una coppia di elettroni e una base è un donatore di una coppia di elettroni: H+ + :OH-
H:OH Ogni acido di Brønsted, dato che fornisce protoni, è anche un acido di Lewis. La reazione fra un acido e una base genera un complesso: A + B
A-B H-A (acido di Brønsted) è il complesso formato dall’acido di Lewis H+ con la base di Lewis A-

Quando si parla di velocità di reazione un donatore di coppie di elettroni è un nucleofilo, un accettore di una coppia di e è chiamato elettrofilo

HYDROGEN ION The nature of the hydrogen ion in solution is not as easy as it seems. The simple explanation is that it is H+. However, there is evidence that the hydrogen ion cannot exist on its own in aqueous solution and many books describe it as a hydrogen ion combined with a water molecule, or H3O+.

pH The concentration of hydrogen ions in aqueous solution varies over a wide range. Soren Sorenson, a Danish chemist, created the pH scale to deal with this wide range in 1909. The origin of the term pH appears to be ‘potens hydrogen’ or ‘the potential of the hydrogen’. Sorenson defined pH as: pH = –log (concentration of hydrogen ions)

Un campione di latte ha pH= 6.5

Titolazioni Acido-Base Le titolazioni acido-base appartengono alla categoria dei metodi analitici chimici; esse fanno parte, in particolare, dei metodi basati sull'analisi volumetrica. Lo schema generale di questi metodi si basa sull'aggiunta, in piccole aliquote successive, di un determinato volume di uno standard a concentrazione (titolo) nota ad un determinato volume di una soluzione a concentrazione sconosciuta, da titolare. La soluzione standard, il titolante, viene aggiunta finché si verifichi una qualche variazione apprezzabile, tale da indicare che il titolante e la sostanza da titolare hanno reagito in maniera completa. Il requisito fondamentale di questo tipo di analisi è che titolante e sostanza da titolare reagiscano in maniera stechiometrica e che la reazione vada a completamento. La titolazione si conclude quando il numero di equivalenti del titolante (1) è uguale al numero di equivalenti della sostanza da titolare(2): si dice allora che si è raggiunto il punto di equivalenza N1*V1 = N2*V2

Vol. iniziale 20 ml HCl 0.1 N - Indicatore: Rosso fenolo NaOH 0.1 N NaOH (ml) Vol. tot. (ml) [HCl] N pH HIn (%) colore  0  20  0.10  1.0  100    + 10  30  0.033  1.5  + 9  39  0.0256  2.6  + 0.9  39.9  0.0025  3.6  + 0.09  39.99  2.5x10-5  4.6 99.8  + 0.05  40.04 NaOH=9.99x10-5 10 0.2                                                                                                                     Al punto di equivalenza, quando gli equivalenti della base forte sono uguali agli equivalenti dell'acido forte, gli unici ioni H+ presenti in soluzione saranno quelli provenienti dall'autoionizzazione dell'acqua e il pH sarà 7

Vol. iniziale 20 ml CH3COOH 0. 1 N Indicatore: Fenolftaleina - NaOH 0 Vol. iniziale 20 ml CH3COOH 0.1 N Indicatore: Fenolftaleina - NaOH 0.1 N NaOH (ml) Vol. tot. (ml) [acido]:[sale] pH HIn (%) col. 20 75:1 2.87 100   + 1 21 19:1 3.47 22 9:1 3.79 + 8 30 1:1 4.74 + 9 39 1:19 6.02 + 0.5 39.5 1:39 6.34 99.9 + 0.4 39.9 1:199 7.04 99.4 + 0.09 39.99 1:1999 8.03 94.9 + 0.01 40.00 1:9400 8.72 79.2 40.01 1:45000 9.4 44.3 + 0.03 40.04 1:178000 10.0 16.6