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In chemistry, an alkali (from Arabic al-qaly "ashes of the saltwort") is a basic, ionic compound salt of an alkali metal or alkaline earth metal chemical element. An alkali also can be defined as a base that dissolves in water. A solution of a soluble base has a pH greater than 7.0. The adjective alkaline is commonly, and alkalescent less often, used in English as a synonym for basic, especially for bases soluble in water. This broad use of the term is likely to have come about because alkalis were the first bases known to obey the Arrhenius definition of a base, and they are still among the most common bases.


Quotes are in chronological order
  • Alkalimetry. ...The object of alkalimetrical operations is to determine the quantity of caustic alkali, or of carbonate of alkali, contained in the potash or soda of commerce. These operations are simple, accurate, rapid, and easy; they may be said to consist in pouring on a weighed portion of the sample of potash or of soda under examination, a certain quantity of an acid of a known strength, until the alkali is saturated, that is to say, until the neutralizing point is hit, which is ascertained by means of litmus paper...
    • A. Normandy, The Commercial Hand-Book of Chemical Analysis: or, Practical Instructions for the Determination of the Intrinsic or Commercial Value of Substances Used in Manufactures, in Trades, and in the Arts. (1865) pp. 15-16.
  • Alkali. (Arabic, al Kali.) A name applied to a well-defined class of bodies characterized by the following properties. They turn red litmus paper blue, completely neutralize acids, they are soluble in water, and their solutions exert a caustic action upon animal matter. The alkalies proper are the oxides of potassium, sodium, rubidium, and cæsium. To these must be added the compound alkali ammonia, the oxide of the hypothetical metal ammonium, which used to be called the volatile alkali, in contradistinction to potash and soda, which were called fixed alkalies. The alkaline earths are the oxides of barium, strontium, calcium, and magnesium. The oxides of some other metals, such as silver, thallium, and lead, are also somewhat soluble in water, and possess slight alkaline properties.
    • G. F. Rodwell, ed., A Dictionary of Science; Comprising Astronomy, Chemistry, Dynamics, Electricity, Heat, Hydrodynamics, Hydrostatics, Light, Magnetism, Mechanics, Meteorology, Pneumatics, Sound, and Statics; Preceded by an Essay on the History of the Physical Sciences (1873) p. 41.

A Dictionary of Chemistry and Mineralogy (1807)[edit]

: With an Account of the Processes Employed in Many of the Most Important Chemical Manufactures to which are Added a Description of Chemical Apparatus, and Various Useful Tables of Weights and Measures, Chemical Instruments, &c. &c. Vol. 1, pp. 37-39, by Arthur Aikin, ‎Charles Rochemont Aikin.

  • Alkali. Alkaline Earth. The class of alkalis is amongst the most antient in chemical science, and one which has stood its ground through all the changes occasioned by modern discoveries, though with some modification.
  • The original application of the term alkali (which is of Arabian origin) was to express the acrid saline residue left in the ashes of the plant kali, after its combustion in open air. This was also very early known to the Greeks and Romans; by the latter termed, lixivial, which term is still retained, lixivium, or ley, meaning properly the soluble salt extracted out of vegetable ashes by the addition of water. From the circumstance of the ashes being the fixed or unvolatilized part of the plant, the rest having been dissipated by the combustion, the lixiviary salt was also called fixed alkali, a term still in universal use.
  • The proper fixed alkalies are of two kinds, the vegetable or potash, and the mineral or soda; the latter is found often native in immense quantities being the basis of rock salt, and is also the principal saline residue of many plants growing on the sea shore; and the former is contained in, and almost entirely procured from, the ashes of vegetables in general, not growing contiguous to the sea. Under the articles Potash and Soda, and Carbonat of Potash and Soda, these most important Salts are fully described.
  • Again, as the volatile ammoniacal salt most curable from most animal matter (also of great antiquity) was found to agree with the other alkalies in taste, and in many chemical properties, though not in fixity, this ammoniacal salt was also termed an alkali, but volatile, in opposition to the two former, which remain unaltered in very considerable heat, and are therefore, comparatively, fixed. The volatile alkali is described under the article Ammonia.
  • Of late years, some of the earths (especially barytes and strontian, which were unknown to antiquity) having been found to possess alkaline properties in no ambiguous degree, these have been by some chemists absolutely associated with the alkalies; by others have received the term alkaline earths, to express this resemblance which in the two above mentioned almost amounts to identity of properties; but in the two others, lime and magnesia, the agreement is only partial.
  • As it is but of little consequence which arrangement be adopted, provided an uniformity be observed, we have throughout the present work restricted the term alkali, to the three antient salts of that name; the two fixed, potash and soda, and the volatile ammonia. Under the appellation alkaline earth, we include the following, Barytes, Strontian, Lime, and Magnesia.
  • We shall now enumerate the properties usually described as belonging to alkalies, distinguishing how far they are possessed by the three salts and the four earths above enumerated.
  • The taste of an alkali is acrid, burning, and nauseous. It acts with so much energy and rapidity on the tongue, as to destroy, if concentrated, the skin of the part which it touches, and hence its extreme causticity. The three alkalies possess this in the highest degree, more rapidly soluble than the earths, and the latter only barytes, strontian, and lime, exhibit this corrosive taste, magnesia being absolutely insipid.
  • All the alkalies, except ammonia, are without smell, or nearly so, a particular urinous odor however arises during the solution with heat, of the other alkalies and earths, magnesia excepted.
  • Ammonia (whose natural state when uncombined, is that of a gas) magnesia, and probably lime, are incapable of crystallization, the other alkalies and alkaline earths are crystallizable.
  • Only the volatile alkali has been decomposed. The others have often been suspected to be also compound, and with some probability, but they have hitherto eluded the attempts of chemists to decompose them.
  • Ammonia alone is volatilized by the application of any degree less than a red heat; when fully red, or at a heat about that of melting copper, the fixed alkalies begin to be dissipated in dense vapours, but the alkaline earths resist any but the extremest degree of heat which has ever been applied.
  • They possess the strongest affinity for acids of all other bodies, uniting with them generally to such a degree as to produce perfect neutralization, or such a state of union, that the characteristic properties of both acid and alkali are lost, and new ones acquired. Most of the combinations with acids are considerably soluble in water, and all crystallizable with more or less ease.
  • The comparative force of affinity for the individual acids possessed by the alkalies and alkaline earths, is not uniformly the same, but generally one or two of the earths stand the highest in force. The affinity of them all for acids is superior to that of the metallic oxyds, and hence they decompose metallic solutions.
  • They all have their peculiar alkaline properties highly modified and lessened, but not entirely taken away, by union with carbonic acid. Hence the states of mild and caustic, the former expressing union with this acid; the latter, the contrary; which obtains in all these substances, though in magnesia alone the difference is scarcely perceptible as it regards the sensible properties.
  • They all unite with sulphur forming compounds soluble in water, of a peculiar fœtid smell, tarnishing the white metals, called sulphurets or hydro-sulphurets, according to circumstances.
    • Note: "Sulphurets" are compunds of sulphur with alkalies, earths, metals, carbon, etc. See William Campbell Ottley, A Dictionary of Chemistry and of Mineralogy (1826) "Sulphurets."
  • They all unite with the fixed oils and animal fats, either into a true soap, or into a saponaceous compound. The fixed alkalies unite more perfectly with the oils, and the alkaline are the only true soaps, being soluble in water. The earthy mixtures with oil are indeed strongly combined, but are insoluble in water.
  • Barytes will combine directly with oils, as M. Vauquelin has found, and when heated with animal matter, will cause it to give out a large quantity of ammonia, and will reduce it to a kind of saponaceous soluble mass.
  • When in solution in a red heat (from which of course the volatile alkali is excepted) they dissolve silex into that beautiful transparent compound called glass. This property of vitrification is the strongest in the alkalies, but has been found by M. Vauquelin to be very powerful in strontian and barytes.
  • With the exception of magnesia, they all powerfully corrode the soft parts of vegetables, and especially animals. Bulk for bulk, the fixed alkalies are by far the most powerful and active in this respect.
  • Lastly they all produce certain changes on some vegetable colours, and this test is perhaps the most striking of all above enumerated, since it is possessed by all without exception, though with some variety. The blue colours of many plants are changed by them to green, of which the syrup or tincture of violets, affords a ready instance; many of the reds, such as that of logwood and litmus, are changed into violet; and the yellow of a great many plants, such as turmeric, rhubarb, or liquorice root, is changed to a brown, or dirty brick red.
  • The latter change seems to be confined to the alkalies when both are carbonated, so that the change of turmeric-yellow into red will distinguish the presence of a carbonated alkali from that of an alkaline earth, held in solution by an excess of carbonic acid. But when pure or caustic, the change is the same in both.
  • The power of all these substances over vegetable colours is very great in proportion to the quantity used, and it should be remarked, that in this instance also, acids and alkalies, or alkaline earths, are in direct opposition to each other, the acid restoring the colour to its original state, after it has been changed by the alkali, and vice versa. Hence a single coloured vegetable may be made a very delicate test for both acids and alkalies; litmus, for example, previously reddened by acids, will be changed into purple by alkalies, and purpled litmus will detect an acid by becoming red.
  • It appears therefore that there is scarcely a single characteristic except the change of certain colours, that will apply equally to all the alkalies and akaline earths, but yet the general resemblance is so great as to justify this classification.
  • Magnesia alone has the least claim to the title of an alkaline earth, and yet its affinity to acids is strong, and its power of changing some of the vegetable colours is very considerable.

Heroes of Science. Chemists. (1883)[edit]

by M. M. Pattison Muir
  • About 1750 Black went to Edinburgh University to complete his medical studies... The attention of medical men was directed at this time to the action of lime-water as a remedy for stone in the bladder. All the medicines which were of any avail in mitigating the pain attendant on this disease more or less resembled the "caustic ley of the soap-boilers" (...caustic potash or soda). These caustic medicines were mostly prepared by the action of quicklime on some other substance, and quicklime was generally supposed to derive its caustic, or corrosive properties from the fire which was used in changing ordinary limestone into quicklime.
    When quicklime was heated with "fixed alkalis" (i.e. with potassium or sodium carbonate), it changed these substances into caustic bodies which had a corrosive action on animal matter; hence it was concluded that the quicklime had derived a "power"—or some said had derived "igneous matter"—from the fire, and had communicated this to the fixed alkalis, which thereby acquired the property of corroding animal matter.
  • Black thought that he might be able to lay hold of this "igneous matter" supposed to be taken by the limestone from the fire; but he found that limestone loses weight when changed into quicklime. He then dissolved limestone (or chalk) in spirits of salt (hydrochloric acid), and compared the loss of weight undergone by the chalk in this process with the loss suffered by an equal quantity of chalk when strongly heated. This investigation led Black to a fuller study of the action of heat on chalk and on "mild magnesia" (or as we now say, magnesium carbonate).
1. It is much decreased in bulk.
2. It loses weight (twelve parts become five, according to Black).
3. It does not precipitate lime from solutions of that substance in acids (Black had already shown that mild magnesia does precipitate lime).
  • He then strongly heated a weighed quantity of mild magnesia in a retort connected with a receiver; a few drops of water were obtained in the receiver, but the magnesia lost six or seven times as much weight as the weight of the water produced.
  • Black then recalls the experiments of Hales, wherein airs other than common air had been prepared, and concludes that the loss of weight noticed when mild magnesia is calcined is probably due to expulsion, by the heat, of some kind of air.
  • Dissolving some of his mild magnesia in acid he noticed that effervescence occurred, and from this he concluded that the same air which, according to his hypothesis, is expelled by heat, is also driven out from the mild magnesia by the action of acid. He then proceeded to test this hypothesis.
  • One hundred and twenty grains of mild magnesia were strongly calcined; the calcined matter, amounting to seventy grains, was dissolved in dilute oil of vitriol, and this solution was mixed with common fixed alkali (potassium carbonate). The solid which was thus produced was collected, washed and weighed; it amounted to a trifle less than one hundred and twenty grains, and possessed all the properties—detailed by Black—of the original mild magnesia. But this is exactly the result which ought to have occurred according to his hypothesis.
  • The next step in the investigation was to collect the peculiar air which Black had proved to be evolved during the calcination of mild magnesia.
  • To this substance he gave the name of "fixed air," because it was fixed or held by magnesia. Black established the existence of this air in the expired breath of animals, and also showed that it was present in the air evolved during vinous fermentation. He demonstrated several of its properties; among these, the fact that animals die when placed in this air.
  • Black did not hamper the advance of chemistry by finding a "principle of alkalinity;" but neither did he give a full explanation of the fact that certain bodies are alkaline while others are not. He set himself the problem of accurately determining the differences in composition between burnt (or caustic) and unburnt (or mild) alkali, and he solved the problem most successfully. He showed that the properties of mild alkalis differ from those of caustic alkalis, because the composition of the former differs from that of the latter; and he showed exactly wherein this difference of composition consists, viz. in the possession or non-possession of fixed air.
  • Strange we may say that this discovery did not induce Black to prosecute the study of caustic alkalis: surely he would have anticipated Davy, and have been known as the discoverer of potassium and sodium.
  • In the time of Stahl the name "salt" was applied... to the substance produced by the union of an acid with an alkali; but the same word was used by the alchemists with an altogether different signification.

A History of Chemical Theories and Laws (1907)[edit]

by M. M. Pattison Muir. A source.
  • The chemical histories of the three classes of compounds acids, salts, and bases, are closely interwoven. Black's quantitative experiments led to the division of alkalis and earths into two classes; mild alkalis and mild earths, and caustic alkalis and caustic earths or quicklimes.
  • [A]t about the time of the discovery of oxygen, salts were thought of as compounds formed by the addition of acids to bases which were generally alkalis or earths; if a mild alkali or a mild earth was the basis, fixed air escaped; if a caustic alkali or earth was the basis, fixed air was not produced.
  • Besides being built on the bases of alkalis and earths, salts could be formed by adding acids to the calces of metals. When Lavoisier had proved calces to be oxides, the theory of the composition of acids and salts was almost complete. Analogy indicated that the alkalis and the earths must be oxides of metals. If experimental investigation should confirm this supposition, the edifice was finished.

Watts' Dictionary of Chemistry (1911)[edit]

Vol. 1. pp. 111-112, by Henry Watts
  • Alkali (Arabic = the ash). This term was originally applied to the ashes of sea-plants; but it was soon extended to include substances which, like the ash of sea-weed, easily dissolved in water, forming solutions which had a soap-like action on the skin, affected the colour of plants, and reacted with acids with effervescence and the production of new substances wherein neither the properties of the acids nor those the alkalis were prominent.
  • Little or nothing was known regarding the composition of alkali until the year 1755 when Black (on the occasion graduating as MD at Edinburgh) published dissertation on 'Magnesia Alba, Quicklime, other Alkaline Substances.' Magnesia alba dissolved in acids with effervescence; but after being strongly heated no effervescence attended the solution of this alkali.
  • The notion of Basil Valentine (end of 15th and beginning of 16th century), that lime when burnt combined with 'matter of fire,' had been accepted by many as an explanation of the difference in the behaviour towards acids of burnt and unburnt lime. If this explanation applied to magnesia it should be possible perhaps to get hold of this 'matter of fire,' which combined with the magnesia alba when that body was heated. But Black found that a given mass of magnesia alba weighed more than the calcined magnesia obtained from it. Hence something was lost instead of gained during the process of heating. This something proved on further quantitative examination to be a gas different from common air; to it Black gave the name of fixed air.
  • The effervescence or non-effervescence of alkalis with acids was proved by Black to accompany the presence or absence of fixed air (carbonic acid). From this time a distinction was clearly drawn between alkalis, which dissolved in acids without effervescence, and carbonated alkalis, the solution of which in acids was accompanied by the escape of carbonic acid gas. It was recognised that whether a caustic or a carbonated alkali dissolved in an acid, the body which remained in solution, and which had no close resemblence either to the acid or the alkali, was one and the same.
  • The properties of the alkalis were supposed by the older chemists to be due to a 'principle of alkalinity,' or sometimes to a 'principle of saltness,' which latter principle was common to acids, alkalis, and the products of their mutual action, i.e. salts.
  • Closely allied to, and sometimes regarded as identical with, the alkalis, was the group of earths. These bodies were known to neutralise acids and affect colouring matters like alkalis, but they were much less soluble in water than the alkalis.
  • It was taught by some chemists that an alkali is hidden in every earth, and by others that an alkali is an earth refined by the presence of acid and combustible matter. Black's exact quantitative investigations tended to disparage all such explanations as these; but it yet remained to find the precise composition the alkalis and the earths.
  • Lavoisier thought that these bodies must be compounds; but, as he had no means of proving this, he classed them with the elements, while suggesting that the earths were probably compounds of oxygen with unknown metals.
  • In 1807 Davy decomposed two alkalis, potash and soda, by passing an electric current through these substances when molten; and a year later he succeeded, by the same agency, in separating the earthy bodies lime, baryta, and strontia, into oxygen and, in each case, a metal.
  • The name alkali is now generally applied to the compounds of hydrogen and oxygen with one or other of the five metals, lithium, sodium, potassium, rubidium, cæsium (v. Alkalis, Metals of the); an aqueous solution of ammonia is also regarded as containing an alkali, viz. a compound of hydrogen and oxygen with the radicle ammonium (v. Ammonium Compounds).
  • The alkalis are classed with the hydroxides, i.e. compounds of hydrogen and oxygen with a third element, rather than with the hydrates, i.e. compounds of water with an oxide or a salt. (v. Hydrates).
  • The general formula of the alkalis is written MOH rather than M2OH2O; M = Li, Na, K, Cs, Rb, or NH.
  • The alkalis are very soluble in water; these solutions neutralise acids forming salts, and also precipitate most of the heavy metals from their solutions in the form of oxides or hydrated oxides; aqueous solutions of the alkalis act corrosively on animal and vegetable substances, and also alter the tint of many colouring matters.
  • Lithia is much less soluble in water than the other alkalis.
  • The solid alkalis are not decomposed by the action of heat alone.

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