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A month back, @CowperKettle had asked this at The Periodic Table chat room:

Does "alkaline" imply the presence of an actual alkali?

Now I've come across the term "alkaline" on numerous occasions, and pretty much all examples that I know of (in Chemistry) actually do involve the presence of an alkali (often NaOH or KOH).

Wikipedia defines an alkali as:

In chemistry, an alkali is a basic, ionic salt of an alkali metal or alkaline earth metal.

Further, it states:

An alkali also can be defined as a base that dissolves in water.

Now when something is a "base", it could mean a lot of things (An Arrhenius base? A Lewis base? A Bronstedt base?), which doesn't really make the Wikipedia definition any clearer.

Take for example, this case:

...mucosal cells of the small intestine secrete bicarbonate ions into the intestinal lumen to render it alkaline...

I know that was from a "Biology perspective", but that raises the following questions:

1) Is there a universally accepted (I guess that would be IUPAC) definition for an alkali? (Since the Wikipedia definition seems a bit oversimplified)

2) Is it "acceptable" to refer to, say, as suspension of a (Lewis) base in water as "alkaline"? And what about that excerpt from my Bio. text book? Is that use of "alkaline" acceptable as well?

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  • $\begingroup$ For the first question: the IUPAC Gold Book does not contain the entry "alkaline" or "alkali". $\endgroup$ – DHMO Jan 25 '17 at 11:00
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    $\begingroup$ Use of alkaline to refer to basic pH is certainly abundant in Biology. I've taught Anatomy/Phyiology as well as basic (alkaline?) Biology, it's commonly used in textbooks. $\endgroup$ – bpedit Jan 25 '17 at 18:17
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Most every English language dictionary definition of alkaline that I've found defines it as the adjective form of alkali and includes something like "having a pH greater than 7". As pointed out in the comments there is no IUPAC definition, so it's going to be difficult to come up with an authoritative scientific answer. Here are some interesting items I've found on the word's origin, usage in different contexts, etc.:

Online Enemology Dictionary:
alkali: late 14c., "soda ash," from Medieval Latin alkali, from Arabic al-qaliy "the ashes, burnt ashes" (of saltwort, which abounds in soda due to growing in alkaline soils), from qala "to roast in a pan." Later extended to similar substances, natural or manufactured. The modern chemistry sense is from 1813.

Cambridge Dictionary :
alkali: chemistry - a substance that has the opposite effect or chemical behavior of an acid. adj. alkaline

Biology Online: alkaline: (Science: chemistry) Having the reactions of an alkali. Origin: L. Alkalinus Relating to or containing an alkali; having a ph greater than 7; alkaline soils derived from chalk or limestone.A substance that has a ph higher than 7.

Merriam-Webster:
alkaline: : of, relating to, containing, or having the properties of an alkali or alkali metal : basic; especially, of a solution : having a pH of more than 7

The almighty Wikipedia, fairly representative of a half-dozen other online definitions:
In chemistry, an alkali (/ˈælkəlaɪ/; from Arabic: al-qaly القلي, القالي , “ashes of the saltwort”) is a basic, ionic 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.

I think in the end its usage is simply tied to the jargon and vocabulary of the context in which it is used. And with so many definitions, most of the ways that we hear it used are probably at least partially correct. And as is so often the case, I think the Wikipedia definition probably comes as close to the best definition as any.

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A Lewis base often also reacts as a Brønsted base, hence the sharing of the word "base", but that is not so by definition. Wrong, wrong, wrong.

"Alkaline" means Brønsted base containing alkali metal ions. The statement "An alkali also can be defined as a base that dissolves in water." is utter nonsense. Ammonia? Phenoles? They are NOT alkalis.

To say that a plain ammonia solution is alkaline is sloppy, at least.

Biology: All biologic tissue is full of sodium and potassium ions, so it's hard to argue against the use you mention.

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