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An excerpt from the online lecture I'm studying is as follows:

1880-Arrhenius defines an acid as a substance that , when dissolved in >water, produces an increased concentration of hydrogen ions.

1900-Naunyn combines Arrhenius' definition with Faraday's previous supposition that anions such Cl- are "base forming" (i.e. an acid) and cations such as Na+ are "acid forming" (i.e. a base) thus acid-base status wasn't determined solely by [H+] but also by a number of other common electrolytes.

Can someone explain to me how sodium behaves like a base and how chloride behaves like an acid (maybe equation that show sodium sequester a proton and another showing chloride somewhat provoking the release of a proton)?

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    $\begingroup$ Wow, I already see why I've never heard about this Naunyn's idea! $\endgroup$
    – Mithoron
    Commented Apr 23 at 18:09
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    $\begingroup$ It's like Lewis, but with acid and base switched. I guess it had to have some progenitors. $\endgroup$
    – Mithoron
    Commented Apr 23 at 18:24
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    $\begingroup$ You should look further into the context of his "idea". A side note: every idea might be worth considering ,But!. Also look into Faraday's reasoning. It might be involved with electrolysis where the anode oxidizing chloride becomes basic anions especially OH- migrate to the anode. H+ migrates to the cathode making it acidic. Also base-forming does not mean acid and acid-forming mean base unless you strictly define the process. $\endgroup$
    – jimchmst
    Commented Apr 23 at 20:20

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First of all remember molecules and their solutions are oblivious of the labels and titles given to them by humans. It is our imperfect knowledge that we cannot fully describe them in words in a highly generalized way...the way mathematicians like to treat their ideas. To us mortals, space is the geometric space, say a 3D room, but for them space is an abstract set with certain properties (hard to visualize!). In the same way, acid and base are old terms and they meant different things to different scientists in their domains. It is a kind of a pigeon hole effect...same words being used to different observation/properties of molecules. Naunyn's work is almost exclusively in German in uncommon and now extinct journals. I could not find his original German paper of early 1900s that describes this definition but most of the sources are secondary. At least it is clear that he was a physiologist. To quote

Ole Siggaard-Andersen (1977) The Van Slyke Equation, Scandinavian Journal of Clinical and Laboratory Investigation, 37:sup146, 15-20, DOI: 10.3109/00365517709098927 Link, subscription required to view https://doi.org/10.3109/00365517709098927

A chemist in a glassworks might prefer a base to be a metal oxide. An organic chemist, or a biochemist, would possibly prefer to let acid and base mean electron acceptor and electron donor respectively, but we should then expect him to say Lewis acid and Lewis base. A physiologist, or a clinician, might favour Van Slyke's description of acid-base regulation and disorders, and would therefore be willing to accept, e.g., sodium ion as a base, which, however, would not be understood by an ordinary chemist.

So certainly sodium ion is not a base for a typical chemist but it is okay for a physiologist of his time. Sodium ion when paired with bicarbonate ion, forms a mildly alkaline solution. Similarly, sodium ion when paired with formate ion, the solution will be weakly alkaline. In this sense, sodium ion is a base to a physiologist...not to chemists.

Chloride ion is an acid in his mind because when it is paired with ammonium ion, it forms an weakly acidic solution.

The definition you cite, is thus specific to physiologists/blood biochemists. It is and was not a generalized idea. Call it a jargon rather!

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