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First, the alloy $(0.5$ to $\pu{2 g})$ should be treated by $\pu{10 mL}$ nitric acid $32\,\%.$ All metals will get dissolved, except tin and silicon, which will be transformed into insoluble dioxide $\ce{SnO2}$ or $\ce{SiO2}$. Dilute in $\pu{100 mL}$ hot water. $\ce{SiO2 + SnO2}$ will make a gelatinous precipitate, that can be eliminated by filtration. Add $\... 14 The state of a compound, solid, liquid, or gaseous is a property which is unrelated to the property / strength of a Brønsted or Lewis acid of said compound. Thus, you can not infer from its physical appearance if an acid is safe enough to be touched by skin; it may be irritant / corrosive even in absence of water (e.g., p-TsOH as a solid acid, triflic acid ... 6 The problem is that there are not many alkaline stains you will find on household objects. Professor Robert Winston just did a demo for attracting attention of the public towards science (so-called outreach experiments). He simply used an acid-base indicator, perhaps thymolphthalein, which is blue in basic medium and colorless in slightly acidic or even in ... 5 Boric acid is a very weak acid with a$\mathrm{p}K_\mathrm{a}$value of 9.2 and does not dissociate in aqueous solution as a Brønsted acid. Yet, in aqueous solutions, it acts as a Lewis acid by accepting a hydroxyl ion to form the tetrahydroxyborate ion$(\ce{B(OH)4-}), as confirmed by Raman spectroscopy (Ref.1): $$\ce{B(OH)3 + 2H2O <=> B(OH)4- + H3O+}... 3 During neutralization of the buffer by a strong base solution, there are ongoing three reversible reactions, maintaining three equilibria:$$ \begin{align} \ce{H2O + A- &<=> OH- + HA} &\quad K_\mathrm{b} &= \frac{\ce{[OH-][HA]}}{\ce{[A-]}} \tag{1}\\ \ce{H2O &<=> OH- + H+} &\quad K_\mathrm{w} &= \ce{[H+][OH-]} \tag{2}\\ \... 2 Fresenius' analytical scheme is also one possibility, although it is not as direct. More information can be found in Harvey's Analytical Chemistry 2.1 Libretext. I bring this up to highlight that even though this is a classical technique, it is still mentioned in some analytical textbooks still in active use. 2 I do not see any reason why the following reaction cannot take place, which will consume\ce{[B(OH)4]-}$and the concerned reaction will move in forward direction to increase the acidity of orthoboric acid$^1\$. According to wikipedia, "With polyols containing cis-vicinal diols, such as glycerol and mannitol, the acidity of the boric acid solution is ...