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Re: A Spindoctoring Classic

pH is the negative of the exponent of the molar concentration of hydrogen ions in aqueous solution (chemists here - yes - I know this isn't the real definition but you know what I am getting at) - a logarithmic, not linear scale. As a result, the 30% number is near enough right (the pH is not being quoted to sufficient accuracy to say otherwise.) The 150% figure is only correct in the worst case scenario, that we take 7.9 to mean 7.85 and 8.2 to mean 8.25. The real misinformation is to suggest that relative changes in pH mean anything at all. A change from pH of 2 to pH of 1 is huge in absolute terms, from 8 to 7 orders of magnitude less, even though the relative change looks the same.

I remember a particularly dim-witted moment in the lab when I couldn't seem to get distilled water that wasn't highly acidic, pH of 4 or 5. I couldn't find anything wrong with the distillation equipment. Fortunately it had my supervisor stumped as well (cue complaints about academic standards these days). The cause of the acidity was of course atmospheric carbon dioxide, the effect of which is much greater on the pH of completely unbuffered distilled water than seawater which is full of all manner of chemical crap.

Re: A Spindoctoring Classic

Pardon my ignorance (my field is electronics) but are you saying that a reduction in pH from 8.2 to 8.1 i.e. moving toward the neutral point, DOES result in an increase in absolute corrosiveness (it's ability to attack a metal or in this case calcium carbonate)? I understand it means an increase in the number of hydrogen ions but I thought that solution had to be genuinely acidic (pH <7)for it to be able to dissolve sea shells.

If I was wrong I owe some people some humble apologies


Re: A Spindoctoring Classic

I think "corrosiveness" is an unhelpful term. The "neutral" pH of 7 is nothing special - it's merely the result of the dissociation constant (10^-7 M) of water molecules in pure water at standard temperature and pressure or thereabouts. Calling solutions with pH<7 acidic and those with pH>7 basic or alkaline is just shorthand.

Seawater is actually extremely corrosive to metals due to the high availability of nonmetallic ions - just ask any north sea oily.

Re: Re: A Spindoctoring Classic

Look, chaps....pardon a mathematician among natural scientists, but if a solution moves towards a pH of 7 it becomes less 'corrosive', surely? If it is neither oxidising nor reductive it will tend to leave things alone?

Re: Re: Re: A Spindoctoring Classic

My hours spent in Stochastic Chemistry are haunting me right now.

My feeble memory is telling me that the fundamental difference between Acids and Bases was the concentration of H+ ions vs OH- ions. Acids had more H+ than OH- and bases had more OH- vs H+, which was their tendency to oxidize or reduce.

My memory also brings back images of mixing Strong Bases with Strong Acids which resulted in lovely exothermic reactions.