Periodycal #5

Litmus & lichens, and the negative pH debate

Jan 17, 2023

Welcome to another edition of the Periodycal newsletter! This month is a somewhat accidentally pH-themed edition, with a new graphic on litmus paper and an unexpected diversion into the merits (or not) of teaching students about pH values outside the usual 0-14 range.

There’s also an update on the website’s logo change (or lack of), a summary of upcoming topical chemistry infographics, and the usual round-up of interesting chemistry news and features.

Litmus and lichens

Infographic on litmus paper. The graphic explains how litmus dyes can be derived from orcinol, itself sourced from species of lichens. The chromophore of the litmus dye is 7-hydroxyphenoxazone. In acidic solutions, the protonated structure is red, while in alkaline solutions the deprotonated structure is blue. — Click to view and download this graphic on the Ci site

The newest graphic on the website takes a closer look at a common pH indicator: litmus paper. I was fascinated to learn a while back that litmus dye is extracted from particular species of lichens, something which I’d somehow never learnt during my years of teaching chemistry. This graphic looks at the origin of the dye and its surprisingly complicated chemical identity, as well as the structural changes that cause the change in its colour.

The pH scale and negative pH

"Today in chemistry history" graphic for 9th January, marking Danish chemist Søren Sørensen's birthday in 1868 and his invention of the pH scale. Sørensen came up with the pH scale to measure hydrogen ion concentrations in solutions while working at the Carlsberg research lab in Copenhagen. The pH scale runs from 0 to 14 and is a way of measuring how acidic or alkaline a solution is. It’s a logarithmic scale, so a change of one represents a ten times increase or decrease in acidity or alkalinity. A neutral solution has an equal concentration of hydrogen and hydroxide ions; this is usually at pH 7, but can vary slightly depending on temperature. — Click to view and download this graphic on the Ci site

I put together the graphic above a little over a week ago as the latest addition to the not-in-any-way regular ‘Today in Chemistry History’ series. It unexpectedly led to a lot of discussion in the comments on social media, particularly around the existence of negative pH values.

Negative pH is, of course, a calculable thing, as are pH values above 14. As this short letter to the Journal of Chemical Education explains, commercially available concentrated hydrochloric acid has a pH of approximately –1.1, while saturated sodium hydroxide solution has a pH of approximately 15.

As for how useful it is to flag this to students? I’m not so sure. I specifically left values below 0 and above 14 off the scale shown in this graphic because there’s a real difficulty in actually measuring them practically. Prior to university-level chemistry, students aren’t likely to need to deal with (and certainly won’t be able to measure) pH values beyond the 0-14 bounds. I can see the argument for students being aware that the scale can exceed the bounds shown here in some cases, but I think that awareness is the only real benefit prior to reaching higher education.

Even at the point where one might be dealing more regularly with substances that would register a negative pH, the Hammett acidity function is a more appropriate measure for dealing with these substances. Whenever we talk about superacids, it’s this figure we’re referring to, not pH — for example, fluoroantimonic acid, the strongest superacid, has a Hammett acidity function of –21, which would be an impossibly high hydrogen ion concentration/activity if interpreted as a pH value.

I’m always keen to hear your thoughts, and if you’re a teacher, how you approach negative pH (if at all!) with your students.

Logo update

Last edition I asked for your input on a potential logo change for the website. Thanks to everyone who voted and/or passed on their thoughts. There was overall a marginal preference for the molecule-style logo, but this wasn’t significantly higher than the number of people who expressed a preference for the current logo, so I’ve decided to stick with the original!

Upcoming chemistry tie-ins

Infographic on the chemistry of popcorn. The infographic highlights some key aroma compounds, including 2-acetyl-1-pyrroline. It also explains what makes popcorn pop: the escape of pressurised water vapour as the kernels are heated. — Click to view and download this graphic on the Ci site

There are a few dates coming up that I’ve got relevant chemistry graphics for, so here’s a quick run-down with links to the graphics in question:

19th Jan: National popcorn day: The chemistry of popcorn
20th Jan: National cheese-lovers day: The chemistry of camembert
23rd Jan: Gertrude Elion’s birthday
28th Jan: International Lego day: What are Lego bricks made of?

Finally, for the cold weather we’re experiencing in the northern hemisphere currently, this graphic in C&EN on the chemistry of snow and ice is topical!

Chemistry news and features

That’s all for this issue! As always, I welcome any thoughts you have on newsletter content or suggestions for new graphics. Let me know in the comments below.

Thanks for reading,

Andy

Mike Ryan

Only with A-level students!

I used to give the definition and let them play with it for acids they had seen. 0.1 M HCl they had seen in titrations and then extended to H2SO4 and then increased the concentration until the negative pHs "appeared". Inevitably someone asked about extremely dilute acids and then water and alkalis, so we dealt with them. A Eureka moment was when they found out for themselves that "neutral = pH7" (at 25 deg C) is more linked to the numbers than to being midway on the UI chart.

Thanks for all you do with Ci

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1 reply by Andy Brunning

McDanik

I agree with Andy - introduce pH's above and below the typical scale, but don't push the issue. This leaves them knowing that life is more complicated than 1st year Chem so they aren't blindsided later, but doesn't belabor the point.

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