Letters April 2019

Weighed and measured

Peter Nelson (Chemistry World, February 2019, p4) raises the same concerns about the revision of the International System of Units (SI) as he has done previously (Chemistry World, October 2017, p4 and Chemistry World, October 2011, p43).

There is nothing ‘new’ about the SI units – instead it is the definitions of four of the SI base units that are changing in order to future-proof the global measurement system. The definition of units with respect to fixed numerical values of fundamental constants allows their realisation across a range of magnitudes of the unit’s scale, and not just with an uncertainty optimised for a specific material artefact. It also allows for realisation according to any workable experiment. A definition of the kilogram (or, for that matter, the mole) based on a fixed number of silicon-28 atoms, as he proposes, is inferior in both respects.

He also suggests that fixing the numerical value of the Avogadro constant will affect the molar mass constant. This is not correct. The fixed numerical value of the Avogadro constant has been carefully chosen to ensure that the molar mass constant is consistent with its historic value to within its newly acquired relative uncertainty of one part in 2 billion (a figure entirely insignificant for practical chemistry).

As for the variation of fundamental constants over time and space, there remains no compelling evidence of this, let alone over the time and length scales that would have any relevance for practical measurements.

Finally, in reply to Franz Wimmer (also Chemistry World, February 2019, p4), the mole is neither just a number, nor is it an arbitrary choice – recent publications on the mole’s redefinition explain why.¹

Richard Brown FRSC
Head of Metrology, National Physical Laboratory, UK

Reference

1 Brown R J C, Metrologia, 2018, 55, L25

Catch the carbon

Direct air capture of CO₂ from the atmosphere is a mad idea. Carbon dioxide is only present at a bit over 400ppm, so a huge amount of energy needs to be used to overcome entropy to capture it. It makes far more sense to capture CO₂ at the points where it is emitted. Nature does the best job, so we should instead be promoting carbon draw-down farming (also producing food) and reforestation (also restoring habitats and protecting ecological diversity). This is only the first half of the story – the CO₂ still has to be stored safely for geological timescales.

The idea to turn the CO₂ into fuel using natural gas or hydrogen (from natural gas or electrolysed water) is a cynical con. The CO₂ captured would be re-released when the fuel is burnt, so really this is just a way of burning hydrogen or natural gas very inefficiently. It would be far simpler, cheaper and more efficient to just use the hydrogen or natural gas directly as fuels. Apologists may say that excess renewable electricity could be used to make the hydrogen. If so, then don’t electrolyse the water and just store the electricity in a battery. Either way, the pretence of turning captured CO₂ into fuel is an illusion.

Why all the convoluted, over-complex thinking, fighting entropy with energy inefficient technology? The most viable long-term storage appears to be injection into basalt (or other alkaline mineral deposits to convert to insoluble carbonates). Here’s another really simple idea – why not do it all in reverse and instead mine, grind and seed the oceans with the basalt. This would take CO₂ out of the seas, precipitating carbonates from the seas (and reducing acidification to benefit coral and crustaceans). This needs research, but could probably be fairly harmless because it mimics natural weathering of alkaline minerals.

Nigel Howard CChem FRSC
Beacon Hill, Australia

Commenter of uncertainty

The word ‘certainty’ in the eye-catching title of the recent report on cryo-electron microscopy (Chemistry World, December 2018, p31) reminded me of a former experience commenting on a research article, in which I criticised the missing uncertainty analysis and overlooked discussion of measurement precision. The response from the authors, independent referees and editor was surprisingly discouraging. It is a disturbing fact that uncertainty analysis is becoming more frequently overlooked in scientific journals.

Simply reporting the average values for considerably scattered data points without providing any information about uncertainty analyses is rather unprofessional. These average values are thus meaningless, if not misleading, to readers in terms of accuracy and precision. Following the comment, one of referees stated that it is not the authors’ responsibility to provide such uncertainty analysis, and, instead, readers should be smart enough to judge results themselves.

I disagree. Uncertainty analysis should be routinely, carefully and professionally carried out for measurements. Without them, it is impossible for common readers to evaluate the precision and accuracy of results.

Yang Gan FRSC
Harbin, China

That would be a registered technical matter

I was very surprised to see the recent article on the under recognition of technicians (Chemistry World, December 2018, p55). When left the UK more than 50 years ago, there was always a route to higher qualifications on a part-time basis. The ability to go from O- or A-Levels through the Ordinary National Certificate or Higher National Certificate to become an associate of the then-Royal Institute of Chemistry paid dividends. I ended up in graduate school before heading moving to the US, but I worked for four years as a technician in the 11 years prior. Do such part-time routes still exist under the auspices of the Royal Society of Chemistry?

David Newman, CChem, FRSC
Wayne, US 

Editor: The Royal Society of Chemistry offers the Registered Science Technician (RSciTech) designation.