Annoying pesticides

With regard to Bárbara Pinho’s article on pesticides, it is wrong to assume that the introduction of DDT was a change from “harmless botany to dangerous chemistry”. While I would never advocate the widespread use of DDT, it is noteworthy that DDT has been marketed as a safe insecticide to replace lead, arsenic, fluoride, and nicotine, which can protect food crops from insects because DDT has very low acute toxicity for humans and mammals. It is impossible to say that lead, arsenic, sodium fluoride, and nicotine are harmless.

When I was in school the library contained two books from the 1950s that are considered shocking nowadays. One on pesticides argued that with the right insecticide choices we could put insect pests in the history books. The other argued on nuclear issues that within a few decades we would have a world-wide nuclear utopia. I think both books made overly optimistic predictions about their subjects.

It is interesting that the Insecticide Book described how lindane could kill DDT-resistant insects, suggesting that DDT resistance was known at the time. The big problem with DDT is its chronic effects. It loses hydrogen chloride with the formation of DDE (1,1-to– (4-chlorophenyl) ethene), which is lipophilic and very stable. DDE can bind to estrogen receptors and disrupt reproduction in birds and reptiles by making their eggs have thinner shells.

Mark Foreman CChem FRSC
Gothenburg, Sweden

Cooler copper catalysts

The highly active platinum-molybdenum carbide water gas shift catalyst (WGS), which initiates the reaction at low temperatures, can enable increased hydrogen release by equilibrating the WGS reaction at a lower temperature. In addition, a more active replacement for existing WGS catalysts could enable smaller converters that use less catalyst with lower pressure drop.

However, I am confused by the claim that copper based catalysts used in industry are ineffective below 300 ° C. Industrial, low-temperature, medium-temperature, copper-based catalysts used in most ammonia plants and some hydrogen flow sheets operate with a converter inlet temperature close to 200 ° C. The minimum operating temperature is often limited by the process gas dew point to prevent condensation of water on the catalyst to be avoided under normal operating conditions. The catalysts bring the WGS reaction into equilibrium with high carbon monoxide conversion until the end of its service life, typically a few years. The WGS reaction is exothermic and the converter outlet temperature is a factor for the WGS equilibrium position reached, the extent of the CO conversion and the carbon monoxide content in the product gas. In general, low temperature shift converters decrease carbon monoxide levels from 2-3 mole percent (dry) to 0.2-0.3 mole percent (dry) for a temperature increase of about 20 ° C, and medium temperature shift converters decrease carbon monoxide levels from 13-15 mol% (dry) to 2-2.5 mol% (dry) with an increase of approx. 100 ° C. Alternative isothermal switching converter designs with on site The cooling is usually done at around 250 ° C.

The article also states that unreacted carbon monoxide in hydrogen from the WGS reaction is a poison for platinum-based fuel cell catalysts, so a catalyst that can produce a pure stream of hydrogen and carbon dioxide would be beneficial. However, the process would require the separation of hydrogen and carbon dioxide and would have to be operated under conditions that reach WGS equilibrium with traces of carbon monoxide low enough to be tolerated by the fuel cell catalysts. This is likely to require low temperature operation using a converter on site Cool down to avoid a rise in temperature. Operating at low temperature may dictate operating at low pressure to control the dew point and avoid condensation of water, which requires recompression of the product hydrogen to a useful delivery pressure.

While catalyst development has excellent potential, the effects of scaling it up to industrial scale must be considered before the benefits can be fully assessed and the claims made in the article can be validated.

Peter Broadhurst CChem MRSC
Hutton Rudby, UK

Shooting for safety reasons

I was amused by the deeply clean comic strip in the lab. When I was doing my PhD at Nottingham University in the early 1960s, we had a simple and very effective way of dealing with strange chemicals created by research. In my lab, we worked on borohydrides and nitrides, many of which were extremely sensitive to air. Often times they were enclosed in ampoules and considered too dangerous to dispose of inside. Behind the new chemistry building was a sandstone cliff about 30 feet high, and at the base these vials were placed on a small pile of sand. The University Rifle Club, led by Col (Dr) Shaw of the division (and former Bisley fame), supported by my close friend Russell Molyneaux, took pot shots, ricochets as a result.

From my lab window, I could see the occasional flash of a successful hit as the contents of the flask burned away harmlessly. I have no idea what happened to the broken glass. Didn’t have to write a risk assessment back then!

Paul Roebuck FRSC
Stanton-on-the-Wolds, UK

John Cornforth

The letter “Living with Hearing Problems” states that John Cornforth, who was profoundly deaf, “was fortunate to have hearing problems after completing his formal education”. It’s not like this. For example, the Guardian obituary on Cornforth records that he began to experience hearing loss at the age of 10 and that as a student at the University of Sydney he “could not fully hear the lectures”.

Clifford Jones FRSC
University of Chester, UK


In ‘The Air We Breathe Out’ (World of chemistry, May 2021, p. 56) The “safest and most practical limit” for blood alcohol should have been 20 mg per 100 ml, and the modern breathalyzer used by law enforcement agencies are not triggered by breath acetone, which is produced by diabetic ketoacidosis . Many thanks to Robert Flanagan CChem FRSC for bringing these mistakes to our attention.


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