Letters: June 2021

Pesky pesticides

Regarding Bárbara Pinho’s article about pesticides, it is wrong to suggest that the introduction of DDT was a change from ‘innocuous botany into hazardous chemistry’. While I would never advocate the widespread use of DDT, it is noteworthy that DDT was marketed as a safe insecticide to replace lead, arsenic, fluorides and nicotine, thus allowing food crops to be protected from insects, as DDT has a very low acute toxicity to humans and mammals. It is impossible to claim that lead, arsenic, sodium fluoride and nicotine are innocuous.

When I was at school, the library contained two books from the 1950s that would be regarded as shocking nowadays. One, on pesticides, argued that with the right choice of insecticides we could banish insect pests to the history books. The other, on nuclear matters, argued that within a few decades we would have a worldwide nuclear powered utopia. I think that both books made excessively optimistic predictions about their topics.

It is interesting that the insecticide book did describe how lindane was able to kill DDT-resistant insects, indicating that DDT resistance was known then. The great problem with DDT is the chronic effects. It loses hydrogen chloride to form DDE (1,1-bis-(4-chlorophenyl) ethene), which is lipophilic and very stable. DDE can bind to estrogen receptors and can disrupt the reproduction of birds and reptiles by causing their eggs to have thinner shells.

Mark Foreman CChem FRSC
Gothenburg, Sweden

Cooler copper catalysts

The highly active platinum-molybdenum carbide water gas shift (WGS) catalyst that initiates reaction at low temperature may allow increased hydrogen output by taking the WGS reaction to equilibrium at lower temperature. In addition, a more active replacement for existing WGS catalysts might allow smaller converters using less catalyst with lower pressure drop.

However, I am confused by the assertion that copper-based catalysts used in industry are ineffective below 300°C. Industrial copper-based low temperature shift and medium temperature shift catalysts, used in most ammonia plants and some hydrogen plant flowsheets operate with a converter inlet temperature close to 200°C. Minimum operating temperature is often constrained by the process gas dew point to avoid water condensation on the catalyst at the usual operating conditions. The catalysts take the WGS reaction to equilibrium until near the end of operational life, typically a few years, with high carbon monoxide conversion. The WGS reaction is exothermic, and the converter exit temperature is a factor in the WGS equilibrium position achieved, the extent of CO conversion and the carbon monoxide level in the product gas. In general terms, low temperature shift converters lower carbon monoxide levels from 2–3 mol% (dry) to 0.2–0.3 mol% (dry) with an approximately 20°C increase in temperature, and medium temperature shift converters lower carbon monoxide from 13–15 mol% (dry) to 2–2.5 mol% (dry) with an approximately 100°C increase. Alternative isothermal shift converter designs with in situ cooling usually operate at about 250°C.

The article also states that unreacted carbon monoxide in the hydrogen from the WGS reaction is a poison for platinum-based fuel cell catalysts, such that a catalyst that can make 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 need to operate under conditions that achieve WGS equilibrium with trace levels of carbon monoxide low enough to be tolerated by the fuel cell catalysts. This is likely to require low temperature operation using a converter with in situ cooling to avoid a temperature rise. Low temperature operation may dictate low pressure operation to control dew point and avoid water condensation, necessitating the recompression of product hydrogen to a useful delivery pressure.

While the catalyst development has excellent potential, the implications of scale up to industrial scale need to be considered before the benefit can be fully assessed and the claims made in the article validated. 

Peter Broadhurst CChem MRSC
Hutton Rudby, UK 

Shooting for safety

I was amused at the laboratory deep clean cartoon strip. When I was a PhD student at Nottingham University in the early 1960s, we had a simple and very effective way of dealing with strange chemicals generated by research. In my lab we were working on boron hydrides and nitrides, many of which were extremely sensitive to air. Often, they were sealed in ampoules and thought to be too dangerous to dispose of inside. Behind the new chemistry building was a sandstone cliff, about 30 feet high, and at the base these ampoules were set up on a small mound of sand. The University Rifle Club, led by Col (Dr) Shaw of the department (and of former Bisley fame), assisted by my close friend Russell Molyneaux, would take pot shots, ricochet being one outcome.

From my lab window I could see the occasional flashes from a successful hit when the flask contents burnt away harmlessly. I have no idea what happened to the shards of glass. Didn’t have to write a risk assessment in those days!

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

John Cornforth

The letter ‘Life with hearing difficulties’ states that John Cornforth, who was profoundly deaf, ‘was fortunate that his hearing difficulty came after he had completed his formal education’. That is not so. It is recorded for example in the Guardian obituary of Cornforth that he started to observe hearing loss at age 10 and that when he was an undergraduate student at the University of Sydney he was ‘unable fully to hear the lectures’.

Clifford Jones FRSC
University of Chester, UK

Correction

In ‘The air we exhale’ (Chemistry World, May 2021, p56) the ‘safest and most practicable limit’ for blood alcohol should have been 20mg per 100ml, and the modern breathalysers used by law-enforcement are not triggered by breath acetone produced by diabetic ketoacidosis. Thanks to Robert Flanagan CChem FRSC for alerting us to these errors.