Throughout the history and prehistory of chemistry, color played a pivotal role. Archeologists found that colored glass-like stones were made both in Egypt and Mesopotamia before 4000 BC; the earliest synthetic pigment (blue ground frit, CaO-CuO-4SiO4) was already produced about 2650 BC[…] The alchemical hierarchy of metals – with lead at the bottom, copper somewhere in the middle, and gold at the top – is grounded on an aesthetic hierarchy of colors (black, red, yellow), which is incidentally preserved in the German flag. It might even be said that the whole obsession with gold, before it was established as a currency by convention, was based on nothing else than on the aesthetic preference of its color and shine. It is also well known that the 19th-century success story of the chemical industry had its main source in the mass production of synthetic dyes[…] their cheap and non-fade dyes rapidly spread all over the world and changed the visual environment in such way that it is fair to speak of an aesthetic revolution.
Joachim Schummer, Aesthetics of Chemical Products – Materials, Models, Molecules, in Hyle, 9, 2003, 76-77
Chemistry, as Schummer reminds us, is first and foremost a sensual experience, and I rejoice whenever there is a chance to run an experiment involving brightly coloured materials. So, when I started my exploration of simple inorganic nitrogen molecules, I did not have many reasons to cheer up: in fact, I thought that NO2 was the only coloured animal to be found in this bestiary1, and not a particularly pleasant one. During my PhD, the custom of cleaning glassware in boiling sulphonitric mixture2 meant that my nose became adept at picking up the distinctive pungent note of NO2 scratching its way into my nostrils like a clawed paw. An orange haze sometimes hovered above the boiling acid mixture, in particular when it was freshly made, or when it was rich in nitric acid (or when it was accidentally overheated). I often stood in awe, feeling safe on the other side of the closed sash of a fumehood as I kept staring at the orange vapours drifting upwards like flames rising from a liquid inferno. After a few months in the lab, I realised that I was able to detect NO2 effortlessly in the exhaust gases puffing out of the Diesel engine of buses, and I developed a vicious habit: as I was cycling behind an accelerating bus, I used to suck in a deep breath, only to wait for that acrid gas to hurt my nose. Possibly, it was a subconscious way of remaining constantly alert to the presence of this very toxic molecule: the nose is a fairly good detector, although it becomes saturated on a very short timescale.
Little did I know that I would have to wait a few extra years to quench my thirst for coloured molecules, and that would involve tearing a page from the inorganic nitrogen family album to dip it into organic chemistry, like a forgotten photographic film brought to life by a developing bath. That torn page reads nitrite.
So here I was, a researcher in an Oxford laboratory, and I was faced with this problem: how to detect and quantify micromolar (µM) levels of nitrite in solution. “Ion chromatography? Yes, why not…if only I could get hold of a column and the right detector before my fellowship ends! No, I can’t bide my time, I need something now”. Suddenly, a name emerged from a dusty drawer, from long-forgotten files of my undergraduate years: diazo coupling.
It was the spring of 2004, a season born of a cold March, when a late snowfall unexpectedly hit Milano on the 10th of that month. I was young and hopeful, energised by that carefree attitude of the twenty-somethings, and feeling worried only about the end of term exams. Chemistry was somewhat more coloured back then, although the dull ochre solid in my round-bottomed flask was the blatant evidence of a multistep organic synthesis gone wrong, a step of which involved using an ice-cold acidic sodium nitrite solution. Where was the bright orange dye3 that I was promised, the first really useful molecule I had ever made? A dripping pipette, the pH shifts, and the final colour is wrong. Matter, the unpredictable foe of every chemist, had got the upper hand once more, and the ugly brownish paste even ended up smearing my cotton sweatshirt. Luckily, this dye does not feature affinity for cellulose fibres, and it disappeared after the first wash, leaving behind only a bizarre whitish halo, as if the dye had bleached the original colour. I still own that tattered sweatshirt, a humbling memento of the inevitable failures inherent to the chemist’s trade.
Fast forward to 2014, and that multicolour drawer brimming with azo dyes was open again; it was nitrite that worked its magic, in the presence of an aryl amine, and so I thought that there must have been some commercially-available reactant for the spectrophotometric determination of nitrite. After skimming through the catalogues of chemical suppliers, I came across what I needed, the modern reincarnation of a time-honoured chemical assay: the Griess test.
On a typical procrastination spree, I went on to read some vintage papers4 by and on Peter Griess5 (1829 -1888). It was hard work (euphemism!), my German being rusty at best, or most likely completely pulverised into disconnected syntagms, and encroached by Dutch (no offence taken on either bank of the Rhine!). What a pity: one might rightfully claim that German is the most chemical language, not only because of some of its structures do remind me of chemical connectivity, but also considering the massive contribution of German chemists to the development of modern chemistry and chemical industry in the 19th century. While working my way through these publications, I came to appreciate the enormous impact made by Peter Griess on the development of organic chemistry, and I realised with astonishment that nobody, during my organic chemistry undergraduate classes, had ever mentioned his name. Every chemist knows Kekulé, but maybe not his contemporary Griess, the first to report on the preparation of aryldiazonium salts (in 1858)6, a reaction that paved the way for the development of synthetic dyes, and a veritable “rainbow revolution”, along with Perkin’s synthesis of mauveine in 1856. Griess’s original description of the diazonium salt prepared from picramic acid,
goes like this, in Hofmann’s translation6: “On passing a current of nitrous acid into an alcoholic solution of picramic acid […] the red liquid assumes at once a yellow colour, and furnishes rapidly a copious deposit of yellow crystals. […] The new body7, for which I propose the provisional name diazodinitrophenol, is soluble in alcohol and ether, and crystallizes from the former solvent in magnificent golden-yellow plates, which detonate on heating. […] on ebullition with water it appears to undergo decomposition; alkalis induce at once a copious evolution of gas […] the gas evolved consisting, according to a minute examination, of perfectly pure nitrogen“.
In addition, my foray into the life of Peter Griess let me peep behind the scenes of his scientific career…
Marburg is surrounded by gorgeous wooded hills, an idyllic setting ideal for a relaxing afternoon stroll away from the hustle and bustle of downtown academic life. Two gentlemen in their forties, walking stick in one hand, the other fiddling with the chain of the pocket-watch hanging from their vests, are chatting in a low voice while following an uphill path at a leisurely pace. The summer strides above them in the arched, cloudless sky.
– I wish I could keep him here, dear August – said one of the two gentlemen – you should really know how gifted this bloke is. He works flat out, he’s got intuition, and he’s committed. I’ve seen him in the lab these months, handling these diazo bodies, nasty stuff, he’s got practical skills, he’s curious, and hungry for knowledge. He could churn out paper after paper, and become a leading scientist in the field, were it not…
– Were it not for…
– Well…his father, his family, they’ve got no money to support him. His father, I don’t know, must be some sort of penniless farmer, a rustic peasant, whatever, he’s hard up. He can’t afford to pay for his son’s career anymore. That’s too bad for Peter, he’s already achieved so much despite his father’s pressure to find a permanent job and to settle down. But now, well, Peter’s got to find the means to keep himself afloat on his own. So I wish I could keep him in my lab, oh, if only I had funding for a research fellowship, then I’d tailor the vacancy on Peter! But no, and not even in the foreseeable future, nothing, nothing at all for him. I can’t hire anyone else, August. I’m worried Peter might end up following his father’s advice and quit academia.
August stops walking and looks straight into his friends’ eyes with a snug gaze, he knows what is next.
-Take him to London with you, to your lab, find a position for him, you won’t be disappointed! Let him work, give him freedom and equipment, and you’ll see what he can do in a couple of months!
Indeed. August is taken aback by his friend’s enthusiasm for this unknown young chemist, but he expected to hear this. He pauses and breathes deeply, while tweaking his moustache with his left hand. Then, trying to look as impassible as ever, he gently taps on his friend’s shoulder with the handle of his walking stick and says:
– Hermann, come on, come on, it’s not so easy! You know my lab in London is packed full as well. Yes, there’s lots of money for research in England these days…finally fat years after the lean ones…but when it comes to staff, there’s no vacancy at all! I’ve brought the most talented youngsters over from Germany, and I’ve already got my dream team…not even the my former supervisor at Giessen8 can boast such a research group. I wonder where could your Peter…
– Ehm… Griess
– Yes, I wonder where your Peter Griess would fit in this picture at all!
– Right, right, I see what you mean August, but let me try and convince you. I’ve got a printout of his latest communication, it’s a stunning piece of work, all new stuff, new reactions he’s stumbled upon and studied with the utmost commitment and passion. Read it tonight before going to bed, and sleep over it. And also, Peter will be in the lab tomorrow morning, very early. I’ve got to teach at 9, but why don’t you come with me, I’ll introduce you to him, and the two of you can have a quick chat. He’s going to show you what he’s working on, he won’t let you down. Please, August!
August is used to Hermann’s sanguine temper, and he does not feel like upsetting him. At any rate, why not taking this opportunity to read a freshly-printed paper and to talk about cutting-edge chemistry with his friend? August will think about this Peter Griess later on. But anyway, he had better be really good…
The two friends turn back, and walk home as the sun is setting.
Even today, how many professors’ chats like this9 can make or break the career of a young scientist? The year was 1858, but it seems like yesterday. Eventually, Griess impressed Hofmann and left (Hermann) Kolbe’s laboratory to join Hofmann’s group at London. This group was a scientific powerhouse, and Hofmann modelled it on his former supervisor’s laboratory at Giessen. Hofmann’s management of his own research group was simply astonishing, and thanks to his young talented coworkers “the London school stayed abreast of the latest chemical thinking on the Continent, and with their help the school became for a while the most productive organic chemical laboratory in the world after Giessen” 10. In a fruitful exchange between academia and industry, most of the young researchers left within three years, when “Hofmann got them better paying jobs, usually with British chemical firms.” 10. So did Peter Griess, who joined a brewery in 1862, in an early example of the typical scenario of the “young talented researcher leaving academia to get a (better-paid), steady job in the private sector and settle down”. A career shift from explosive diazonium salts to pale ale, and the relocation from London to Burton-upon-Trent: these must have been quite challenging times for Griess, who, to make matters worse, was not really proficient in English 6.
I should stop beating about the bush, let us go back to the lab. The classical Griess test involves a two-step reaction:
The first time I carried out this simple test, the bright colour filled me with such joy that I wrote a smiley in my laboratory journal (who on Earth has ever said that laboratory journals should be impersonal, devoid of emotions?). I kept staring at the lively pink solution dancing in the plastic tube…
…and I could not help thinking that lonely nitrite had finally found love in the twin embrace of the sulphanilic and napthilic moieties, the happy, colourful ending of nitrite’s tale. No sooner had I thought this than I started humming a well-known tune:
…Quand il me prend dans ses bras
il me parle tout bas
je vois la vie en rose…
(Edith Piaf, La vie en rose)
This “pink” (or, as my student used to say, “purple”), is nothing but a charming illusion created by the brain and its optical sensor, the eye. As far as light is concerned, wavelengths (symbol: λ) replace perception, and a suitable instrument (spectrophotometer) can record the following absorption spectrum for the azo dye obtained as a result of the Griess test. The peak maximum corresponds to λmax = 540 nm
I shall resist the temptation of boasting that my eye, like my nose, is also highly trained at recognising the faintest pinkish hue in very diluted samples. Honestly, I have been able to detect the colour unambiguously only at nitrite concentrations between 2.5 and 5 µM (not too bad!), when the measured absorbance ranges between 0.06 and 0.11 (for a 1 cm light path).
If you think that you have seen such ‘fluorescent’ pink in the lab before, you are probably right: Griess pink looks stunningly similar to the colour of a solution of another, but completely unrelated, dye, Rhodamine B.
Its absorption spectrum in ethanol shows a λmax at 542.8 nm, almost bang on, just a bit redder. I first met this dye during research on soft matter when I was an Erasmus student in 2006…but this is another story, another blog post.
Let’s go back to diazo dyes and nitrite. Since I wanted to determine the concentration of the analyte in my samples, I moved on to record the absorbance as a function of nitrite concentration, fitting the data with a linear fit routine to obtain a calibration “curve”. Then I sat down and stopped for a while, looking at the plot, and I somehow felt like a rambler who is about to set off on a long walk well-equipped with a detailed map. There is something reassuring about these straight lines shooting upwards from the interception of the Cartesian axes and cutting through the neatly arranged experimental points like the trajectory of a crossbow bolt piercing through a line of apples. Calibration curves trace a path, carving out an ordered, knowable space from the untidy, bewildering world that surrounds us. This treasure island affords quantifiable information, while all around rough seas, the messy business of life, threaten to wreck our frail boat. Instead, a calibration curve is a safe haven where one can lie at anchor and fathom the depth of the sea from a vantage point. There is yet another interesting aspect: calibration curves associate an observable phenomenon or quantity with a number (plus experimental error), and here I see a clear connection with the Pythagorean philosophical stance, which sought Nature’s harmony in the realm of numbers and proportions. Could calibration curves possibly be unconscious reincarnations of Pythagorean thought?
The unpredictable twists and turns of life do bend our trajectory from the linear path charted by calibration curves.
In 1869, a few years after moving to Burton-on-Trent, Peter Griess married Louisa Anna Mason, the daughter of the most renowned medical doctor in Burton. Tying the knot, settling down, starting his own family: we can only speculate what all of this could mean to this forty-year-old chemist in the second half of the 19th century. A dream coming true? The accomplishment of the bourgeois life path expected of all respectable men? The make-or-break attempt to try and integrate, once and for all, in the English lifestyle that Griess found so foreign to him?
Several letters provide a glimpse into Griess’s family life: he comes across as a caring husband and father. His wife soon fell seriously ill, eventually being unable to leave the house, and she was even bedridden for months. Griess “lovingly devoted to her all of his free time”, and “his ill wife was the focus of his loving care for years”4. Despite the joy from his sons’ achievements, Griess was filled with a deep longing for his homeland11, but his wife’s illness and the upbringing of his children prevented him from travelling back to Germany as often as he would have wished to. In particular, he had to turn down a tantalising job offered by his friend Heinrich Caro, by then a major figure at BASF.
Sadly, rarely can we resort to a calibration curve to find our bearings, as the experience of life is all too often an imperfect reaction, like that brownish dye I made in 2004. Seldom is there a colourful nemesis for nitrite.
1. To be honest, sodium nitrite is a (sickly) pale yellow crystalline solid, but nothing impressive enough.
2. A mixture of concentrated sulphuric acid and nitric acid. Yes, sulphuric, I like writing it the old way, it looks more ‘alchemical’.
3. I think it was a dye of the “Acid Orange” family, but I cannot recall its exact name.
4. See the bibliography at the bottom of the Wikipedia page on Peter Griess. In particular, Bopp, A., v. Hofmann, A. W. and Fischer, E., Zur Erinnerung an Peter Griess, Ber. Dtsch. Chem. Ges., 1891, 24: 1006–1078, makes for a very engaging read, full of first-hand insight into Griess, the chemist and the man.
5. Yes yes yes, I know, the purists are right. It ought to be written *Grieß*. I don’t feel like scaring the non-German reader this time…keep it simple.
6. Available in English as a report to the Proceedings of the Royal Society of London, translated by Griess’s mentor August Wilhelm von Hofmann: On New Nitrogenous Derivatives of the Phenyl- and Benzoyl-Series, Proc. R. Soc. Lond. 1857-1859 9, 594-597. Hoffmann was working at the Royal College of Chemistry at that time and shortly afterwards he offered Griess a position at his laboratory in London.
7. Körper in the German original. Body, as referred to a substance, is literally correct, although it might look a bit weird nowadays.
8. Justus von Liebig’s chemical laboratory was based at the University of Giessen (yes, purists, another ß gone)
9. The original German “transcript” of this chat, and the follow-up, can be found on pages 1023-1026 Zur Erinnerung an Peter Griess
10. William Henry Perkin, too, was working under Hofmann’s supervision at the Royal College of Chemistry when he serendipitously synthesised mauveine. More information in John J. Beer, A. W. Hofmann and the founding of the Royal College of Chemistry, in J. Chem. Educ., 1960, 37, p 248.
11. “Die Sehnsucht nach der alten Heimat hat Peter Griess nie ganz verlassen“, R. Wizinger-Aust, Peter Griess und seine Zeit, Angew. Chem. , 1958, 7, p 199