{"id":816,"date":"2023-08-14T20:32:25","date_gmt":"2023-08-14T20:32:25","guid":{"rendered":"https:\/\/sites.rutgers.edu\/motley-emblem\/?p=816"},"modified":"2023-10-07T16:38:58","modified_gmt":"2023-10-07T16:38:58","slug":"dyebath-acidity-and-litmus","status":"publish","type":"post","link":"https:\/\/sites.rutgers.edu\/motley-emblem\/dyebath-acidity-and-litmus\/","title":{"rendered":"Dyebath Acidity and Litmus"},"content":{"rendered":"
\"\"
Like a conjuring trick: three hues from one solution of brazilwood dye.<\/figcaption><\/figure>\n

Many organic dyes, what the eighteenth-century colour-makers called “vegetable” colors, are sensitive to the alkalinity or acidity of the dyebath.\u00a0 I have elsewhere shared the range of bright reds and purples that can be got from brazilwood dye, by progressively adding just a few grams of mildly acidic alum; I have also repeated experiments from eighteenth-century recipe-books on the yellows of buckthorn and the purplish extract of violets.\u00a0 And once, while I was trying to figure out what was meant by “strong” aqua fortis, I happened upon a property of cochineal which the eighteenth-century color-makers also already knew. \u00a0Instead of the bright red scarlet I was after, I got something that looked like toxic lemonade.\u00a0 My strong aqua fortis, in short, was much, much stronger than theirs.<\/p>\n

\"\"<\/a>
Robert Boyle, at the Science History Institute<\/figcaption><\/figure>\n

For colourmakers, this property was useful for perfecting the hues of pigments.\u00a0 In the hands of chemist Robert Boyle, however, these secrets suggested a tool for analytical chemistry.\u00a0 Boyle was writing in the 1660’s, and, by his moment, certain techniques were already well known.\u00a0 It was well-known among alchemists that the “syrup” of violets<\/a> could be transformed from red to blue by the addition of oil of vitriol, then back again, by the lees of wine.\u00a0 Colormakers knew that a juice extracted from turnsole\u2014a low, shrubby annual\u2014could be shifted using lemon juice or distilled urine; medieval illuminators depended on this trick to derive a range of colors from a single dyestuff.\u00a0 And he furthermore learned from the dyers that other vegetable liquors, like cornflower or privet berries, or buckthorn, cochineal, and brazilwood, could be similarly adjusted\u2014though, in each case, each artisanal tradition depended on different additives, like alewort or fermented bran, or slaked lime or pearl ash.<\/p>\n

\"\"
Boyle’s vegetable-dye trick sorts solutions into acids (blue) and alkalis (orange\/red).<\/figcaption><\/figure>\n

Boyle’s insight was to collect all these observations in the same place\u2014noticing that each process contained the kernel of the same sort of reaction.\u00a0 He moreover saw that all aqueous solutions could therefore be described in terms of their effects on these organic substances, that the effect, as it were, sorted reagents into categories.\u00a0 He had taken a few trade secrets and alchemical experiments, and used them to make broad observations about the invisible qualities of solutions.\u00a0 Boyle doesn’t seem to have been the first person to have made this leap.\u00a0 But his insight was nevertheless an important waypoint in the history of what would become litmus paper, that rough-and-ready tool still among the most important utensils in the chemist’s laboratory.<\/p>\n

\"\"<\/a>
Two cochineal scarlets achieved only by applying different mordants: aluminum (left) and a tin solution called stannic chloride (right).<\/figcaption><\/figure>\n

But what causes the color-shifting trick: that remained a mystery.\u00a0 Boyle knew that it only works on organic dyestuffs; it has no effect on mineral pigments, like bice<\/a> or the mercuric sulfide called “vermillion.”\u00a0 What is more, this process is different than what happens when vegetable dyes are mordanted, as when we add alum to brazilwood in the first step of the modern laking process.\u00a0\u00a0 While different mordants produce different effects (i.e. the difference between Dutch and Venetian Scarlet) adding a mordant triggers a transformation that happens once, and is mostly irreversible.\u00a0 But adjusting the pH level of a dyebath can shift the color in ways that can be easily reversed: a few milliliters of muriatic acid, for instance, nudges a brazilwood dye towards blue, but a gram or two of alkaline pearl-ash prods it back toward the red.<\/p>\n

Here’s where this post gets “wonky,” as the economists say, as in, “preoccupied with arcane details or procedures in a specialized field.”\u00a0 Skip the next four paragraphs if you’d like to get to the punchline.<\/p>\n

\"\"<\/a>
The structure of carminic acid, a typical organic compound.<\/figcaption><\/figure>\n

Boyle speculated that acids and bases “insinuated themselves into the pores” of the “tingeing particles,” thus changing the “asperity” of their surface (and their reflectance properties). \u00a0That’s a good start. \u00a0But with the advantage of insights made possible by Boyle’s analytical tool, we can now sharpen up our sense of what’s going on, in a molecular sense.\u00a0 An acid is a liquid which, slightly counterintuitively, is prone to donate protons; although acids burn, and can sort of melt things like metals away in to liquid, they are actually, at a molecular level, just very, very aggressive donors.\u00a0 An acid has an excess of protons, and when another substance is dissolved in it, it is especially active in trying to deliver its extra positive charge.\u00a0 An alkali is the opposite; it is a liquid which wants to pry protons away from anything dissolved in it.\u00a0 It is for this reason that acids and alkalis neutralize each other: acids donate protons which alkalis gladly accept.<\/p>\n

Organic dyes are large chains of carbon rings, studded with extra protons in the form of hydrogen (H) or hydroxyl groups (-OH); they are, therefore, particularly apt to be modified by acids and alkalis, since those rings can accept more protons, or the ones it already has, can be cracked off.\u00a0 Put an organic molecule in an alkaline dyebath, and it cracks some of those limbs off.\u00a0 Put in an acidic one, and it adds a few extra.\u00a0 In either case, the total structure of the whole molecular compound is subtly changed.<\/p>\n

Now: when a photon, the smallest quantum of light, strikes a dye molecule, it is absorbed by an electron; that extra energy shifts it into a higher orbit.\u00a0 But the electron can only stay in that higher orbit briefly\u2014and when it finally collapses back into its lower, more comfortable state, it sheds a photon in compensation, which is of a wavelength exactly equal to the energy it loses.\u00a0 In the case of single atoms, these wavelengths can be calculated based on the structure of the nucleus and its electrical charge.\u00a0 But in the case of organic compounds, the effects get complex.\u00a0 Because carbon is such a good conductor, some of the electrons of an organic compound are shared across the total molecular structure.\u00a0 When a photon strikes that molecule, therefore, it excites an electron which is part of that whole cloud-like field; and when that cloud sheds that energy, it is one electron somewhere in the whole cloud which emits that photon.<\/p>\n

\"\"<\/a>Changing an organic compound, therefore, by adding or stripping away a proton, effects subtle changes in the whole molecule’s total reactivity.\u00a0 Stripping away a proton means that the whole compound has more of those relatively free-floating electrons; the molecule becomes more reactive, and therefore liable to release higher-energy, shorter wavelength photons.\u00a0 Shorter wavelengths shift towards blue.\u00a0 Conversely, adding a proton\u2014by putting it in an acidic environment, for instance\u2014renders the whole compound relatively electron-poor, in the sense that there are fewer electrons that aren’t tightly bound to protons.\u00a0 This produces a less-reactive system, which will emit lower-energy photons.\u00a0 Lower energy means longer, slower wavelengths: more red.<\/p>\n

This, then, is the punchline, the explanation of that color-shifting effect.\u00a0 An acidic solution makes the dye molecule less reactive.\u00a0 The photons it emits are less energetic: more red.\u00a0 The alkaline one makes it more reactive.\u00a0 The photons it emits are more energetic: more blue.\u00a0 Now, it isn’t quite as simple as this; the colors we see don’t simply slide along a spectrum, for reasons that partly have to do with how the eye perceives colors.\u00a0 But the direction of the shift with organic dyes is mostly consistent: acidic is towards red, and alkaline, towards blue.<\/p>\n

\"\"<\/a>
Caltech chemist Arnold O. Beckman’s “Acid Meter,” designed and built to test the acidity of California lemons.<\/figcaption><\/figure>\n

I mention this all at some length\u2014the color shifting trick, and its relationship to the pH scale\u2014in part because I’ve done this research, and written up these findings, under funding by the Beckman Center for the History of Chemistry, which is (in effect) pH money.\u00a0 Arnold Beckman<\/a> made his first fortune developing the portable analytical tool which superseded Boyle’s invention\u2014and it is largely from that device, and the company Beckman founded to build analytical instruments, that the Beckman Center received its major endowment.\u00a0 Exactly two floors beneath my carrel is the public wing of the same institution, the Science History Museum, where hangs the so-called “Shannon” portrait of Robert Boyle.\u00a0 Here, a certain historical circuit is completed– or, I mean, *almost* completed; Boyle, flanked by his major publications, gazes serenely down on exhibits which include the history of modern dyestuffs, and on the early history of chemical instruments.\u00a0 It is one of the great disappointments to anyone interested in the history of pH instruments that Boyle’s effigy cannot gaze upon Beckman’s instrument; his eyeline is blocked by a concrete pillar, which no amount of alchemical tricks have yet succeeded in making transparent.<\/p>\n","protected":false},"excerpt":{"rendered":"

Many organic dyes, what the eighteenth-century colour-makers called “vegetable” colors, are sensitive to the alkalinity or acidity of the dyebath.\u00a0 I have elsewhere shared the range of bright reds and … Read More<\/a><\/p>\n","protected":false},"author":136,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[1],"tags":[],"class_list":["post-816","post","type-post","status-publish","format-standard","hentry","category-uncategorized"],"acf":[],"yoast_head":"\nDyebath Acidity and Litmus - The Motley Emblem<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/sites.rutgers.edu\/motley-emblem\/dyebath-acidity-and-litmus\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Dyebath Acidity and Litmus - 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My last project was a born-digital museum of eighteenth-century cognitive models; visit at www.mindisacollection.org. I am currently engaged in recreating a marbled page from Laurence Sterne's novel _Tristram Shandy_. 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