|A depiction of the character Mittler "arriving at the Terrace" (HB:238), from part 1: chapter 2, drawn by German illustrator Philipp Johann (check?).|
The chemical origin of the character Mittler seems to not have been specified by Goethe specifically—as, of course, contrary to his usual practice, he famously destroyed all the notes and drafts to the construction of this novella. He does state, however, that the theory of the novella is based on the 1775 affinity chemistry work of Torbern Bergman—and affinity chemistry itself originated in the work of English chemistry-physicist Isaac Newton. Hence, one can discern the character of Mittler in the following 1678/79 letter to Irish chemist Robert Boyle, a letter which, according to American science historian William Newman, was preoccupied with the phenomenon of elective affinity among chemicals, in which Newton states: 
“There is a certain secret principle in nature by which liquors are sociable to some things and unsociable to others. Thus water will not mix with oil but readily with spirit of wine or with salts.”
Furthermore, as summarized by Newman, Newton elaborates: 
“Just as water ‘elects’ to mix with ethyl alcohol or with salts, so it ‘chooses’ not to mix with oil, Similarly, water will sink into wood while quicksilver will not, but quicksilver will penetrate and amalgamate with metals, which water will not. Likewise aqua fortis (nitric acid) will dissolve silver and not gold, while aqua regis (mixed nitric and hydrochloric acid) will dissolve gold and not silver. Nonetheless these rules are not written in stone: ‘but a liquor which is of itself unsociable to a body may by a mixture of a convenient mediator be made sociable. So molten lead which alone will not mix with copper or with Regulus of Mars, by the addition of tin is made to mix with either.”
Hence, according to Newton, chemicals, or humans as Goethe viewed things, can have their “elections” and “choices” to one another modified by the actions of a “convenient mediator” so as to make previous unsociable entities “sociable”, as Newton says.
The character of Mittler famously is representative of a certain point of view or "marriage philosophy". The following is Mittler’s famous speech, as found quoted in Walter Benjamin’s 1921 “Goethe’s Elective Affinities” essay: 
"Anyone who attacks the state of marriage," Mittler cried, "who undermines this foundation of all moral society by word or deed, will have to reckon with me; or else, if I cannot better him, I will have nothing to do with him. Marriage is both the base and the pinnacle of culture. It makes barbarians tame, and it gives the most cultivated of people an opportunity to demonstrate their gentleness. It must be indissoluble; it brings so much luck that individual misfortunes cannot be weighed against it. And why speak of misfortune? Misfortune is really impatience that comes over people from time to time, and then they like to see themselves as unlucky. If you let the moment pass, you will think yourself fortunate that something that has stood the test of time still exists. There is no sufficient reason for separation. The human condition is so highly charged with joy and sorrow that one cannot calculate what two spouses owe each other. It is an infinite debt that can be paid only in eternity. It may be unpleasant at times—I can well believe it—but that is right and proper. Are we not also married to our conscience, which we would often like to get rid of, since it is more disagreeable than any man or woman could ever be?"
Mittler, in this sense, seems to embody the Bible-backed "till death do us part" view of religion, in the sense that divorce, for whatever reason, is an "immoral" action. Goethe, in this sense, when contrasting the dogmatic view of Mittler with the natural view of the separations and combinations that occur in chemistry, seems to be setting up Mittler as an Aunt Sally for someone only to knock down in the novel or possibly for future generations to knock down.
American reaction theory chemist Roald Hoffmann argues that the character Mittler, “the mediator”, whose central point was to never enter any house where there was not a dispute to settle or difficulties to put right, was the role model for a catalyst or human catalyst.  In Goethe’s mind, however, Mittler more likely was modeled on the theory of “mediating affinity”, a species or substance the brings about an action in or between two other chemical species.  This argument is by virtue of the fact that the theory of catalysis was not yet solidified as a scientific concept by 1809; but was only beginning to come into theoretical form through the work of Goethe’s personal chemist Johann Dobereiner, someone he did not meet until 1810.
● Elective Affinities (characters)
● Footnote 2.5
1. Hoffmann, Roald. (1995). The Same and Not the Same (§34: Catalyst, pgs. 179-; Goethe, 58, 88-89, 179-80, 256). Columbia University Press.
2. (a) Thims, Libb. (2007). Human Chemistry (Volume One), (preview). Morrisville, NC: LuLu.
(b) Thims, Libb. (2007). Human Chemistry (Volume Two), (preview), (ch. 10: "Goethe's Affinities", pgs. 371-422). Morrisville, NC: LuLu.
3. Newman, William R. (2003). Gehennical Fire: the Lives of George Starkey, and American Alchemist in the Scientific Revolution (Elective affinity, pgs. 231-34). University of Chicago Press.
4. Benjamin, Walter. (1921). “Goethe’s Elective Affinities” (scribd), first published by Hugo von Hofmannsthal in the Neue Deutsche Beitrage (1924/25); in: Selected Writings, Volume 1: 1913-1926 (Elective Affinities, pgs. 297-360; Mittler quote, pg. 300-301). Harvard University Press, 1996.