HerbalEd has a brief description of the remedy here
http://www.herbaled.org/THM/Compounds/neutral.html and cites that it was developed in the mid 1850s, though I have found reference to it as early as 1830
Ellingwood's Therapeutists from 1917 on page 439 Credits the concoction to Edward Beach, though I am still researching as to whether he created the formula...Here is text from the book
The compound syrup of rhubarb and potassium is one of the most effective weapons in the armamentarium of the eclectic physician in aiding Nature in the readjustment of perverted functions, and in the re-establishment of the metabolic processes upon which health depends. In neutralizing cordial is to be found the sightly appearance, and agreeable odor, the pleasant taste, and above all, the very satisfactory effect of its administra tion. This mixture is of eclectic origin and is yet chiefly used by our own school.
It was originally prepared by Dr. Wooster Beach in this manner: Pulverized rhubarb. Pulv. salaratus. Pulv. peppermint plant; equal parts. To a large teaspoonful of this mixture add one-half pint boiling water; when cool, strain, sweeten with loaf sugar and add a teaspoonful of brandy. Dose. — One to two tablespoonfuls every one-fourth to two hours, according to indications.
Some fun examples of the formula in use
Recipe that was written out by a pharmacy in 1907
This is a great resource of a handwritten book from 1846 http://www.rae.org/hale/halebook3.html
There is an article on the exploration of the remedy being amended to be sugarless
Journal of Therapeutics and Dietetics, Volume 2edited by Pitts Edwin Howes page
Eclectic Medical Journal, Volume 66
— ECLECTIC MATERIA MEDICA AND THERAPEUTICS. BY HARVEY WICKES FELTER, M. D. NEUTRALIZING CORDIAL. — " See that your patients have a bottle of neutralizing cordial to take to the country with them." — Eclectic Med. Review, July, 1906). Brief and to the point, the foregoing suggestion is pregnant with foresight, and will save many a life and add to the pre- scriber's reputation. We have many times declared that if we could have but one medicine for the stomach and bowel complaints of the summer season, it would be the neutralizing cordial, or compound syrup of rhubarb and potash. We have never gone on a trip to parts any distance from home, or attended a doctors' convention, with its banquet of luxuries and a change of water, without taking with us a liberal quantity of the cordial. The bottle is generally emptied by those who have failed, like the foolish virgins, to go prepared for the very emer gencies for which neutralizing cordial is the very best corrective yet devised — disorders of stomach and bowels, caused by overfeeding or change of water. In general, neutralizing cordial is so well known that but mere reference to its properties are necessary. It has three especial qualities that we must note. Rhubarb, through its specific adaptability to irritation of mucous surface, makes
the cordial the ideal gastric sedative, for in such cases there is marked irritation, as shown by the red dened and pointed tongue. With most of these cases there is a fermentative state, with sourish and burning eructations, and often the bowel discharges contain sour and fermented material. For this condition there is no more pleasing antacid and corrective than potassium bicar bonate, though should the tongue show more pallor than redness, sodium bicarbonate may answer a better purpose. The aromatic qualities of the cordial derived from the peppermint oil and herb make it grateful as a carminative, and render it especially pleasant for children. Now, the cordial to which I have been referring is that devised by Prof. Frederick J. Locke, of the Eclectic Medical Institute, and based on the original prepared by Dr. Wooster Beach. We often hear Eclectic phy sicians speak freely of neutralizing cordial, and we sometimes wonder whether they are aware of various preparations that have passed, and still pass, under that name.