Organic acid

“ According to Sherman, these organic acids have a lower fuel value, per gram, than carbohydrates, but are reckoned as such in computing a food in which they exist. The function of these acids is chiefly that of neutralizing the acids formed in the body in metabolism. Being base-forming in character, they function after absorption and oxidation in the body as potential bases—the base associated with the acid in their ash combining with carbonic acid to form carbonates, which act as above described. ”

“ There is a deal to be said in favour of the use of another organic acid, formic acid. It is equally as safe as acetic acid, and quite efficacious; the only drawback is that, taking all things into consideration, it is very slightly more expensive. ”

“ This indicator is affected by even so weak an acid as carbonic acid, which must be removed by boiling the solution before titration. It is the indicator most generally employed for the titration of organic acids.
In general, it may be stated that when a strong acid, such as hydrochloric, sulphuric or nitric acid, is titrated against a strong base, such as sodium hydroxide, potassium hydroxide, or barium hydroxide, any of these indicators may be used, since very little hydrolysis ensues. ”

“ Other organic acids are undoubtedly formed as well as certain aromatic products. While the production of acid as a result of fermentative activity is usually accompanied with a development of flavor, the flavor is not directly produced by the formation of acid. ”

“ For clinical purposes the most convenient tests are those which depend upon certain changes in color produced in reagents which enable us to distinguish inorganic from organic acids. In the gastric juice the only inorganic acid which comes into consideration is hydrochloric acid, and the most important organic acid is lactic. ”

“ Organic Acids.—Organic acids are characterized by the presence of the monovalent group—CO·OH, termed the carboxyl group, in which the hydrogen atom is replaceable by metals with the formation of salts, and by alkyl radicals with the formation of esters. The basicity of an organic acid, as above defined, is determined by the number of carboxyl groups present. Oxy-acids are carboxylic acids which also contain a hydroxyl group; similarly we may have aldehyde-acids, ketone-acids, &c. ”

“ Where there is a tendency to the deposition of uric acid in the body, they hinder its formation. Citric, tartaric, malic and other organic acids exist in fruits in combination with potash and other bases, as well as in the free state. The free acids in fruits, when eaten, combine with the alkalies in the intestinal tract, and are absorbed by the body and pass into the blood, not as acids, but as neutral salts. Here they are converted into potassium carbonate or some other carbonate. Fruit acids never make the blood acid but the reverse. ”

“ The acid must be free of ammonia and be kept in such a way as not to absorb ammonia from the atmosphere of the laboratory. To guard against danger of error from such an impurity frequent control determinations should be made. In control experiments one or two grams of pure sugar should be used as the organic matter. If the acid employed contain traces of ammonia the necessary corrections should be made in each analysis. The flask having been charged is placed on a wire gauze over a small flame. ”

“ On the other hand, it is known that plants always form and contain organic acids, and these must be regarded as derivatives of carbonic acid, as is seen by all their reactions, of which we will shortly treat. For this reason it might be thought that the carbonic acid absorbed by the plants first forms (according to Baeyer) formic aldehyde, CH2O, and from it organic acids, and that these latter in their final transformation form all the other complex organic substances of the plants. Many organic acids are found in plants in considerable quantity ”

“ Make strongly alkaline with lime water, 25 grams of the milk, and evaporate to dryness on the water bath. Destroy the organic matter by igniting the residue. Dilute with 15 cc. of water and acidify with hydrochloric acid. Then add 1 cc. of the concentrated acid. Dip a piece of delicate turmeric paper in the solution; and if borax or boric acid is present, it will have a characteristic red color when dry. Ammonia changes it to a dark blue green, but the acid will restore the color. ”

“ The pure concentrated commercial acid has a density which usually reaches 1.842, and its boiling point is about 640° F. The absolutely pure acid is perfectly colorless, but usually even that used in laboratories has a peculiar grayish color, due to slight traces of organic matter. Sulphuric acid is exceedingly hydroscopic, and when exposed to the air it rapidly increases in bulk, owing to absorption of atmospheric moisture. ”

“ The mixture is brought to a temperature of approximately 210 F., and it may even for a few moments exceed that temperature, but it should be kept below the boiling point, for excessive heat injures or breaks down the secretin molecule and impairs or destroys its activity. Although I prefer to use hydrochloric acid, I would have it understood that other acids—both organic or inorganic—may be employed, provided that the percentage of acidity is regulated to prevent a chemical change in the secretin, and further provided, of course, that the acid has no injurious effect on the human system. ”

“ Organic carbon, during this process, is converted chiefly into carbon dioxid, but small quantities of formic, propionic, and butyric acids are also produced. Any organic sulfur which is present is converted into acid. No hydrogen or nitrogen is eliminated in a free state. While slight alkalinity is favorable to the development of this bacterium, yet it may be propagated in a feeble sulfuric acid solution when the acid is less than one per cent. ”

“ The general characters of sulphuric acid are as follows:—When pure, it is a colourless, or, when impure, a dark brown to black, oily liquid, without odour at common temperatures, of an exceedingly acid taste, charring most organic tissues rapidly, and, if mixed with water, evolving much heat. If 4 parts of the strong acid are mixed with 1 part of water at 0°, the mixture rises to a heat of 100°; a still greater heat is evolved by mixing 75 parts of acid with 27 of water. ”

“ In chemistry the term denotes a substance which combines with an acid to form a salt. In inorganic chemistry such compounds are almost invariably oxides or hydroxides, and water in eliminated during the combination; but in organic chemistry many compounds exist, especially ammonia derivatives, which directly combine with acids. Chemical bases are consequently antithetical to acids; and an acid is neutralized by a base with the production of a salt. They reverse certain colour reactions of acids, e.g. ”

“ First, at least three earthy salts-the sulphates of lime, alumina, and potassa. Second, two organic and one simpler acid—acetic (absolute vinegar) , meconic (one of the most powerful irritants which can be applied to the intestines through the bile) , and sulphuric. All these exist uncombined in the gum, and free to work their will on the mucous tissues.
A green extractive matter, which comes in all vegetal bodies developed under sunlight, next deserves a place by itself, because it is one of the few organic bodies of which no rational analysis has ever been pretended. ”

“ The many forms of organic acids—as the malic, tartaric, oxalic, citric, etc.—and a variety of alkaloids and other bodies are also products of destructive metabolism, which may be deposited in the various tissues as waste materials, or some of them may be changed by soluble ferments into forms which may be again utilized in the economy of the plant. The organic acids and tannin of green fruits, for example, are converted into sugar in the process of ripening by ferments formed from the protoplasm of the fruit cells. ”

“ It is almost always used in those cases where mineral acids are objectionable. To convert, for example, a solution of a substance in hydrochloric acid into a solution of the same in acetic acid, alkali should be added in excess and then acetic acid. Many compounds are insoluble in acetic acid, which are soluble in mineral acids, such as ferric phosphate, ferric arsenate, zinc sulphide, calcium oxalate, &c., so that the use of acetic acid is valuable in some separations. The commercial acid is strong enough for most purposes, and is used without dilution. ”

“ And the alcohols, with their allies, have no great power of resisting decomposition. These bodies, formed by the union of oxygen, hydrogen, and carbon, are also, as a class, chemically inactive. Formic and acetic are doubtless energetic acids; but the higher members of the fatty-acid series are easily separated from the bases with which they combine. Saccharic acid, too, is an acid of considerable power; and sundry of the vegetable acids possess a certain activity, though an activity far less than that of the mineral acids. ”

“ It is known that the acids lose those properties by which they reveal themselves as acids, by the addition of the inflammable substance, as sulphur, the elastic acid of nitre, regulus of arsenic, sugar, and the like, plainly shew. I am inclined to believe that fire-air consists of a subtle acid substance united with phlogiston, and it is probable that all acids derive their origin from fire-air. Now, if this air penetrates into plants, these must attract the phlogiston, and consequently the acid, which manifests itself as aerial acid, must be produced. ”

“ We all know how dangerous acid is in water, causing that water to act on many substances, the iron of iron vessels, the lime in soil or rock, etc., bringing iron and lime respectively into solution. Now the atmosphere contains carbonic acid, and carbonic acid occurs in the earth, being evolved by decomposing vegetation, etc. Carbonic acid is also soluble to a certain, though not large extent, in water. As we shall see, water charged with carbonic acid attacks certain substances insoluble in pure water, and brings them into solution, and thus the water soon becomes hard. ”

“ Before the power of the fixed air in subduing putrefaction was known, the efficacy of fruits, greens, and fermented liquors, was commonly ascribed to the acid in their composition and we have still reason to believe that the acid concurs in operating that effect. If it be alleged that mineral acids, which contain little or no fixed air, have been tried in the scurvy with little success, I would answer, that I doubt that in those trials they have never been sufficiently diluted ”

“ The action of the acid on various non-living matters is as follows: poured on all vegetable earth, there is an effervescence, arising from decomposition of carbonates; any grass or vegetation growing on the spot is blackened and dies; an analysis of the layer of earth, on which the acid is poured, shows an excess of sulphates as compared with a similar layer adjacent; the earth will only have an acid reaction, if there has been more than sufficient acid to neutralise all alkalies and alkaline earths. ”

“ The damage which can be done by strong acids is well known; if a jar of sulphuric acid is overturned, and some of it falls on the skin, it eats its way into the flesh and leaves an ugly sore; if it falls on carpet or coat, it eats its way into the material and leaves an unsightly hole. The evil results of an accident with acid can be lessened if we know just what to do and do it quickly, but for this we must have a knowledge of bases, the second group of chemicals. ”

“ Acids may be considered as a peculiar class of burnt bodies, which during their combustion, or combination with oxygen, have acquired very characteristic properties. They are chiefly discernible by their sour taste, and by turning red most of the blue vegetable colours. These two properties are common to the whole class of acids; but each of them is distinguished by other peculiar qualities. Every acid consists of some particular substance, (which constitutes its basis, and is different in each,) and of oxygen, which is common to them all. ”

“ The alembics are covered with conical capitals, ending in bent tubes, which conduct the acid vapours into lead pipes of condensation; and the furnaces are erected under a proper hood. As the cold acid is inoperative, it must be set a boiling, at which temperature it gives up one atom of its oxygen to the metal, and is transformed into sulphurous acid, which escapes in a gaseous state. Some of the undecomposed sulphuric acid immediately combines with the oxide into a sulphate, which subsides, in the state of a crystalline powder, to the bottom of the vessel. ”

“ This acid, united to the mineral alkali, makes Rochelle salt.
Every kind of wood, when distilled, or burned, yields a peculiar acid; and it is the vapour of this acid that is so offensive to the eyes in the smoke of wood.
A peculiar acid is obtained from most [Pg 57] vegetable substances, especially the farinaceous ones, and from sugar, by distillation with the nitrous acid. This seizes upon the substance with which the acid was united, and especially the phlogiston adhering to it, and then the peculiar acid of sugar crystallizes. ”

“ If the alkaline solution be titrated with a strong acid, the hydrions present in a very small amount of the acid suffices to reverse the colour; a weak acid, however, must be added in considerable excess of the quantity properly required to neutralize the solution, owing to its weak dissociation. This indicator is therefore only useful when strong acids are being dealt with, while its strongly acid nature renders it serviceable for both strong and weak bases. ”