Part 6

Acetic acid is produced either by the partial dehydrogenation and subsequent oxidation of bodies containing its elements, or by their destructive distillation. The first is effected--by their exposure, in a finely divided state, to the action of air or atmospheric oxygen, as in the quick process of making vinegar; or--by submitting them, in combination with ferments, to contact with a free supply of atmospheric air, as in the old field process of making vinegar; or--by exposure to the direct action of chemical or mechanical oxidizing agents, as condensed air (platinum-black process), chromic and nitric acid, &c. In general, it is alcohol more or less dilute, particularly as it exists in fermented liquors, which is thus converted into acetic acid. In the second process (destructive distillation), wood is the substance usually employed, and heat is the agent which develops the acid.

The conversion of alcohol into acetic acid is not immediate and direct. The atmospheric oxygen first oxidises two atoms of its hydrogen, aldehyd and water being formed; and this aldehyd uniting with one atom of oxygen produces one molecule of ACETIC ACID. The changes are represented in the following equations:--

Alcohol. Oxygen. Aldehyd. Water. =1.= C_{2}H_{6}O + O = C_{2}H_{4}O + H_{2}O

Aldehyd. Oxygen. Acetic Acid. =2.= C_{2}H_{4}O + O = HC_{2}H_{3}O_{2}

After the first formation of aldehyd, the two processes, unless artificially checked, go on simultaneously, as long as any undecomposed alcohol is present.

The conversion of alcohol into acetic acid, although greatly accelerated by the presence of nitrogenised organic matter (according to Mulder, of a fungus--the _Mycoderma Vini_ or Vinegar Plant), is rather a case of eremacausis (slow combustion) than of fermentation. Acetification effects combination, as shown by the foregoing equations, whereas fermentation resolves complex bodies into simpler ones, _e.g._ sugar into alcohol and carbonic anhydride. Moreover, the presence of ferments is not essential to the change, since pure alcohol becomes acetified when exposed to the oxidising agents already named.

Another remarkable distinction between acetification and fermentation is, that the former requires the continued presence of atmospheric oxygen; whilst the vinous fermentation after being once established, proceeds perfectly without it.

During the oxidation of the alcohol of vegetable solutions, some of the other organic matters present also suffer change. A white gelatinous mass (_mother of vinegar_)[5] is commonly deposited; but this is a secondary result of the process, and not, as formerly supposed, one essential to it. In ordinary cases acetification occurs only at or near the surface of the liquid; which accounts for the length of time required for the operation under the old process of 'fielding,' and the shorter time in which it is accomplished by the improved process of Mr Ham. It proceeds favorably at temperatures ranging from 60° to 90° Fahr.; and most rapidly at 95° Fahr. (Liebig). In the 'quick process' of making vinegar a temperature of 90° to 92° is generally aimed at; but it often rises to 100°, or even to 105°, Fahr. As the temperature falls acetification proceeds more slowly, and at 46 to 50° Fahr. it ceases altogether (Liebig).

[Footnote 5: It has generally been asserted that this substance contains vibriones, and other low forms of organised life; but Mulder describes it, under the name _mycoderma aceti_, as a plant of the order 'fungi.' It is formed at the expense of the constituents of the vinegar, and often causes whole vats of it to pass into water.]

Aldehyd (see _above_) is an exceedingly volatile substance, and easily dissipated by a slight heat. It is, therefore, of the highest importance to duly regulate the temperature, as well as the supply of air, during acetification. In the 'quick process' of making vinegar the loss from this cause is always considerable, and often very great. This loss may be diminished by passing the heated air, as it escapes from the acetifier, through a porcelain or silvered copper worm or refrigerator, set in a chamber containing water of a temperature not higher than 40° to 45° Fahr.; the connection being made at the _lower_ end of the worm, whilst the upper end is open to the air. On the small scale, as in the platinum-black process, the loss may be almost entirely prevented by causing the upper air tube to pass through a vessel containing ice or a freezing mixture; or by uniting it with the lower end of a Liebig's condenser.

In liquors undergoing the vinous fermentation, a portion of the newly formed alcohol is invariably acetified whenever the temperature rises above 51° Fahr.; and at a higher temperature, this proceeds with a rapidity often highly injurious to the quality of the liquor. In this way there is frequently a useless loss of the alcohol, which is rendered more apparent by the incipient, and sometimes the actual, souring of the liquor.

=ACETIM'ETRY=. _Syn._ ACETOM'ETRY; ACÉTOMÉTRIE, Fr.; ACETIME'TRIA, &c., L. The art or process of determining the quantity of pure acetic acid in vinegar, or in any other liquid. The plans generally adopted for this purpose are--

I. From the saturating power of the acid, as in the common methods of acidimetry:--

1. The molecular weight of commercially pure bicarbonate of potash, in crystals, being 100, whilst that of absolute acetic acid is 60, it is evident that every ten grains of the bicarbonate will exactly equal 6 grains of the acid. To apply this practically, we have only to exactly neutralise 100 gr. of the vinegar or solution under examination with the bicarbonate, observing the usual precautions; then, as 10 is to 6, so is the number of grains used, to the per-centage strength required. In this, as in other like cases, it is convenient to form a test-solution with the bicarbonate, by dissolving it in sufficient water to fill the 100 divisions of any simple form of 'acidimeter,' as _a_, _b_, or _c_; when the quantity of the solution, and, consequently, of the salt used, may be read off at once from the graduated portion of the tube. Still greater accuracy may be obtained by dissolving the bicarbonate in exactly 1000 gr. of distilled water contained in a 'Schuster's alkalimeter,' previously very carefully weighed; in which case each grain of the test-solution will indicate 1/10th of a grain, or 0·1% of absolute acetic acid, whilst every 10 grains will be equal to 1 grain, or 1%.

The test-solution may also be prepared from bicarbonate of soda, or from the carbonates of soda or potash, care being taken that the quantity of the salt dissolved be in proportion to its molecular weight.

2. (Brande.) A small piece of white marble, clean and dry, is weighed, and then suspended by a silk thread in a weighed sample (say 100 or 1000 grs.) of the vinegar or acid under examination; the action being promoted by occasionally stirring the liquid with a glass rod, until the whole of the acid is saturated, as shown by no further action on the marble being observable on close inspection. The marble is then withdrawn, washed in distilled water, dried and weighed. The loss in weight which it has sustained will be nearly equal to the acetic acid present, or strictly, as 50 (marble) to 60 (absolute acetic acid). The only precautions required are, to avoid striking the piece of marble with the rod whilst stirring the solution, or causing loss of substance in it after its withdrawal; and to allow ample time for the action of the acid on it. If the sample consists of strong acid, it should be diluted with twice or thrice its weight of water before suspending the marble in it.

3. (Ure.) 100 grains of the sample under examination is slightly reddened with tincture of litmus, and ammonia of the sp. gr. 0·992 is added drop by drop (from an acetimeter holding 1000 water-gr. measure, divided into 100 divisions) until precise neutralisation is effected, indicated by the blue colour of the litmus being restored. The number of the divisions of the acetimeter used, multiplied by 60, and the first two right-hand figures of the product cut off as decimals, gives a number which represents the exact quantity of absolute acetic acid in the sample. In practice, it is found more convenient to keep the test-ammonia ready tinged with litmus.

The mode of estimating the per-centage of acetic acid in beers, when finding their original gravities, is a slight modification of the above. A test-solution of ammonia is prepared of such a strength that a given bulk of it will exactly neutralise one per cent. of absolute acetic acid in an equal bulk of beer, so that, if 100 fluid grains of the solution are sufficient to neutralise the acid in 1000 fluid grains of beer, such beer contains one tenth per cent. of acid. A solution of ammonia, diluted with distilled water until it has the sp. gr. ·9986 at 60°, is of the exact strength required.

An acetimeter holding 1000 grains, and graduated downwards to 100 equal divisions, is filled to 0 of the scale with the test-ammonia, which is then added, drop by drop, to 1000 measured grains of the beer, until neutralisation takes place. Every division of the acetimeter (corresponding to ten fluid grains), so emptied, indicates ·01 per cent. of acetic acid in the beer. The progress of the neutralisation is tested from time to time with a slip of reddened litmus paper, which should be suffered to become faintly blue before ceasing to add the ammonia. By this method the exact per-centage of absolute acetic acid in any sample may be accurately determined. The only precaution necessary is to be certain that the 'test-ammonia' has the required sp. gr. (·9986). Test-solutions may also be prepared with pure potash or pure soda.

II. From the specific gravity of the liquid after it has been neutralised with hydrate of lime:--

Common hydrate of lime (freshly slaked lime), in powder, is added gradually to the sample under examination, until it is saturated, when the sp. gr. of the resulting clear solution of acetate of lime is taken by Taylor's ACETIMETER. This instrument is so adjusted and graduated as to float at the mark on the stem called 'proof,' in a solution containing 5% of absolute acetic acid (No. 24 vinegar). For vinegars stronger than proof small weights are provided, each of which indicates an additional 5 per cent. To ascertain the per-centage of real acid, 5% must therefore be added to the acetimeter number. Thus, without being loaded, the instrument, floating at the 'proof mark,' indicates a vinegar of 5%; with one weight, a vinegar of 10%; with two weights, 15%, and so on. According to this system of notation, each 5% is called a 'vinegar.' An acid of 10% is said to contain two vinegars; one of 15%, three vinegars, &c. It is also common to speak of the degrees of the acetimeter as proof or over-proof. Thus, No. 24 vinegar is said to be proof; one of 5 acetimeter degrees, 5 over-proof; one of 10 degrees, 10 over-proof, &c. For malt and wine vinegars, which contain gluten and mucilage, this method is not strictly accurate, as a portion of these substances escapes precipitation by the lime, and consequently alters the specific gravity. A small weight marked 'M' is generally supplied with the acetimeters for trying such vinegars.

III. From the specific gravity:--

The sp. gr. of the sample (carefully determined by any of the usual methods) is sought in one of the following Tables, when the corresponding per-centage content of acetic acid is at once seen.

This method furnishes reliable results only with pure, or nearly pure solutions which do not contain much above 50% of glacial acid, or which have a sp. gr. not higher than 1·062. It is also more to be depended on for weak solutions than strong ones. By carefully diluting a strong acid with an equal weight, or twice or thrice its weight of water, and allowing the mixture to again acquire its normal temperature, the sp. gr. may be taken as a guide in all cases in which great accuracy is not required. When such dilution is made it only becomes necessary to multiply the indication furnished in the Tables by 2, 3, or 4, as the case may be. As, however, authorities are not agreed as to the precise sp. gr. of the monohydrate or glacial acid, and of its solutions, extreme accuracy must not be expected by this method.

TABLE I.--_Adapted to the Specific Gravities of common vinegar_. By Messrs J. and P. TAYLOR.

per sp. gr. cent. 1·0085 contains of anhydrous or real acetic acid 5 1·0170 " " 10 1·0257 " " 15 1·0320 " " 20 1·0470 " " 30 1·0580 " " 40

TABLE II.--_Exhibiting the quantity of_ ABSOLUTE _or_ GLACIAL ACETIC ACID (HC_{2}H_{3}O_{2}), _in acetic acid of successive strengths_. By Mr COOLEY.

+---------+-------+---------+-------+---------+-------+---------+-------+ |Absolute | |Absolute | |Absolute | |Absolute | | | Acetic |Sp. Gr.| Acetic |Sp. Gr.| Acetic |Sp. Gr.| Acetic |Sp. Gr.| | Acid, | | Acid, | | Acid, | | Acid, | | |per cent.| |per cent.| |per cent.| |per cent.| | +---------+-------+---------+-------+---------+-------+---------+-------+ | _Pure | | | | | | | | | acid_, | | 75 |1·0731 | 49 |1·0593 | 23 |1·0320 | | or 100 |1·0630 | 74 |1·0732 | 48 |1·0582 | 22 |1·0311 | | 99 |1·0648 | 73 |1·0728 | 47 |1·0568 | 21 |1·0292 | | 98 |1·0663 | 72 |1·0721 | 46 |1·0557 | 20 |1·0275 | | 97 |1·0677 | 71 |1·0718 | 45 |1·0553 | 19 |1·0264 | | 96 |1·0685 | 70 |1·0713 | 44 |1·0544 | 18 |1·0253 | | 95 |1·0696 | 69 |1·0711 | 43 |1·0535 | 17 |1·0241 | | 94 |1·0704 | 68 |1·0708 | 42 |1·0525 | 16 |1·0229 | | 93 |1·0708 | 67 |1·0702 | 41 |1·0518 | 15 |1·0218 | | 92 |1·0715 | 66 |1·0701 | 40 |1·0513 | 14 |1·0200 | | 91 |1·0721 | 65 |1·0693 | 39 |1·0502 | 13 |1·0173 | | 90 |1·0726 | 64 |1·0692 | 38 |1·0492 | 12 |1·0172 | | 89 |1·0729 | 63 |1·0685 | 37 |1·0482 | 11 |1·0161 | | 88 |1·0730 | 62 |1·0679 | 36 |1·0473 | 10 |1·0150 | | 87 |1·0731 | 61 |1·0675 | 35 |1·0460 | 09 |1·0131 | | 86 |1·0732 | 60 |1·0672 | 34 |1·0449 | 08 |1·0121 | | 85 |1·0733 | 59 |1·0665 | 33 |1·0439 | 07 |1·0102 | | 84 |1·0734 | 58 |1·0662 | 32 |1·0425 | 06 |1·0085 | | 83 |1·07343| 57 |1·0653 | 31 |1·0413 | 05 |1·0071 | | 82 |1·0735 | 56 |1·0645 | 30 |1·0402 | 04 |1·0057 | | 81 |1·0738 | 55 |1·0641 | 29 |1·0392 | 03 |1·0042 | | 80 |1·0743 | 54 |1·0632 | 28 |1·0380 | 02 |1·0025 | | 79 |1·0742 | 53 |1·0628 | 27 |1·0364 | 01 |1·0012 | | 78 |1·0740 | 52 |1·0616 | 26 |1·0352 | _Pure |1·0000 | | 77 |1·0739 | 51 |1·0610 | 25 |1·0341 | water._ | | | 76 |1·0736 | 50 |1·0602 | 24 |1·0330 | | | +---------+-------+---------+-------+---------+-------+---------+-------+

_Concluding remarks_. Before applying the above processes, account should be taken of any mineral acid which may be present in the sample, such being not unfrequently added to vinegar to impart artificial strength; and in those depending on the sp. gr., gum, gluten, &c., must also be allowed for. The methods depending on the saturating power of the acid will be found appropriate to acetic acid of all strengths, when unadulterated with the mineral acid. The method based on the sp. gr. is also very convenient, and is sufficiently accurate for distilled vinegars and for pure acids of moderate strength.

It is found that the decimal fraction of the sp. gr. of pure or nearly pure vinegar is doubled by its conversion into acetate of lime. Thus, 1·0085 in vinegar becomes 1·0170 when converted into a solution of acetate of lime. In malt vinegar, however, 0·005 may be deducted from the sp. gr. for mucilage and gluten. The quantity of foreign matter present in vinegar may therefore be approximatively ascertained, by deducting the decimal of the sp. gr. of the solution of acetate of lime from double that of the decimal part of the sp. gr. of the vinegar. Thus:--the sp. gr. of a sample of vinegar being 1·014, and after saturation with hydrate of calcium 1·023, the sp. gr. of the pure vinegar would be 1·009, and that due to foreign matter ·005. For--

·028 - ·023 = ·005

and--

1·014 - ·005 = 1·009

The reason why proof-vinegar is called, in commerce, No. 24, is that 1 fl. oz. of it requires exactly 24 gr. of pure anhydrous carbonate of soda to neutralise it. Weaker vinegars are represented in the same 'notation' by the Nos. 22, 20, 18, &c., according to their respective strengths estimated by their saturating power.

=ACETINE.= An essence for the removal of corns. Concentrated vinegar (1·04 sp. gr.) slightly tinged with fuchsine, 15 grms. (Hager.)

=ACETINE, HOCHSTETTER'S.= Prepared by J. C. F. Witte, Berlin. A remedy for corns, warts, and hard skin. Diluted vinegar, coloured with blue carmine, 16 grms. (Schälder.)

=ACETOLATS.= [Fr.] _Syn._ ESPRITS ACÉTIQUES. In _French pharmacy_, medicated vinegars obtained by distillation.

=ACETOLES.= [Fr.] In _French pharmacy_, medicated vinegars obtained by maceration.

=ACETOUS FERMENTATION.= See ACETIFICATION.

=ACETUM.= [L.] Vinegar.

=ACETYL.= _Syn._ ACETYLE. A name originally given to a hypothetical body, having the formula C_{2}H_{3}, and regarded by Berzelius as the radical of the acetates and their congeners. The acetyl of Gerhardt (C_{2}H_{3}O) is, however, according to that chemist, the true radical of the acetates. Williamson, in order to remove the confusion of terms occasioned by the application of the same name to compounds of different composition, proposed the title of othyl for the radical C_{2}H_{3}O.

=ACHAR.= See PICKLES.

=ACEIILE'INE= (-k[)i]l-). A peculiar bitter principle obtained from achillé a millefolium (Linn.), or yarrow.

=A'CHOR=, (-k[)o]r). [Gr.] See SCALD-HEAD.

=ACHROMAT'IC= ([)a]k-ro-). _Syn._ ACHROMATIQUE, Fr. In _optics_, devoid of colour; bodies that transmit light without decomposition, and consequently, without the formation of coloured rings or fringes; applied to compound lenses, prisms, &c., and to instruments fitted with them.

=ACRO'MATISM.= _Syn._ ACHROMATISME, Fr. In _optics_, the state of being achromatic; the absence of coloured fringes in the images of objects seen through a lens or prism.

Light is not homogeneous, but decomposable by refraction, absorption, or reflection, into coloured rays of unequal refrangibility. A ray of white light, in passing through a glass prism, is entirely separated into the coloured rays forming the 'prismatic spectrum,' and when it passes through a lens, an analogous resolution into coloured rays still occurs, though not so readily observed, and that to an extent often incompatible with distinct vision. Now, if a convex lens be regarded as a number of prisms united by their bases round a common centre, and a concave lens, as a similar number of prisms with their apices in contact, the action of lenticular and prismatic glasses on light will be reduced to a common principle. A beam of light thrown on a simple converging lens not only suffers refraction at the spherical surface (SPHERICAL ABERRATION), but the different coloured rays of which it is composed, from the causes mentioned, being unequally bent or refracted, diverge from their original course (CHROMATIC ABERRATION), forming as many foci on the axis of the lens as there are colours, and fall separately, instead of together, on the eye or object which receives them. Hence arise the coloured fringes or halos that surround objects viewed through ordinary glasses, and which form the great impediments to the construction of perfect lenses. This effect, like the refractive power and focal distance, varies in degree in different diaphanous substances.

The correction of the chromatic aberration of lenses is commonly effected by combining two, or more, made of materials possessing different 'dispersive' powers. Thus, the spectrum formed by flint glass is longer than that formed by crown glass, for the same deviation. When the two are combined, so as to form a compound lens, the one tends to correct the 'dispersion' of the other. On this principle ACHROMATIC GLASSES are generally formed in this country. A convex lens of crown glass is combined with a weaker concave lens of flint glass, the latter counteracting the dispersion of the former, without materially interfering with its refractive power. The resulting combination is not absolutely achromatic, but is sufficiently so for all ordinary purposes. According to Dr Blair, a compound lens perfectly achromatic for the intermediate, as well as for the extreme rays, may be made by confining certain fluids, as hydrochloric acid, between two lenses of crown glass. In order to produce nearly perfect achromatism in the object-glasses of telescopes, microscopes, cameras, &c., a concave lens of flint glass is commonly placed between two convex lenses of crown or plate glass, the adjacent surfaces being cemented with the purest Canada balsam, to prevent the loss of light by reflection from so many surfaces.

_Obs._ The production of perfect achromatism in lenses is a subject not less fraught with difficulty than with practical importance to the astronomer, the mariner, the microscopist, and the photographer; and it has hence engaged the attention of the leading mathematicians and artists of Europe up to the present time. All the larger object-glasses lately manufactured are said to consist of only two lenses; the resulting achromatism proving sufficiently exact for all useful purposes. Those of recent production have come chiefly from the workshops of Dollond, of London, and the opticians of Bavaria and Switzerland. The achromatism of prisms depends upon the same principles, and it is effected in the same way as that of lenses.

=ACIC'ULAR.= Needle-shaped; slender or sharp pointed; spicular; in _botany_, applied to leaves, and in _chemistry_, to crystals. The last are also sometimes termed ACIC'ULÆ.

=ACID=, _Syn._ ACIDUM, L.; ACIDE, Fr.; ACIDO, Ital.; SÄURE, G. In familiar language, any substance possessing a sour taste. In _chemistry_, substances are said to be acid, or to have an acid reaction, when they are capable of turning blue litmus red. In _chemistry_, also, the term acid is applied to a very large class of compounds containing hydrogen (hydrogen salts), and in which one or more atoms of that element may be replaced by an equivalent quantity of a metal or other basic radical; _e.g._--

1. The one atom of hydrogen in hydrochloric acid (HCl) may be replaced by sodium, producing the salt sodium chloride (NaCl).

2. The one atom of hydrogen in nitric acid (HNO_{3}) may be replaced by silver, producing the salt silver nitrate (AgNO_{3}).

3. One atom of hydrogen in acetic acid (HC_{2}H_{3}O_{2})[6] may be replaced by the basic radical ammonium (NH_{4}), producing the salt ammonium acetate (NH_{4}C_{2}H_{3}O_{2}).

[Footnote 6: Symbols indicating the number of atoms of replaceable hydrogen occupy the foremost position in the formulæ of acids, as shown in the text.]