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.]
Acids which, like those mentioned in the foregoing examples, contain
one atom of replaceable hydrogen are called monobasic; those which
contain two such atoms (_e.g._ sulphuric acid, H_{2}SO_{4}; tartaric
acid, H_{2}C_{4}H_{4}O_{6}),[7] dibasic; those which contain three
such atoms (_e.g._ phosphoric acid, H_{3}PO_{4}; citric acid,
H_{3}C_{6}H_{5}O_{7}),[7] tribasic; and so on with acids of higher
basicity. Acids of greater basicity than unity are frequently termed
polybasic.
[Footnote 7: See footnote, p. 26.]
Besides containing replaceable or basic hydrogen, acids are further
characterised by the property of combining with alkaloids to form salts;
_e.g._--
Sulphuric Acid. Quinia.
H_{2}SO_{4} + 2C_{24}H_{24}N_{2}O_{2} =
Quinia Sulphate.
(C_{20}H_{24}N_{2}O_{2})_{2} . H_{2}SO_{4}
Acetic Acid. Morphia.
HC_{2}H_{3}O_{2} + C_{17}H_{19}NO_{3} =
Morphia Acetate.
C_{17}H_{19}NO_{3} . HC_{2}H_{3}O_{2}
=Dibasic Acids.= See ACID.
=Fatty Acids.= Acids separable from fats or oils; _e.g._ stearic acid,
oleic acid, butyric acid, &c.
=Inorganic Acids.= Same as MINERAL ACIDS (which _see_).
=Mineral Acids.= Acids chiefly or wholly derived from the mineral kingdom.
In _medicine_, sulphuric, hydrochloric, and nitric acids, are commonly so
called.
=Monobasic Acids.= See ACID.
=Organic Acids.= Acids formed by, or derived from organic substances;
_e.g._ acetic acid, tartaric acid, uric acid, &c.
=Polybasic Acids.= See ACID.
=Pyro-acids.= Acids resulting from the decomposition by heat of other
acids, _e.g._ gallic acid, when heated, yields pyro-gallic acid.
=Tribasic Acids.= See ACID.
=ACIDIFICA'TION.= [Eng., Fr.] _Syn._ ACIDIFICA'TIO, L. In _chemistry_, the
act, process, or state of acidifying, or of making, becoming, or
impregnated with acid.
=ACIDIM'ETER.= _Syn._ ACIDOM'ETER; ACIDIME'TRUM, &c., L.; ACIDIMÈTRE, Fr.
An instrument or apparatus employed in acidimetry.
The ordinary acidimeters of the chemist are small tubes, constructed to
hold exactly 1000 grains of distilled water, at 60° Fahr., within the
limits of their scale, which is accurately graduated into 100 divisions.
They are used to contain the alkaline solutions (TEST-LIQUORS, NORMAL or
STANDARD SOLUTIONS) employed in the following processes.
Beaumé's Acidimeter, and others of the same class, are HYDROMETERS, and
are described under that 'head.'
=ACIDIM'ETRY.= _Syn._ ACIDOM'ETRY; ACIDIME'TRIA, &c., L.; ACIDIMÉTRIE, Fr.
The estimation of the strength or quantity of acid, in a free state,
contained in any liquid. It is the reverse of 'alkalimetry.' Acidimetrical
assays are understood to refer to the relative strengths of the same acids
(_i. e._, the quantity of real acid of the same kind contained in the
solutions examined), and not to the comparative strengths of acids of
different composition or names.
_Acidimetrical processes._ These are founded chiefly on the capacity of
the acids to saturate the bases; and, in some of the liquid acids, on the
specific gravity.
_a._ VOLUMETRICALLY:--
1. The sample of the acid to be examined (100 gr., or any convenient
aliquot part thereof) is placed in a suitable glass vessel, and if it be
one of the stronger acids, diluted with six or eight times its weight of
water, or if solid (as oxalic, or citric acid), dissolved in a like
quantity. This liquid is then exactly neutralised with an alkali.
This point is usually determined, by the addition of a small quantity of
litmus solution, which turns just blue when the solution is neutralised,
but when a carbonate is used for the alkaline solution, the acid must be
boiled a short time after each addition to expel the carbonic acid. The
quantity of the alkaline solution consumed for this purpose represents an
equivalent quantity of acid, and thus gives us the acid content of the
sample under examination. The common practice is to dissolve one
equivalent of the alkaline test in grains or grammes in water, and to make
up the solution to exactly 1000 parts by measure (_i. e._, 1000
'water-grains' or grammes), so as to accurately fill the 100 divisions of
an acidimeter; when the quantity, in grains or grammes, of the sample
tested, bears the same proportion to the equivalent number of the acid
under examination, that the number of acidimeter divisions of the
test-liquor consumed bear to the per-centage of acid sought.
Thus:--suppose 50 gr. of a sample of sulphuric acid take 25 acidimeter
divisions (300 parts or water-grains measure) of the test-liquid to
neutralise it, what is its content of real acid?
The equivalent of sulphuric acid is 49 (half its atomic weight); so, by
the rule of proportion,
50 : 49 :: 25 : 24-1/2
It therefore contains 24-1/2 parts of real sulphuric acid, in 50.
If the 1000 parts or grain-measures, instead of the number of the
acidimeter divisions, be taken for the calculation, it will, of course, be
necessary to point off the first right-hand figure of the result as a
decimal. Thus; repeating the above example--
50 : 49 :: 250 : 24·5
Or, since the equivalent of the test-liquid is 100, it will bear the same
proportion to the equiv. of the acid examined as the number of the
acidimeter divisions of the test-liquid consumed in neutralising 100 gr.,
do to the per-centage sought. Thus:--50 gr. of hydrochloric acid take 45
acidimeter divisions to effect neutralisation, what is its real
strength?--The equiv. of hydrochloric acid is 36·5: therefore--
100 : 36·5 :: 45 : 16·425%
and, since only 50 gr. (instead of 100 gr.) were examined--
16·425 × 2 = 32·85%
Some operators prefer employing 100 gr. instead of the equivalent weights
of the given tests in making their test-solutions, in which case each gr.
or 1000th part represents 1/10th, and each acidimeter degree 1 gr. of the
alkali or carbonate employed; when a similar proportion will obtain to
that first above given.
In technical analysis it is more convenient if the number of acidimeter
divisions of the 'test-liquid' consumed express the per-centage strength
of the acid, without further calculation. For this purpose the number of
grains of the acid taken for the assay should correspond to the equivalent
number of such acid (see _Table_ I, below); or to some convenient aliquot
part of it, as the 1/2, 1/4, 1/5, or 1/10th; the per-centage answer, in
the last case, being doubled, quadrupled, &c., according to the aliquot
part taken. The reason of this is obvious.
For the test-solutions, ammonia, and the dry and crystallised carbonates
and bicarbonates of potash and soda, are used, and are made by dissolving
in water their constituents except ammonia, of which 1000 grains, or one
litre, of solution of specific gravity 0·992 contains exactly one
equivalent.
53 grains (or grammes) of pure anhydrous carbonate of soda, prepared by
gradually heating to redness the crystallised salt, constitute one
equivalent (half the atomic weight), and 69 grains (or grammes) of pure
dry carbonate of potash. Of the crystallised salt 143 grains of carbonate
of soda will be required, and 84 grains (grammes) of the crystallised
bicarbonate of soda, and 100 of the crystallised bicarbonate of potash.
Occasionally solutions containing in one thousand parts, 50 of pure
carbonate of lime or chalk, or 28 of pure caustic lime, are used.
Besides these, a process known as Kiefer's is practised, and an ammoniacal
solution of oxide of copper is employed as the 'test-liquor,' and the
'point of neutralisation' is known by the turbidity observed as soon as
the free acid present is completely saturated.
The normal solution or test-liquor is prepared by adding to an aqueous
solution of sulphate of copper, pure ammonia water, until the precipitate,
which at first forms, is just redissolved, carefully avoiding excess. Or
better, by adding a rather strong solution of sulphate of copper, to a
quantity of a rather strong solution of ammonia containing exactly 17 gr.,
or one equiv. of pure ammonia, as long as the precipitate which forms is
redissolved on agitation; the resulting liquid being afterwards diluted
with pure distilled water, until it accurately measures 1000 water-grains,
or fills 100 divisions of an acidimeter, at 60° Fahr. In either case, the
strength of the resulting 'test-solution' must be carefully determined by
means of standard sulphuric acid, and adjusted, if necessary.
This method answers well with all the stronger acids (excepting oxalic
acid), even when dilute; and it has the advantage of not being affected by
the presence of a neutral metallic salt with an acid reaction, as sulphate
of copper, or of zinc.
Besides this process a solution of lime in sugar may be used, as proposed
by M. Peligot, and made as follows:--
Pure caustic lime is carefully slaked by sprinkling with water, and 50
grains (or grammes), made up by water to a milky solution, and 100 grains
of pure sugar candy dissolved in 1000 grains of water, are added, and the
whole well shaken. It is allowed to settle in a closed bottle, and the
clear solution poured off and diluted, until 1000 grains neutralise
exactly 100 grains of pure hydrochloric acid of sp. gr. 1·1812. Of course
it only answers with acids whose calcium salts are readily soluble in
water.
_b._ GRAVERMETRICALLY:--
The test-liquors or standard solutions of the above methods are made up so
as to _weigh_ exactly 1000 grains, instead of to 'measure' 100 acidimeter
divisions. Every grain of the test-liquor thus represents 1/10th gr. of
alkali; and every 10 gr., 1 gr. of alkali; or respectively, 1/10th per
cent. and 1 per cent. The vessel used for containing the solutions is
carefully weighed whilst empty, and 1000 gr. being placed in the opposite
scale, the test-solution, containing exactly one equivalent of base, is
poured in, and the whole made up with distilled water (if necessary) so as
to restore the balance to an equilibrium. After the process of
neutralisation, the acidimeter, with its contents, is again placed in the
scales; its previous weight still remaining there. The number of grains
required to restore the equilibrium of the balance (_i.e._, the loss of
weight), gives the exact weight of the test-liquor consumed. In all other
respects the process is the same as in the 'volumetrical method' already
described.
Another method for estimating the strength of the sample of acid is by
weighing the amount of carbonic acid expelled during saturation. (Method
of Fresenius and Will.) This depends on the weight of gaseous carbonic
acid which a given weight of the acid-sample under examination is capable
of expelling from pure bicarbonate of soda (or of potash), which is
estimated by the loss of weight in the acidimeter, or apparatus, after the
gas, rendered perfectly dry by passing through sulphuric acid, has escaped
into the air.
TABLE I.--_Weights of the respective acids equivalent
to the given weight of the principal bases, hydrogen
being taken as unity._
{51 Acetic acid (anhydrous).
{60 " " (crystallised or glacial).
{99 Arsenious acid (dry).
{35 Boracic acid (anhydrous).
17 gr. of pure ammonia.[8] } {62 " " (crystallised).
31 " anhydrous soda.[9] } {22 Carbonic acid (dry).
40 " hydrate of soda.[9] } {67 Citric acid (crystallised).
53 " dry carbonate of soda.[10] } {85 Gallic acid (dried at 212°).
143 " crystallised carbonate of } {94 " " (crystallised).
soda.[11] } {127-1/2 Hydriodic acid (dry or gaseous).
84 " crystallised bicarbonate } {27 Hydrocyanic acid (anhydrous).
of soda. } {36-1/2 Hydrochloric acid (dry or gaseous).
47 " anhydrous potassa.[9] } {109 " " (liquid, sp. gr. 1·162).
56 " hydrate of potassa.[9] } are {166-1/2 Iodic acid.
69 " dry carbonate of potassa.[10]} exactly {54 Nitric acid (anhydrous).
100 " crystallised bicarbonate } neutralised {63 " " (liquid, _monohydrated_, sp. gr.
of potassa. } by { 1·517 to 1·521).
50 " {pure chalk. } {67-1/2 " " (liquid, _sesquihydrated_, sp. gr.
{pure marble. } { 1·5033 to 1·504).
28 " pure caustic lime. } {72 " " (liquid, _binhydrated_, sp. gr.
37 " hydrate of lime (fresh). } { 1·486).
44 " dry carbonic acid (when } {90 " " (liquid, sp. gr 1·42).
the bicarbonate of } {36 Oxalic acid (anhydrous).
potassa or soda is } {63 " " (crystallised).
used for testing in } {72 Phosphoric acid (anhydrous).
the process of Fresenius } {81 " " (glacial).
and Will). } {50 Succinic acid (dry or anhydrous crystals).
22 " dry carbonic acid (when } {59 " " (ordinary crystals).
a dry carbonate is } {40 Sulphuric acid (anhydrous).
used). } {49 " " (liquid, _monohydrated_, sp.
{ gr. 1·8485).
{75 Tartaric acid (crystallised).
{212 Tannic acid (carefully dried).
[Footnote 8: 1000 water-grains measure of pure liquor of ammonia, sp. gr.
0·992, contains exactly 17 gr., or 1 equiv. of pure gaseous ammonia. A
standard liquor of this strength may be most conveniently prepared by
cautious dilution of a stronger solution, until a hydrostatic bead,
corresponding to the sp. gr., floats indifferently in the middle of the
new solution, at 60° Fahr. By keeping two hydrostatic beads in the
solution--the one made barely to float, and the other barely to sink--we
shall always be able to detect any change of strength or temperature which
it may suffer; since the "loss of a single hundredth part of a grain of
ammonia per cent., or the difference of a single degree of heat, will
cause the beads to" vary their positions. To preserve its integrity it
must be kept in a well-stoppered bottle. (See below.)]
[Footnote 9: These substances, as well as 'test-solutions' containing
them, must be perfectly free from carbonic acid, and must be carefully
preserved to prevent the absorption of carbonic acid from the atmosphere.
Mohr states that a dilute solution of either of them is best preserved in
a flask or bottle well closed with a cork fitted with a small bulb tube
(resembling a chloride of calcium tube), filled with a finely triturated
mixture of sulphate of soda and caustic lime, and bearing a very thin open
tube in the exit aperture. Fresenius, and most other foreign chemists,
prefer 'test-solutions' of pure soda. With test-solutions containing
caustic alkalies, exact neutralisation of an acid is not only more easily
effected, but more readily perceived, particularly when either solution is
tinted with litmus.]
[Footnote 10: Prepared by gradually heating the pure crystallised
carbonate to redness. From being uniform in composition, and easily
procured or prepared, they are much employed; preference being usually
given to the soda-salt.]
[Footnote 11: The crystals must be free from attached water, but not the
least effloresced.]
_Oper._ A determined amount of the acid under examination is accurately
weighed into the flask _A_ (see _engr._); and if it be a concentrated
acid, or a solid, it is mixed with or dissolved in 6 or 8 times its weight
of water. The little glass tube (_e_) is then nearly filled to the brim
with pure bicarbonate of soda, in powder, and a fine silken thread is tied
round the neck of the tube, by means of which it can be lowered down into
the flask (_A_), so as to remain perpendicularly suspended when the cork
is placed in the latter; the cord being held between the cork and the
mouth of the flask. The flask (_B_) is next about half filled with oil of
vitriol, and the tubes being arranged in their places, as represented in
the _engr._; and time having been allowed for the mixture of acid and
water to cool completely, after the increase of heat caused by mixing, the
whole apparatus is very accurately weighed. The cork in the flask (_A_) is
then slightly loosened, so as to allow the little tube containing the
bicarbonate of soda to fall into the acid, and is again instantly fixed
AIR-TIGHT in its place. The evolution of carbonic acid now commences, and
continues until the acid in the flask (_A_) is neutralised. When this
takes place, which is easily seen by no bubbles being emitted on shaking
the apparatus, the flask (_A_) is put into hot water (120° to 130°
Fahr.), and kept there, with occasional agitation, until the renewed
evolution of gas has completely ceased. The little wax stopper is then
taken off the tube (_a_), the apparatus taken out of the hot water, wiped
dry, and suction applied, by means of a perforated cork, or a small
india-rubber tube, and the mouth, to the end of the tube (_d_), until the
sucked air no longer tastes of carbonic acid. The whole is then allowed to
become quite cold, when it is replaced in the balance (the other scale
still containing the original weights), and weights added to restore the
equilibrium.
[Illustration:
(_A_) A wide-mouthed flask, capable of holding 2-1/2 to 8 oz.,
containing sample for trial (_f_).
(_B_) Ditto, capable of holding 1-1/2 to 2 oz., partly filled with
oil of vitriol (_g_).
(_a_, _c_, _d_) Tubes fitting air-tight in the flasks by means of
the corks (_i_) and (_j_).
(_b_) Piece of wax fitting air-tight on the end of _a_.
(_e_) Small tube capable of holding about 1 drachm of powdered
bicarbonate of soda or potash.
(_h_) Open end of the tube (_d_).
(_k_) Silk cord fastened to the tube (_e_).]
The loss of weight represents the exact quantity of dry carbonic
anhydride, or anhydrous carbonic acid gas, that has been expelled from the
bicarbonate of soda, by the action of the acid in the sample examined.
The quantity of real acid it contained is then deduced by the following
calculation:--One equivalent of gaseous carbonic anhydride, or anhydrous
carbonic acid (= 44) bears the same proportion to one equivalent of the
acid in question, as the amount of the carbonic anhydride expelled does to
the amount of the acid sought. Thus, suppose a dilute sulphuric acid
expels 3 gr. of carbonic anhydride, the arrangement is--
44 : 49 :: 3 : 3·349
Consequently the sample operated on contained 3·5 (nearly) grains of true
sulphuric acid.
Instead of the above calculation, we may multiply the weights of the
respective acids required to expel 1 gr. of carbonic acid (as exhibited in
the following table) by the number of gr. of dry carbonic acid evolved
during the above operation. The product represents the per-centage
strength, when 100 gr. of the acid have been examined. When only 50, 25,
20, or 10 gr. have been tested, this product must, of course, be doubled,
quadrupled, &c., as the case may be.