KEY:_Cooley's Cyclopædia of Practical Receipts and Collateral Information in the Arts, Manufactures, Professions, and Trades..., Sixth Edition, Volume I

A - Factors or Multipliers for converting the weight of carbonic

acid expelled into real strengths.

B - Quantity (in grains) to be taken, so that the per-centage

value of the sample tested shall be shown in the terms of

any of the denominations given, by the weight of the evolved

Carbonic Acid reckoned--

C - in tenths of a grain.

D - Whole numbers and decimals.

E - Nearest common numbers.

F - in quarter-grains.

G - in half-grains.

H - in grains.

-----------------------+-------+-------------------------------------------+

| | B |

| |--------------------^----------------------+

NAMES, &c | | C | | | |

| |------^--------| | | |

| A | D | E | F | G | H |

-----------------------+-------+------+--------+---------+--------+--------+

(pure, gaseous) | ·77273| 7·727| 7-3/4 | 19-1/3 | 38-5/8 | 77-3/0 |

| | | | | | |

(neutral, anhydrous) |1·77273|17·727|17-3/4 | 44-5/16 | 88-5/8 |177-1/4 |

| | | | | | |

crystallised) |1·9773 |19·773|19-3/4 | 49-7/16 | 98-7/8 |197-3/4 |

| | | | | | |

ammonia (translucent) |2·6818 |26·818|26-13/16| 67-1/10 |134-1/10|268-1/5 |

| | | | | | |

(crystallised) |3·5909 |35·909|35-9/10 | 89-13/16|179-5/8 |359-1/10|

| | | | | | |

POTASSA (anhydrous) |2·1364 |21·364|21-1/2 | 53-1/2 |107 |213-3/8 |

| | | | | | |

Hydrate of potassa |2·54546|25·455|25-5/11 | 63-5/8 |127-1/4 |254-1/2 |

| | | | | | |

(anhydrous) |3·1364 |31·364|31-3/8 | 78-1/2 |157 |313-1/2 |

| | | | | | |

(granulated) |3·7727 |37·727|37-1/2 | 94-3/10 |188-5/8 |377-1/4 |

| | | | | | |

(crystallised) |3·9545 |39·545|39-5/8 | 99 |198 |395-1/2 |

| | | | | | |

(crystallised) |4·5454 |45·454|45-1/2 |113-3/4 |227-1/2 |454-1/2 |

| | | | | | |

SODA (anhydrous) |1·4091 |14·09 |14-1/10 | 35-1/4 | 70-1/2 |141 |

| | | | | | |

Hydrate of soda |1·8182 |18·182|18-1/5 | 45-1/2 | 91 |182 |

| | | | | | |

(anhydrous) |2·4091 |24·091|24-1/10 | 60-1/4 |120-1/2 |241 |

| | | | | | |

(crystallised) |6·5 |65· |65 |162-1/2 |325 |650 |

| | | | | | |

(dry; theoretical) |2·9091 |29·091|29-1/10 | 72-1/2 |145 |290 |

| | | | | | |

soda (Ph. L., 1836) |3·7273 |37·273|37-1/4 | 93-1/4 |186-1/2 |373 |

| | | | | | |

commercial) |3·7954 |37·954|38 | 94-7/8 |189-3/4 |379-1/2 |

| | | | | | |

(crystallised) |3·8182 |38·182|38-1/5 | 95-1/2 |191 |382 |

| | | | | | |

anhydrous) | ·6818 | 6·818| 6-13/16| 17-1/20 | 34-1/10| 68-1/5 |

| | | | | | |

caustic) |3·4773 |34·773|34-4/5 | 86-7/8 |173-7/8 |347-3/4 |

| | | | | | |

LIME (pure, caustic) |1·2727 |12·727|12-3/4 | 31-3/4 | 63-5/8 |127-1/4 |

| | | | | | |

anhydrous) | ·90909| 9·091| 9-1/11 | 22-3/4 | 45-1/2 | 91 |

-----------------------+-------+------+--------+---------+--------+--------+

In this ingenious method of alkalimetry it is absolutely necessary that

the whole of the alkali in the specimen tested should be in the state of

neutral carbonate. If a sample of potash contains any caustic alkali (as

the potashes and pearlash of commerce generally do), Fresenius and Will

direct it, previously to being tested, to be triturated with its own

weight of pure quartzose sand, and about one third of its weight of

carbonate of ammonia; and the resulting mixture, placed in a small iron

capsule, or a porcelain crucible, to be moistened with water, and exposed

to a gentle heat until it becomes quite dry, and all the ammonia is

expelled. If the sample contains any bicarbonate or sesquicarbonate, it

must be heated to dull redness before being placed in the apparatus and

tested. In the case of crude soda (particularly soda ash), the proportion

of carbonate of ammonia should be equal to at least one half the quantity

operated on. With both alkalies, if the sample contains sulphides,

sulphites, or hyposulphites, the same method is to be followed, except

that solution of ammonia, instead of water, is to be employed for

moistening the powder. To remedy the error which would arise from the

apparent amount of carbonic anhydride liberated during the assay, being

swelled by the disengagement of 'sulphuretted hydrogen' or sulphurous acid

from these substances, a small quantity of neutral (_i. e._ yellow)

chromate of potash may be added to the alkaline solution in the flask

(_A_); by which they will be converted into sulphates, sulphur, and water,

which will remain in the apparatus, the carbonic acid only being evolved.

"As most sorts of soda of commerce contain one or other of the substances

(just) named, and as it is far more simple to add at once some chromate of

potassa to the soda solution, than to test the latter for either of the

three salts, it is always advisable to make it a rule, in the examination

of SODA, to add some chromate of potassa." (Fresenius.)

If the sodium or other carbonate under analysis contains much chloride,

the addition of more sulphuric acid than necessary must be avoided, and

the carbonic anhydride expelled by gently heating over a warm bath, and

not by the addition of excess of acid.

[Illustration]

To obviate the difficulties, and to give greater precision and delicacy to

volumetrical assays, the instrument known as Mohr's ALKALIMETER, or Mohr's

BURETTE, and which is figured in the margin, may be employed. By means of

it the test-acid in the graduated tube (_a_) may be added to the alkaline

solution in (_f_), in any quantity at a time, however minute, by merely

pressing the handles of the clamp (_d_) with the thumb and finger. The

terminal tube (_e_) has its lower orifice very small, and it is connected

with the burette by means of a small piece of vulcanised india-rubber

tube, on which the clamp (_d_) acts. (See _engr._) The inner cylindrical

part of the arm (_b_) is lined with cork, to prevent injury to the glass

burette, and to hold it the more firmly.

Generally the alkali in the specimen examined may be in either the caustic

or carbonated state, or it may consist of any mixture of caustic alkali,

or carbonates; but it is absolutely necessary for accurate results, that

it should be free from sulphides, sulphites, and hyposulphites, as

sulphuric acid acts upon these substances as well as on carbonates. The

presence of chlorides does not interfere with the accuracy of the assay,

unless a higher degree of heat is employed than that necessary for the

expulsion of the absorbed carbonic acid. The SODA-ASH of commerce

generally contains all these substances besides common salt, sulphate of

soda, and insoluble matter, which do not interfere. Rough samples of

POT-ASHES and PEARL-ASH also generally contain some sulphides, though not

a large quantity. Various plans have been proposed to avoid this source of

error. The best is that of MM. Fresenius and Will, given above, in which

the value of the carbonates is estimated by their yield of carbonic

anhydride.

The difference between an assay of a sample of the unprepared alkali and

of another which has been treated as above, indicates the quantity of

impurities contained in them under the forms just referred to. The

presence of these substances in the commercial alkalies may be detected by

the following tests:--

_Sulphides._ The addition of sulphuric acid causes the evolution of an

odour like that of rotten eggs. The sample in solution yields a black

precipitate with acetate of lead. But the most delicate test is the

splendid violet-blue colour with nitro-prusside of sodium.

_Sulphites and Hyposulphites._ A solution of the alkali, insufficient for

saturation, being added to sulphuric acid tinged reddish yellow with

bichromate of potash, occasions a greenish tinge (owing to the formation

of oxide of chromium), when these are present. Hydrochloric acid added to

a clear solution, after some time, causes a turbidity and odour of

sulphurous anhydride.

_Chlorides_ yield a copious curdy precipitate with nitrate of silver,

soluble in ammonia, and reprecipitated by excess of nitric acid.

The amount of pure caustic alkali in a sample of alkali is best determined

by Fresenius's method, as follows:--The total amount of pure alkali, both

caustic and carbonated, expressed in per-cents. of carbonate of soda or

carbonate of potassa, is ascertained by any of the usual methods. The

apparent quantity of alkali per cent. is then determined, without previous

treatment of the sample with carbonate of ammonia, by the method of Will

and Fresenius (p. 86). The difference between the results indicates the

per-centage of dry caustic alkali present; or if the volumetric method be

in use, it can be often fairly estimated by adding the first portions of

the test-acid very gradually to the sample, carefully observing the

effect. When the effervescence at length commences the weight or measure

of the test-liquor expended shows the quantity of pure caustic alkali

under treatment (nearly). The result depends upon the fact, that little or

no carbonic-acid gas is expelled from the liquid on the addition of the

test-acid, until the caustic portion is very nearly neutralised.

The quantity of WATER or MOISTURE, per cent., present in an alkaline

carbonate, is indicated by the loss of weight which 100 gr. suffer on

gentle ignition in a loosely-covered iron dish or platinum crucible. So

also with samples containing caustic alkali, except that here the water of

hydration (= 1 equiv. = 9) is not expelled from the 'caustic' portion, and

must therefore be determined by calculation.

Other matters deserving the serious attention of the operator are--hitting

the exact point of neutralisation, and--preparing the test-acids of the

proper strength. The method of effecting the former correctly has been

already referred to in this article, and is also fully noticed under

ACETIMETRY and ACIDIMETRY.

_Test-acids_ may be very simply prepared by gradually diluting

concentrated sulphuric acid with water until it is reduced to the proper

strength; the dilution being made in a glass vessel containing a

'hydrostatic bead' exactly corresponding to the desired specific gravity

of the dilute acid. When the proper point is reached, and the mixture has

again acquired the normal temperature of 60° Fahr., the bead rises from

the bottom of the vessel, and floats about indifferently in the middle of

the liquid. The sp. gr. may then he carefully ascertained by means of an

hydrometer or a specific gravity bottle; after which the strength must be

accurately determined by means of a standard solution of either pure

anhydrous carbonate of soda or pure caustic soda. An acid of any given

strength or saturating power may also be prepared in the following

manner:--49 parts of commercial sulphuric acid (oil of vitriol), sp. gr.

1·825, contain nearly 40 parts or 1 equiv. of anhydrous sulphuric acid; if

we, therefore, wish to prepare a dilute acid containing in every 1000

grains weight, or measure, exactly 1 equiv. of hydrated sulphuric acid, we

have only to make 49 gr. of such acid up to 100 gr. weight or measure with

pure water. After it has recovered the proper temperature, its sp. gr., or

rather its saturating power, must be carefully tried, and, if necessary,

readjusted. As, however, it very often happens that the oil of vitriol

employed is not so strong as that above referred to, it is better first to

test its strength with pure anhydrous carbonate of soda, and to calculate

the quantity required by the Rule of Proportion. Every 53 gr. of the dry

carbonate are equal to 40 gr. of 'dry sulphuric acid.' Suppose we find the

oil of vitriol to contain only 72% of hydrated acid, then--

100 : 40 :: 72 : 55·55

or, instead of only 40 gr., fully 55-1/4 gr. will be required, which are

to be made up with water to 1000 gr., as before. Finally, the diluted acid

must be very carefully re-tested, and if found correct, at once put into a

well-stoppered bottle, and labelled, for use. Too much care cannot be

taken to ensure the test-liquid, whether for alkalies or acids, being of

the proper strength, of which the specific gravity alone is an

insufficient proof. In practice, so small a quantity only of test-acid as

that referred to above is, of course, seldom made; but as any larger

quantities are mere multiples of the smaller one, the necessary

proportions to be employed are easily calculated. The common plan is to

prepare one or more gallons or quantities of 10 lbs. each, and to preserve

the liquid in stoppered green glass 'Winchester-quart bottles,' so that it

may be always ready for use.

Although, as may be inferred from the text, sulphuric acid is generally

used as the standard acid, yet oxalic acid in pure crystals is recommended

by M. Mohr, and answers admirably, and is prepared and used exactly in the

same manner.

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