RE-AGENTS.--ACIDS, ETC.
~Acetic Acid~, H[=A=c] or C_{2}H_{4}O_{2}. (sp. gr. 1.044, containing 33
per cent. real acid).--An organic acid, forming a class of salts,
acetates, which are for the most part soluble in water, and which, on
ignition, leave the oxide or carbonate of the metal. 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.
~"Aqua Regia"~ is a mixture of 1 part by measure of nitric acid and 3
parts of hydrochloric acid. The acids react forming what is practically
a solution of chlorine.[6] The mixture is best made when wanted, and is
chiefly used for the solution of gold and platinum and for "opening up"
sulphides. When solutions in aqua regia are evaporated, chlorides are
left.
~Bromine~, Br. (sp. gr. 3.0). Practically pure bromine.--It is a heavy
reddish-brown liquid and very volatile. It boils at 60° C., and,
consequently, must be kept in a cool place. It gives off brown
irritating vapours, which render its use very objectionable. Generally
it answers the same purpose as aqua regia, and is employed where the
addition of nitric acid to a solution has to be specially avoided. It is
also used for dissolving metals only from ores which contain metallic
oxides not desired in the solution.
~"Bromine Water"~ is simply bromine shaken up with water till no more is
dissolved.
~Carbonic Acid~, CO_{2}.--A heavy gas, somewhat soluble in water; it is
mainly used for providing an atmosphere in which substances may be
dissolved, titrated, &c., without fear of oxidation. It is also used in
titrating arsenic assays with "iodine" when a feeble acid is required
to prevent the absorption of iodine by the alkaline carbonate. It is
prepared when wanted in solution, by adding a gram or so of bicarbonate
of soda and then as much acid as will decompose the bicarbonate
mentioned. When a quantity of the gas is wanted, it is prepared, in an
apparatus like that used for sulphuretted hydrogen, by acting on
fragments of marble or limestone with dilute hydrochloric acid.
~Citric Acid~ (H_{3}[=C=i] or C_{6}H_{8}O_{7}.H_{2}O) is an organic acid
which occurs in colourless crystals, soluble in less than their weight
of water. The solution must be freshly prepared, as it gets mouldy when
kept. It forms a comparatively unimportant class of salts (citrates). It
is used in the determination of phosphoric acid, chiefly for the purpose
of preventing the precipitation of phosphates of iron and alumina by
ammonia, and in a few similar cases. The commercial crystals are used;
they should be free from sulphuric acid and leave no ash on ignition.
~Hydrochloric Acid~, HCl in water, (sp. gr. 1.16. It contains 32 per
cent. of hydrogen chloride).--It is sometimes called "muriatic acid,"
and when impure, "spirit of salt." The acid solution should be
colourless and free from arsenic, iron, and sulphuric acid. It forms an
important family of salts, the chlorides. It is the best acid for
dissolving metallic oxides and carbonates, and is always used by the
assayer when oxidising agents are to be avoided. The acid is used
without dilution when no directions are expressly given to dilute it. It
has no action on the following metals: gold, platinum, arsenic, and
mercury; it very slightly attacks antimony, bismuth, lead, silver, and
copper. Tin is more soluble in it, but with difficulty; whilst iron,
zinc, nickel, cobalt, cadmium, and aluminium easily dissolve with
evolution of hydrogen and the formation of the lower chloride if the
metal forms more than one class of salts. All the metallic oxides,
except a few of the native and rarer oxides, are dissolved by it with
the formation of chlorides of the metal and water.
~Dilute Hydrochloric Acid~ is made by diluting the strong acid with an
equal volume of water. This is used for dissolving precipitates obtained
in the general course of analysis and the more easily soluble metals.
~Hydrofluoric Acid, HF.~--A solution in water may be purchased in
gutta-percha or lead bottles. It is of variable strength and doubtful
purity. It must always be examined quantitatively for the residue left
on evaporation. It is used occasionally for the examination of
silicates. It attacks silica, forming fluoride of silicon, which is a
gas. When the introduction of another base will not interfere with the
assay, the substance may be mixed in the platinum dish with fluoride of
ammonium, or of potassium, or of calcium, and hydrochloric acid,
instead of treating it with the commercial acid. It is only required in
special work. The fumes and acid are dangerous, and, of course, glass or
porcelain vessels cannot be used with it.
~Iodine, I.~--This can be obtained in commerce quite pure, and is often
used for standardising. It is very slightly soluble in water, but
readily dissolves in potassium iodide solution. It closely resembles
chlorine and bromine in its properties, and can be used for dissolving
metals without, at the same time, attacking any oxide which may be
present. It is chiefly used as an oxidizing agent in volumetric work,
being sharp in its reactions and easily detected in minute quantities.
It cannot be used in alkaline solutions, since it reacts with the
hydrates, and even with the carbonates, to form iodides and iodates.
Iodine is soluble in alcohol.
~Nitric Acid, HNO_{3}.~ (Sp. gr. 1.42; boiling point 121° C.; contains
70 per cent. by weight of hydrogen nitrate).--It is convenient to
remember that one c.c. of this contains 1 gram of real acid. It combines
the properties of an acid and of an oxidising agent. One c.c. contains
0.76 gram of oxygen, most of which is very loosely held, and easily
given up to metals and other oxidisable substances. Consequently it will
dissolve many metals, &c., upon which hydrochloric acid has no action.
All sulphides (that of mercury excepted) are attacked by it, and for the
most part rendered soluble. It has no action on gold or platinum, and
very little on aluminium. The strong acid at the ordinary temperature
does not act on iron or tin; and in most cases it acts better when
diluted. Some nitrates being insoluble in nitric acid, form a protecting
coat to the metal which hinders further action. Where the strong acid
does act the action is very violent, so that generally it is better to
use the dilute acid. When iron has been immersed in strong nitric acid
it not only remains unacted on, but assumes a _passive_ state; so that
if, after being wiped, it is then placed in the dilute acid, it will not
dissolve. Tin and antimony are converted into insoluble oxides, while
the other metals (with the exception of those already mentioned)
dissolve as nitrates. During the solution of the metal red fumes are
given off, which mainly consist of nitrogen peroxide. The solution is
often coloured brown or green because of dissolved oxides of nitrogen,
which must be got rid of by boiling. Generally some ammonium nitrate is
formed, especially in the cases of zinc, iron, and tin, when these are
acted on by cold dilute acid. Sulphur, phosphorus, and arsenic are
converted into sulphuric, phosphoric, and arsenic acids respectively,
when boiled with the strong acid.
~Dilute Nitric Acid.~--Dilute 1 volume of the strong acid with 2 of
water.
~Oxalic Acid~, H_{2}[=O] or (H_{2}C_{2}O_{4}.2H_{2}O.)--This is an organic
acid in colourless crystals. It forms a family of salts--the oxalates.
It is used in standardising; being a crystallised and permanent acid, it
can be readily weighed. It is also used in separations, many of the
oxalates being insoluble. For general use make a 10 per cent. solution.
Use the commercially pure acid. On ignition the acid should leave no
residue.
[Illustration: FIG. 32.]
~Sulphuretted Hydrogen.~ Hydrosulphuric acid, SH_{2}.--A gas largely
used in assaying, since by its action it allows of the metals being
conveniently classed into groups. It is soluble in water, this liquid
dissolving at the ordinary temperature about three times its volume of
the gas. The solution is only useful for testing. In separations, a
current of the gas must always be used. It is best prepared in an
apparatus like that shown in fig. 32, by acting on ferrous sulphide with
dilute hydrochloric acid. When iron has to be subsequently determined in
the assay solution, the gas should be washed by bubbling it through
water in the smaller bottle; but for most purposes washing can be
dispensed with. The gas is very objectionable, and operations with it
must be carried out in a cupboard with a good draught. When the
precipitation has been completed, the apparatus should always be washed
out. The effect of this acid on solutions of the metals is to form
sulphides. All the metallic sulphides are insoluble in water; but some
are soluble in alkaline, and some in acid, solutions. If sulphuretted
hydrogen is passed through an acid solution containing the metals till
no further precipitation takes place, a precipitate will be formed
containing sulphides insoluble in the acid. On filtering, adding ammonia
(to render the filtrate alkaline), and again passing the gas, a further
precipitate will be obtained, consisting of sulphides insoluble in an
alkaline solution, but not precipitable in an acid one; the filtrate may
also contain sulphides not precipitable in an acid solution, which are
soluble in an alkaline one; these will be thrown down on neutralising.
Again, the metals precipitated in the acid solution form sulphides which
may be divided into groups, the one consisting of those which are
soluble, and the other of those which are not soluble, in alkalies. This
classification is shown in the following summary:--
1. _Precipitable in an acid solution._
(a) Soluble in Alkalies.--Sulphides of As, Sb, Sn, Au, Pt, Ir, Mo, Te,
and Se.
(b) Insoluble in Alkalies.--Sulphides of Ag, Pb, Hg, Bi, Cu, Cd, Pd, Rh,
Os, and Ru.
2. _Not precipitated in an acid solution, but thrown down in an alkaline
one._
Sulphides of Mn, Zn, Fe, Ni, Co, In, Tl, and Ga.
These can again be divided into those which are dissolved by dilute
acids and those which are not.
3. _Not precipitated in an acid or alkaline solution, but thrown
down on neutralising the latter._
Sulphides of V and W.
Sulphuretted hydrogen is a strong reducing agent. Ferric salts are
thereby quickly reduced to ferrous; in hot solutions nitric acid is
decomposed. These changes are marked by a precipitation of sulphur, and
the student must be careful to pass the gas sufficiently long, and not
be too hasty in concluding that no sulphide will form because it does
not at once make its appearance. The best indication that it has been
passed long enough is the smell of the gas in the solution after
shaking.
~Sulphurous Acid~, H_{2}SO_{3}.--The reagent used may be regarded as a
saturated solution of sulphur dioxide in water. It may be purchased, and
keeps for a long time. It may be made by heating copper with sulphuric
acid and passing the gas formed into water. The heat should be withdrawn
when the gas is coming off freely. It is used as a reducing agent, and
should not be diluted.
~Sulphuric Acid~, H_{2}SO_{4}. (Sp. gr. 1.84, containing 96 per cent. of
real acid, H_{2}SO_{4}.)--This acid forms insoluble sulphates with salts
of lead, strontium, and barium. It has a high boiling point, 290° C.,
and, when evaporated with salts of the more volatile acids, converts
them into sulphates. When nitrates or chlorides are objectionable in a
solution, evaporation with sulphuric acid removes them. In working with
this acid caution is necessary, since, on mixing with water, great heat
is evolved; and, if either the acid or water has been previously heated,
a serious accident may result. In diluting the acid it should be poured
into cold water. Glass vessels containing boiling sulphuric acid should
be handled as little as possible, and should not be cooled under the
tap. The action of diluted sulphuric acid on metals closely resembles
that of dilute hydrochloric acid. Magnesium, aluminium, iron, zinc,
nickel, cobalt, manganese, and cadmium dissolve, with evolution of
hydrogen, in the cold acid, or when warmed. The action of hot and strong
sulphuric acid is altogether different; it acts as an oxidising agent,
and is itself reduced to sulphur dioxide or even to sulphur. The
following metals are attacked in this way:--copper, bismuth, mercury,
silver, antimony, tin, and lead. Gold, platinum, and arsenic are not
affected. This property is made use of in parting silver from gold and
platinum. Metallic sulphides are similarly attacked; but this method of
opening up minerals has the disadvantage of giving rise to the formation
of anhydrous sulphates of iron, &c., which are not readily dissolved
when afterwards diluted. The use of sulphuric acid in assaying is (for
these reasons) to be avoided. Its chief use is as a drying agent, since
it has a strong affinity for water. Air under a bell jar may be kept dry
by means of a basin of sulphuric acid, and gases bubbled through it are
freed from water-vapour.
~Dilute Sulphuric Acid.~--This is made by diluting 1 volume of the
strong acid with 4 of water.
~Tartaric Acid~, H_{2}[=T] or C_{4}H_{6}O_{6}.--A crystallised organic
acid, soluble in less than its own weight of water, or in less than
three parts of alcohol. It is used for the same purposes as citric acid
is. The solution is made when required.
BASES, SALTS, &c.
~Alcohol~, C_{2}H_{6}O. (Commercial alcohol of sp. gr. 0.838; it
contains 84 per cent. by weight of alcohol.)--It should burn with a
non-luminous flame and leave no residue. It is used for washing
precipitates where water is inapplicable, and for facilitating drying.
~Ammonia~, NH_{3}. (Commercial ammonia, a solution having a sp. gr. of
0.88 to 0.89, and containing about 33 per cent. of ammonia.)--It is used
as an alkali (more commonly than soda or potash), since an excess of it
is easily removed by boiling. The salts of ammonium formed by it may be
removed by igniting, or by evaporating in a porcelain dish with an
excess of nitric acid. It differs in a marked way from soda or potash in
its solvent action on the oxides or hydrates of the metals. Salts of the
following metals are soluble in an ammoniacal solution in the presence
of ammonic chloride:--copper, cadmium, silver, nickel, cobalt,
manganese, zinc, magnesium, sodium, potassium, and the alkaline earths.
~Dilute Ammonia~ is made by diluting 1 vol. of commercial ammonia with 2
of water. The dilute ammonia is always used; but in assays for copper a
stronger solution (1 of strong ammonia to 1 of water) is required.
~Ammonic Carbonate~ (Am_{2}CO_{3}) is prepared by dissolving one part of
the commercial sesquicarbonate of ammonia in four parts of water, and
adding one part of strong ammonia.
~Ammonic Bicarbonate~ (HAmCO_{3}) is prepared by saturating a solution
of the sesquicarbonate of ammonia with carbon dioxide.
~Ammonic Chloride~, AmCl.--Use the commercial salt in a 20 per cent.
solution in water. The salt should leave no residue on ignition.
~Ammonic Molybdate.~--The solution is prepared as follows:--Dissolve 100
grams of the powdered commercial salt in 200 c.c. of dilute ammonia, and
pour the solution in a slow stream into 750 c.c. of dilute nitric acid;
make up to 1 litre, and allow the mixture to settle before using. It is
used for the purpose of separating phosphoric oxide from bases and from
other acids, and also as a test for phosphates and arsenates. In using
this solution the substance must be dissolved in nitric acid, and a
considerable excess of the reagent added (50 c.c. is sufficient to
precipitate 0.1 gram P_{2}O_{5}); when the phosphate is in excess no
precipitate will be got. The precipitate is phospho-molybdate of
ammonia.
~Ammonic Nitrate~ (AmNO_{3}) is used in the separation of phosphoric
oxide by the molybdate method, and occasionally for destroying organic
matter. It is soluble in less than its own weight of water. The solution
is made when wanted.
~Ammonic Oxalate~ (Am_{2}C_{2}O_{4}.2H_{2}O) is used chiefly for the
separation of lime. The solution is made by dissolving 15 grams of the
salt in 100 c.c. of water.
~Ammonic Sulphide~ may be purchased in the state of a strong solution.
It is yellow, and contains the disulphide, S_{2}Am_{2}. It serves the
same purpose as is obtained by passing a current of sulphuretted
hydrogen through an ammoniacal solution; but has the disadvantage of
loading the solution with sulphur, which is precipitated when the
solution is subsequently acidified. It is useful for dissolving the
lower sulphide of tin (SnS).
~Baric Carbonate~ (BaCO_{3}) is sometimes used for precipitating the
weaker bases. It should be prepared when wanted by precipitating a
solution of baric chloride with ammonic carbonate and washing. The moist
precipitate is used without drying.
~Baric Chloride~, BaCl_{2}.2H_{2}O.--A crystallised salt, soluble in
2-1/2 parts of water. It is used for the detection and separation of
sulphates. Make a 10 per cent. solution.
"~Black Flux.~"--A mixture of finely divided carbon with carbonate of
potash or with carbonates of potash and soda. It is prepared by heating
tartar or "rochelle salt" until no more combustible gas is given off.
One gram will reduce about 2 grams of lead from litharge.
~Borax~, Na_{2}B_{4}O_{7}.10H_{2}O.--It is chiefly used as a flux in dry
assaying, as already described. It is also used in testing before the
blowpipe; many metallic oxides impart a characteristic colour to a bead
of borax in which they have been fused.
~Calcium Chloride.~--The crystallised salt is CaCl_{2}.6H_{2}O; dried at
200° C. it becomes CaCl_{2}.2H_{2}O, and when fused it becomes
dehydrated. The fused salt, broken into small lumps, is used for drying
gases. It combines with water, giving off much heat; and dissolves in a
little more than its own weight of water. Strong solutions may be used
in baths in which temperatures above the boiling-point of water are
required. One part of the salt and 2 of water give a solution boiling at
112°, and a solution of 2 parts of the salt in 1 of water boils at 158°.
The salt is very little used as a reagent.
~Calcium Fluoride~ or "~Fluor Spar~," CaF_{2}.--The mineral is used as a
flux in dry assaying; it renders slags which are thick from the presence
of phosphates, &c., very fluid. Mixed with hydrochloric acid it may
sometimes be used instead of hydrofluoric acid.
~Calcium Carbonate~, CaCO_{3}.--It is precipitated in a pure state by
ammonic carbonate from a solution of calcium chloride. It is used for
standardising. In the impure state, as marble or limestone, it is used
in the preparation of carbonic acid.
~Calcium Hydrate~ or ~"Lime Water."~--This is used in testing for carbon
dioxide and in estimating the amount of that gas present in air. It may
be made by slaking quicklime and digesting the slaked lime with water.
One hundred c.c. of water at 15° C. dissolves 0.1368 grams of the
hydrate (CaH_{2}O_{2}), and hot water dissolves still less. "_Milk of
lime_" is slaked lime suspended in water.
~Cobalt Nitrate~ (Co(NO_{3})_{2}.6H_{2}O) is used in a 10 per cent.
solution for the detection of oxides of zinc, aluminium, &c.; on
ignition with which it forms characteristically coloured compounds.
~Copper~, Cu.--Pure copper, as obtained by electrolysis, can be
purchased. This only should be used.
~Copper Oxide~, CuO.--It occurs as a black, heavy, and gritty power, and
is used for the oxidation of carbon and hydrogen in organic substances.
It should be ignited and cooled out of contact with air just before
using, since it is hygroscopic. Oxide of copper which has been used may
be again utilised after calcination.
~Copper Sulphate~ (CuSO_{4}.5H_{2}O) contains 25.4 per cent. of copper.
It is used in the outer cell of a Daniell-battery. The commercial salt
is used for this purpose. The re-crystallised and pure salt is used for
preparing the anhydrous sulphate, which is used for detecting moisture
in gases. For this purpose it is dried at 200° C. till no trace of
green or blue colour remains. It must be prepared when wanted. It may be
conveniently used in the form of pumice-stone, saturated with a solution
of the salt and dried. Traces of moisture develop a green colour.
~Ferric Chloride~, Fe_{2}Cl_{6}. (When crystallised,
Fe_{2}Cl_{6}.6H_{2}O.)--The solution is prepared as described under
iron. The commercial salt contains arsenic, and, since the chief use of
ferric chloride is for the determination of this substance, it must be
purified (_see_ under ARSENIC).
~Ferric Sulphate~ (Fe_{2}(SO_{4})_{3}) is a yellowish white deliquescent
salt. It is used as an indicator in volumetric silver assaying, and for
the separation of iodine from bromine. It may be purchased as iron alum,
Am_{2}Fe_{2}(SO_{4})_{4}.24H_{2}O. But it is best prepared by adding
strong sulphuric acid to ferric hydrate in equivalent proportions. Use
it as a solution containing 2 or 3 per cent. of iron.
~Ferrous Sulphate~, FeSO_{4}.7H_{2}O.--The granulated form is best, and
can be purchased pure. It is used for standardising. It keeps better in
crystals than in solution. It is readily soluble in water, but the
solution is best made with the help of a little free acid. As a re-agent
use a 10 per cent. solution. The crystals should be clear bluish-green;
if their colour is dark green, brown, or blue, they should be rejected.
~Ferrous Sulphide~ (FeS) is used for the preparation of sulphuretted
hydrogen. It may be purchased and broken in small lumps, nut-size, for
use.
"~Fusion Mixture~" (K_{2}CO_{3}.Na_{2}CO_{3}) is a mixture of potassic
and sodic carbonates in the proportions of 13 of the former to 10 of the
latter, by weight. It is hygroscopic. A mixture of the bicarbonates is
better, being purer and less apt to get damp.
~Gallic Acid~ (C_{7}H_{6}O_{5}.H_{2}O) is an organic acid, occurring as
a pale fawn-coloured crystalline powder, soluble in 100 parts of cold
water, or in 3 parts of boiling water. It is used for the determination
of antimony. A 10 per cent. solution in warm water is made when
required.
~Hydrogen~ (H) is a gas. It is obtained by acting on zinc with dilute
hydrochloric or sulphuric acid. It is used as a reducing agent, and for
providing an atmosphere free from oxygen. It reduces metallic oxides at
a high temperature. It must be freed from water; and special precautions
should be taken to prevent an admixture with air. It is generally
required in a current which can be continued for an hour or more without
interruption. The preparation can be conveniently carried out in the
apparatus shown (fig. 33). A quart bottle is half filled with sheet
zinc, and connected with bulbs filled with sulphuric acid, and with a
calcium chloride tube. The last is connected with the apparatus through
which the gas has to be passed. Dilute hydrochloric acid mixed with a
few cubic centimetres (20 c.c. to 1 pint) of stannous chloride sol. to
fix any dissolved oxygen, is placed in the funnel, and let into the
bottle by opening the stopcock when required. Care must be taken to let
the hydrogen escape for some time before starting the reduction.
[Illustration: FIG. 33.]
~Gold~, Au.--Gold, obtained by cupelling and "parting," is for most
purposes sufficiently pure. It is best kept in the shape of foil. When
the purer metal is required, gold should be dissolved in aqua regia, the
solution evaporated to a paste, diluted, allowed to stand, and filtered.
The filtered solution is acidified with hydrochloric acid, warmed, and
precipitated with sodium sulphite. The precipitate is collected, washed,
and fused on charcoal.
~Iron~, Fe.--The soft wire (thin) is used for standardising. Rods are
used in dry assays as a desulphurising agent. Steel must not be used,
since it is not pure, and contains a variable amount of iron.
~Lead~, Pb.--Granulated lead or lead-foil is used in the dry assay for
silver and gold, and in the preparation of lead salts. It can be
obtained very pure, but always contains more or less silver, 1 or 2
milligrams in 100 grams. The amount of silver it contains must be
determined and recorded.
~Lead Acetate~ (Pb[=A=c]_{2}.3H_{2}O, or
Pb(C_{2}H_{3}O_{2})_{2}.3H_{2}O) is used as a test, specially for the
detection and estimation of sulphuretted hydrogen. Prepare a 10 per
cent. solution for use.
~Lead Nitrate~ (Pb(NO_{3})_{2}) can be purchased pure. It is used for
standardising.
~Lead Dioxide~ (PbO_{2}) occurs as a dark-brown powder. It is used as an
oxidizing agent and for absorbing sulphurous oxide. It can be prepared
by digesting red lead with warm dilute nitric acid; washing and drying
the residue.
"~Litharge~," PbO.--It can be purchased as a yellow heavy powder. It is
used in dry assaying as a flux, as a desulphurising agent, and also as a
source of lead. It always contains some silver, the amount of which must
be determined.
~Litmus.~--This is an organic colouring matter which is turned red by
acids and blue by alkalies. For ordinary purposes it is best used as
litmus paper, which may be purchased in small books. A solution is
prepared by digesting 15 or 20 grams of the commercial litmus in 100
c.c. of water on the water bath. After being allowed to settle, it is
filtered and made just faintly red with acetic acid. Then there is
added a drop or two of a solution of soda and 10 c.c. of alcohol. It
should be kept in a loosely-covered bottle.
~Magnesia~, MgO.--It may be purchased as "calcined magnesia." It is used
for making "magnesia mixture," and should be kept in a corked
wide-mouthed bottle.
"~Magnesia Mixture.~"--Dissolve 22 grams of magnesia in about a quarter
of a litre of dilute hydrochloric acid, avoiding excess. Add 5 grams of
magnesia, boil, and filter. Add 300 grams of ammonic chloride, and 250
c.c. of strong ammonia; and dilute with water to 2 litres. It should be
kept in a stoppered winchester.
~Magnesium Sulphate~, MgSO_{4}.7H_{2}O.--It can be purchased very pure,
and is occasionally used as a standard salt.
~Manganese Dioxide~, MnO_{2}.--It is used in the preparation of
chlorine. The commercial article is not pure, but is sufficiently so for
this purpose.
~Marble~, CaCO_{3}.--Fragments of the white crystalline variety only
should be used. It is used as a source of lime and of carbon dioxide.
~Mercury~, Hg.--This can be purchased pure. It should have a bright
surface, flow without a tail, and leave no residue on ignition. It is
used as a standard; for amalgamation; and as a confining liquid in gas
analysis.
~Mercuric Chloride~ (HgCl_{2}) may be purchased pure. Make a 5 per cent.
solution in water. It is used for destroying an excess of stannous
chloride; for removing sulphuretted hydrogen from solution; and as a
test for stannous salts.
~Microcosmic Salt~, HAmNaPO_{4}.8H_{2}O.--When fused NaPO_{3} is formed.
It is used in testing for metallic oxides and silica before the
blowpipe. The crystals are sometimes used as a standard for phosphoric
acid.
"~Nessler's Solution.~"--Mode of preparation: Dissolve 35 grams of
potassium iodide in 100 c.c. of water; dissolve 17 grams of mercuric
chloride in 300 c.c. of water, and pour this solution into that of the
iodide till a permanent precipitate is produced; make up to 1 litre with
a 20 per cent. solution of potash; add mercuric chloride till a
precipitate is again formed; allow to settle and decant. It is used for
detecting ammonia.
~Nitre.~--This is potassium nitrate.
~Platinum Chloride~, 2HCl.PtCl_{4}. (In the crystallised form it has
6H_{2}O).--It may be made as follows:--Take 5 grams of clean platinum
scrap and dissolve in a flask at a gentle heat in 50 c.c. of
hydrochloric acid with the occasional addition of some nitric acid;
evaporate to a paste; and then dissolve in 100 c.c. of water. It is used
for separating and determining potassium.
~Phenolphthalein~ is an organic compound used as an indicator; more
especially in determining the weaker acids, it cannot be used in the
presence of ammonia. Dissolve half a gram in 100 c.c. of dilute alcohol.
~Potassium Bicarbonate~, KHCO_{3}.--It may be purchased pure; on
ignition it leaves the carbonate, K_{2}CO_{3}, which may be used as a
standard.
~Potassium Cyanide~, KCN.--It is used in the dry assay as a reducing
agent. The commercial salt is very impure. Purchase that sold as
potassic cyanide (gold) which contains about 95 per cent. of KCN. It is
used for copper assaying and occasionally in separation. Make a 10 per
cent. solution when wanted.
~Potassium Bichromate~, K_{2}Cr_{2}O_{7}. It may be purchased nearly
pure. It is used as an oxidising agent, for determining iron; and as a
test solution. For this last purpose a 10 per cent. solution is
prepared.
~Potassium Chlorate~ (KClO_{3}) can be purchased pure. It is used with
hydrochloric acid as a substitute for aqua regia.
~Potassium Ferrocyanide~ (K_{4}Fe(CN)_{6}.3H_{2}O), or "yellow prussiate
of potash," is used as a test; as an indicator; and for the
determination of zinc. Make a 5 per cent. solution.
~Potassium Ferricyanide~ (K_{6}Fe_{2}(CN)_{12}), or "red prussiate of
potash," is used for testing; and as an indicator. Make a 5 per cent.
solution when wanted, as it decomposes on keeping.
~Potassium Hydrate~, KHO. Purchase that purified with alcohol. It is an
alkali, and is used for absorbing carbonic acid, &c.
~Potassium Iodide~, KI. It may be purchased nearly pure. It is used as a
test and for dissolving iodine. It should be used in a 10 per cent.
solution freshly made. The solution decomposes on exposure to light,
with separation of iodine.
~Potassium Nitrate~ (KNO_{3}) can be purchased pure. It is used in the
dry way as an oxidizing agent. It is very fusible. It decomposes at a
low temperature into potassium nitrite (KNO_{2}) and free oxygen; and at
a higher temperature leaves potash (K_{2}O). It oxidizes sulphur and
carbon with explosive violence. This action may be moderated by mixing
the nitre with carbonate of soda, common salt, or some other inert body.
~Potassium Nitrite~, KNO_{2}.--The commercial article is not pure, but
is sufficiently so for the purpose required. A saturated solution is
used in the separation of cobalt; the solution is made when wanted.
~Potassium Permanganate~, KMnO_{4}.--This salt can be purchased
sufficiently pure. It is much used as an oxidizing agent.
~Potassium Bisulphate~ (KHSO_{4}) is used as a dry reagent for opening
up minerals. It fuses; and at a much higher temperature is converted
into potassium sulphate with loss of sulphuric acid.
~Potassium Sulphocyanate~ (KCNS) is used for the detection and
determination of traces of ferric iron; as also in the separation of
silver and copper from some of the other metals. Make a 10 per cent.
solution. It should show no colour on the addition of hydrochloric acid.
"~Red Lead~" (Pb_{3}O_{4}) is used in the dry assay as a flux instead of
litharge, from which it differs in containing a little more oxygen. When
acted on by nitric acid a brown residue of lead dioxide is left, nitrate
of lead going into solution. Like litharge it always carries silver;
about 2 milligrams in 100 grams.
~Silver~, Ag.--Pure silver in foil is required as a standard. It may be
prepared as follows:--Dissolve scrap silver in dilute nitric acid and
decant off from any residue; dilute the solution with hot water and add
hydrochloric acid until there is no further precipitate, stir; allow the
precipitate to settle; decant and wash; dry the precipitate, mix it with
twice its bulk of carbonate of soda and fuse the mixture in a crucible
until tranquil; clean the button and roll or hammer it into foil.
~Sodium Acetate~, NaC_{2}H_{3}O_{2}.3H_{2}O.--The crystals may be
purchased sufficiently pure. Make a 20 per cent. solution in water. It
is used for replacing mineral acids by acetic acid.[7]
~Sodium Acetate and Acetic Acid.~--A solution is used in the
determination of phosphates and arsenates; 100 grams of the salt is
dissolved in 500 c.c. of acetic acid, and diluted with water to one
litre.
~Sodium Bicarbonate~ (NaHCO_{3})is used as a flux in dry methods. On
ignition it leaves the carbonate (Na_{2}CO_{3}), which is used as a
standard reagent. Make a 20 per cent. solution of the carbonate for use.
It should be free from chlorides or sulphates, or if impure the amount
of impurities must be determined.
~Sodium Hydrate~, NaHO. It may be purchased in sticks, which should be
kept in a well-corked bottle. It is sometimes called "caustic soda." It
is a strong alkali. It is used for neutralizing acid solutions and for
separations where ammonia is unsuitable. Make a 5 per cent. solution for
use.
~Sodium Hyposulphite~, Na_{2}S_{2}O_{8}.5H_{2}O.--It may be purchased
pure. It is generally known as "hypo." It is used as a standard.
~Sodium Sulphite~ (Na_{2}SO_{3}.7H_{2}O) is used as a reducing agent.
~Sodium Phosphate~, Na_{2}HPO_{4}.12H_{2}O. The crystals may be
purchased pure, but they effloresce in dry air with loss of water. It
is used as a standard and for precipitating magnesia, &c. Make a 10 per
cent. solution.
~Stannous Chloride~, SnCl_{2}.2H_{2}O.--The crystals are best purchased.
If kept dry and free from air they are fairly permanent. A solution is
made by dissolving 20 grams in 10 c.c. of hydrochloric acid and diluting
to 1 litre. The solution is not permanent. It is a strong reducing
agent, and is chiefly used in solution for this purpose.
~Tin~, Sn.--Grain tin should be purchased. It is not pure, but contains
99.5 per cent. of the metal. The chief impurity is copper. It can be
used as a standard. When acted on with hot hydrochloric acid it slowly
dissolves (more rapidly in contact with platinum) and forms stannous
chloride.
~Uranium Acetate~, UO_{2}(C_{2}H_{3}O_{2})_{2}.H_{2}O.--It is best
purchased in crystals. The solution is used for the determination of
phosphates and arsenates. A solution of 3 per cent. strength is
occasionally used as an indicator.
~Uranium Nitrate~, UO_{2}(NO_{3})_{2}.6H_{2}O.--This salt is very
soluble in water and is sometimes used instead of the acetate, which is
somewhat difficult to dissolve.
"~Water~," H_{2}O.--Spring or well water is sufficiently pure for most
purposes, 100 c.c. will leave a residue of from 10 to 30 milligrams, so
that where a salt has to be dissolved out, evaporated, and weighed it
should be replaced by distilled water. Rain water, melted snow, &c.,
always leave less residue than spring water; but in other respects they
are often dirtier. Distilled water is best prepared in the office, a
glass or tin condenser being used.
~Zinc~, Zn.--It is sold in a granulated form or in sticks. It generally
contains over 1 per cent. of lead, with a little iron and arsenic. It is
used for separating metals from their solutions, and generally as a
reducing agent. For the preparation of hydrogen, and in most other
cases, scrap sheet zinc may be used.
~Zinc Oxide~, ZnO.--The commercial oxide sometimes contains carbonate.
~Zinc Sulphate~, ZnSO_{4}.7H_{2}O.--It is occasionally used as a
standard, and can be purchased nearly pure.
FOOTNOTES:
[6] 3HCl + HNO_{3} = Cl_{2} + NOCl + 2H_{2}O.
[7] NaC_{2}H_{3}O_{2} + HCl = H_{4}C_{2}O_{2} + NaCl.