IMPURITIES OF WATER.
=Properties of Water.=—When pure, water =is colourless=, or bluish
when seen in large quantity. It should be quite =inodorous=. If,
after keeping it for some time in a perfectly clean vessel, or if on
heating it a smell is developed, the water is bad. Its =taste= should
be pleasant and sparkling from the atmospheric gases dissolved in it.
Bitterness generally indicates the presence of sulphate of magnesium
(Epsom salts). Saltness is always a suspicious property, except in
water obtained in the neighbourhood of salt mines or brine springs, or
near the sea. It should be soft to the =touch=, and should dissolve
soap easily. It should be bright and =clear=, and contain no suspended
matters. Clear water is not necessarily pure, but turbid water is
always to be rejected; the only exception being the brownish-tinged
water from moors, which is not hurtful. In all other cases, printed
matter should be legible through at least 18 inches of water in a
clear glass cylinder. Thoroughly dissolved organic matter is less
dangerous than suspended; the turbidity of water is therefore of great
importance. But water may be bright and sparkling and apparently
perfectly clear, and yet highly dangerous. The most important of the
physical properties of water in regard to health are the absence of
smell and turbidity, and these can be ascertained by even the most
inexperienced. The chemical tests for the more important impurities are
given (pages 85 to 87).
The impurities of water may be classed under four heads—gaseous,
mineral, vegetable, and animal.
The gases ordinarily present in water cannot properly be regarded as
impurities, inasmuch as they are always present, and greatly increase
its palatableness. The dissolved nitrogen and oxygen bear to each other
the proportion 1·42 to 1; where sewage contamination occurs, the oxygen
will be diminished or disappear, owing to oxidation of the organic
matter.
The amount of carbonic acid gas in water varies greatly. It may be
considerable in chalk waters, and in contaminated well-water.
=Mineral Impurities.=—Mineral impurities are dissolved by water in
its course through the soil, and so will vary with the character of
the latter. 1. The water obtained from _granitic_ formations contains
very little mineral matter, often not more than two to six grains per
gallon. _Clay slate water_ is also generally very pure, as is the
water from hard _trap rocks_. 2. The water from _millstone grit_ and
_hard oolite_ is very pure, often containing only four to eight grains
per gallon, chiefly calcium and magnesium sulphate and carbonate. 3.
_Soft sand-rock waters_ usually contain thirty to eighty grains per
gallon of sodium salts, with a little lime and magnesia. 4. _Loose
sand and gravel waters_ vary greatly. They may be almost free from
mineral matter, or the solids may be more than seventy grains per
gallon, including much organic matter. 5. Waters from the _lias clays_
vary somewhat, but commonly contain a large quantity of calcium and
magnesium sulphates. 6. _Chalk waters_ generally contain from seven
to twenty grains of calcium carbonate, with smaller quantities of
other salts. 7. _Limestone and magnesian limestone waters_ differ
from the last, in containing more calcium sulphate and less calcium
carbonate, as well as much magnesium sulphate and carbonate in the
dolomite districts. 8. _Selenitic waters_ contain calcium sulphate
in considerable quantities. 9. _Clay waters_ usually possess the
characters of water from surface wells, and are objectionable. 10.
_Alluvial waters_ generally contain a large amount of various salts,
including the various calcium, magnesium, and sodium salts. 11.
_Artesian well water_ varies greatly in composition. It may contain
a large amount of sodium and potassium salts, or a small quantity of
iron, or calcium salts.
The commonest and most important mineral constituent of water is
calcium carbonate, next to this calcium sulphate. These two salts
are the chief causes of =hardness of water=. For practical purposes
as regards use in domestic matters and in manufactures, the most
important classification of waters is into _hard_ and _soft_. The
degree of hardness varies within wide limits—from rain-water, which has
no hardness at all, to the water from new red sandstone rocks which
sometimes possesses a hardness of 90 degrees; or wells in the gravel,
in which it may be as much as 152 degrees.
The following _classification of waters_, according to the degree
of hardness, beginning with the least hard and gradually increasing
in hardness, is from the sixth report of the Rivers Pollution
Commissioners:—1. Rain-water. 2. Upland surface. 3. Surface from
cultivated land. 4. River. 5. Spring. 6. Deep-well. 7. Shallow-well
water.
Calcium carbonate is the most common cause of hardness, and the
hardness produced by it is remediable by boiling or chemical means.
Calcium carbonate (chalk) is rendered soluble in water, by the carbonic
acid contained in the latter, a double bicarbonate being thus formed.
The air contained in the interstices of the soil through which water
passes, often contains 250 times as much carbonic acid as ordinary air.
The water, in percolating through the soil, dissolves this carbonic
acid, and thus is able to take up a considerable amount of chalk.
The amount of hardness in any given water is expressed in degrees, one
degree being equivalent to a grain of calcium carbonate in a gallon
of water. =Clarke’s soap test= is employed to detect the amount of
hardness. It consists of a solution of soap of a known strength. Soft
water will form a lather at once with this; hard water will only form
a lather after all the calcium salt is neutralised. The amount of
Clarke’s solution required before a lather is produced, will give an
estimate of the amount of hardness.
=To Determine the Total Hardness= take 70 c.c. of the water and place
in a stoppered bottle. From a burette run in a sufficient quantity of
the standard soap solution (of which 1 c.c. equals 1° of hardness), to
produce a lather on shaking the water, which remains unbroken after
standing five minutes. Thus, if 7·5 c.c. of the soap solution were
required, the hardness is 6°·5, as 1 c.c. of the solution is required
to produce a lather in soft water. The 6°·5 means 6·5 milligrammes of
calcium carbonate in 70 c.c. or 6·5 grains in a gallon of the water.
=To Determine the Permanent Hardness= boil 70 c.c. of the water in a
flask for half-an-hour; allow the precipitated carbonates of calcium
and magnesium to settle. Some of the latter will be re-dissolved.
Carefully decant, and make up the liquid to the original 70 c.c.
with distilled water. Filter through fine filter paper and estimate
hardness as above.
The amount of soap wasted in consequence of the hardness of water is
very great. Thus, in the case of water of one degree of hardness, as
every gallon contains one grain of chalk, 7,000 gallons would contain
7,000 grains—that is, a pound. But every grain of chalk wastes 8 or
9 grains of soap; therefore, a pound of chalk, contained in 7,000
gallons, would waste about 8½ pounds of soap. But nearly all waters
are harder than this, and they not uncommonly possess a hardness of
20° or more. If the hardness be 20°, the waste would be 170 pounds of
soap. This quantity would be easily used annually in a family of seven
or eight persons, if we include the washing of clothes. The amount of
money thus wasted can be easily estimated.
Not only does soft water require less soap, but it is much more
suitable for making tea and soup, and for boiling meat and
vegetables—both time and fuel being saved. The reason why better tea is
made when a little carbonate of soda is added to the water is that the
chalk is by this means precipitated.
Carbonate of calcium is precipitated from water by boiling it; carbonic
acid being driven off, the neutral salt falls to the bottom of the
vessel. This is the origin of the “fur” inside kettles, which lessens
their conductivity to heat, and renders necessary a greater consumption
of fuel.
The chalk may also be removed by adding to the water, while still
in the reservoir, some milk of lime—that is, quicklime made into a
milky solution with water. This is done on a large scale at various
waterworks. The reaction may be expressed thus:—
Calcium bicarbonate + calcium oxide = calcium carbonate + calcium
carbonate.
The calcium carbonate, as it is precipitated, carries down with it
organic and other matters, thus clearing and purifying the water.
The hardness due to calcium sulphate is not removable by boiling. It
is, therefore, called =permanent hardness=, to distinguish it from the
_temporary hardness_ of chalk waters, which is removable by boiling.
It may, however, be partially removed by the addition of washing soda
to the water, as well as the nitrate and chloride of calcium which are
also present. The magnesium salts are not removable by boiling or
soda. This is shown by the fact that the “fur” inside kettles does not
usually contain magnesium salts.
The amount of hardness varies greatly in different waters. In the deep
wells in magnesium limestone, it varies from 14°-57°; in the deep
wells from chalk beds, it varies from 13° to 27° and may be higher.
In the water from Bala Lake, Wales, the temporary hardness is 0°·1,
the permanent hardness 0°·3; in the Loch Katrine water there is no
temporary hardness, 0°·9 permanent hardness; in the water from the new
red sandstone (Nottingham), the temporary hardness is 9°·6, permanent
10°·2; in a chalk spring at Ryde, temporary hardness 16°·7, permanent
3°·9 (Wanklyn). The total hardness in the metropolitan water supplies
from the rivers Thames and Lea, varies from 13°·2 (Southwark Company)
to 14°·6 (New River Company); in the Kent Deep Wells 20°·1; in deep
wells from the chalk at Brighton it varies from 12° to 13°. In all
these, the hardness is chiefly temporary.
The amount of permanent hardness is always great in water from clays,
as the London, Oxford, Kimmeridge, and Lower Lias clays; or in places
where there are large deposits of calcium sulphate, as at Montmartre,
near Paris (hence the name Plaster of Paris, given to desiccated
calcium sulphate). Water from fissures in the clay often contains,
also, a large amount of organic matter.
=Chlorides= are always present in small quantities in water. As a rule
the presence of more than 1 grain per gallon, _i.e._ ·7 parts per
100,000 of water, indicates contamination with some animal refuse,
unless the water is derived from new red sandstone, or brine springs,
or from the neighbourhood of the sea. This rule does not, however, hold
universally good. The absence or the presence of only a minute quantity
of chlorides indicates the probable absence of animal contamination;
but in exceptional cases waters of the highest organic purity may
contain more chlorides than the same bulk of sewage.
=To determine the amount of Chlorine= take 70 c.c. of the water,
add a few drops of solution of potassium monochromate (KCrO₄). From
a burette run in gradually a standard solution of silver nitrate
(of such a strength that 1 c.c. of the solution is equivalent to 1
milligramme of chlorine). The silver solution forms milky chloride of
silver (AgCl) by combination with the chlorine of the chlorides in the
water. When all the chlorine is thus combined, the next drop of the
silver solution forms a deep red tint with the chromate. The number of
c.c.’s of the silver solution required to produce this effect, equals
the number of milligrammes of chlorine in 70 c.c. of water, or the
number of grains of chlorine in a gallon of water. To convert this
into parts per 100,000, divide by 7 and multiply by 10.
To express the amount of chlorine in terms of common salt (NaCl),
multiply the parts per 100,000 of chlorine by 1·65.
=Nitrates= in any water are suspicious; but their import varies with
the circumstances under which they occur. A minute quantity of ammonium
nitrate is present in nearly all waters; and the water of deep wells,
especially of wells in the chalk, which, as a rule is perfectly free
from sewage, may be highly charged with nitrates. Nitrates, when
derived from sewage, represent a completely oxidised condition of
its nitrogenous matter. Crude sewage generally contains no nitrates.
=Nitrites= as a rule indicate more recent contamination, and therefore
greater danger than nitrates. The presence of more than a trace of
phosphates is a strong indication of contamination with sewage matter.
=To determine the amount of Nitrites and Nitrates= the best known
methods are by the indigo, the phenol-sulphuric, the aluminium, or the
zinc-copper couple tests. For nitrites the metaphenylene-diamine test
is employed (page 85). The following _qualitative tests_ will suffice
for elementary work.
=Nitrates.= An equal amount of a solution of brucine is added to the
suspected water in a test-tube, then a little pure sulphuric acid is
poured down the side of the tube. A pink zone is produced if nitrates
are present in considerable amount.
=Nitrites.= A few drops of each of diluted sulphuric acid and of
metaphenylene-diamine solution give a red colour with water after
standing for a few minutes, if nitrites are present.
=Lead= is an occasional contamination of slightly acid waters. The
purest and most oxygenated waters act most readily on lead; as
also those containing organic matter, nitrates or nitrites. Waters
containing chlorides also act on lead, the chloride of lead being
sufficiently soluble to produce poisonous symptoms. Upland surface
waters derived from moorlands in certain districts, _e.g._ around
Sheffield, have been found to be capable of dissolving considerable
lead from lead service-pipes. The water taken first from the tap in
the early morning is the most heavily charged with lead. Such waters
are very soft; but other moorland soft waters do not dissolve lead.
It is the water having a slightly acid reaction which possesses this
property. The source of this acid, whether sulphuric acid from the
products of combustion in a neighbouring town, or an organic acid,
is uncertain. The plumbo-solvent action of such water is greatest in
autumn, when the amount of acid is at its maximum. The property of
dissolving lead is removed by passing the water on a large scale over
filters of sand, spongy iron, chalk, or limestone. The addition of
a small quantity of carbonate of soda has the same effect. In such
districts the use of tin-lined iron pipes for domestic services has
been recommended, but these are liable to fracture when bent. Pipes
consisting of an outer case of lead and an inner pipe of tin with a
layers of asbestos between have also been placed on the market. (See
also page 68.)
Hard waters have the least action on lead; a coating of insoluble
carbonate of lead being formed on the interior of the pipe, which
prevents any further action. Thus the use of lead pipes for water
containing carbonates or sulphate, or calcium phosphate, is
comparatively safe. Hard water containing carbonic acid gas under
pressure will dissolve a small amount of carbonate of lead; this
explains the cases of lead poisoning from soda water which was formerly
supplied in syphon bottles with lead tubes.
Lead is dissolved much more easily by water if other metals are in
contact with it, as iron, zinc, or tin, galvanic action being thus set
up. Zinc pipes containing some lead are very dangerous, especially with
the distilled water used on board ships.
=To determine the presence of lead in water=, place a given quantity,
say 100 c.c. in a white dish, and stir with a rod dipped in a solution
of ammonium sulphide; if the water becomes coloured, this is generally
due to the presence of iron or lead. If the colour remains after
adding a drop or two of hydrochloric acid, lead is present.
=To determine the amount of lead=, a standard solution of lead acetate
containing 1∕10 milligrammes of lead in 1 c.c., is made by dissolving
·183 gramme of crystallised lead acetate in a litre of distilled
water. Place 100 c.c. of the water to be examined in a Nessler glass,
acidify by a few drops of acetic acid; now add 1∕2 c.c. of a saturated
solution of ammonium sulphide. A brownish-black discoloration is
produced if lead is present. To a second Nessler glass, containing 100
c.c. of distilled water, the same amounts of acetic acid and ammonium
sulphide are added, and then a sufficient quantity of the standard
lead solution is added, until the tints of the contents of the two
Nessler glasses are identical. The amount of the standard solution
added being known, we know the amount of lead in 100 c.c., and the
amount per litre (1,000 c.c.) will be tenfold. Thus if 2 c.c. of the
solution were required for matching colours, there were ·2 parts of
lead per 100,000 of water, or ·14 grains per gallon.
=Traces of Iron= are sometimes present in water, giving it an
astringent taste. Such water is apt to turn brown; and tea made from it
is very dark.
=Organic Impurities.=—Organic impurities may be either vegetable or
animal, the latter being by far the most dangerous. The water from
moorlands is often brown, but this is not noxious. Growing plants,
again, may be beneficial to water, by absorbing dissolved organic
matter, and aiding its oxidation. Decaying vegetable matter is
objectionable in water, and may set up diarrhœa.
The most important organic impurity of water has an animal origin—from
sewage; the liquid or solid excreta (_i.e._ the urine or fæces) gaining
accidental access to the water. Besides sewage, the eggs of various
intestinal worms have been swallowed with water; and in a few cases,
even leeches. But whatever the source of the organic matter contained
in water, it contains nitrogen as an essential constituent; and tends
under the influence of warmth, and therefore especially in summer,
to undergo putrefactive changes, owing to the action of bacteria.
These split up the more complex molecules of organic matter into
simpler matter; ammoniacal compounds and salts, of which the most
important are nitrites and nitrates, being final products of their
activity. The detection of nitrates, and still more of nitrites, is
important, as they may indicate the occurrence of previous sewage
contamination. These products are quite harmless in water, except as
an indication that the water has been polluted, and that possibly a
certain proportion of the nitrogenous matter in the form of the complex
organic matter forming the germs of such diseases as enteric fever, may
still be present. Organic matter may be _suspended_ or _dissolved_,
the former being most dangerous to health. The germs or microbes
causing disease consist of suspended, _i.e._ particulate matter. The
amount of organic matter is determined by the amount of free ammonia
and albuminoid ammonia which are present (Wanklyn’s process), by
Frankland’s combustion process, or by Forschammer’s oxygen process; all
of which give indications, rather than an exact estimate of its amount.
METHODS OF WATER ANALYSIS.
The following scheme of =qualitative examination= may be followed,
when an immediate opinion is required as to a water. It can only be
trusted when the examination shows pollution. The following results
will be obtained, for instance, when a minute quantity of urine is
added to a gallon of water.
(1) The water has a faint odour.
(2) Its colour is greenish yellow in bulk.
(3) On adding a few drops of Nessler’s solution, a deep yellow colour
appears.
(4) A few drops of an acid solution of permanganate of potassium
become yellow when added to it.
(5) Acidify some of the water in a test-tube with nitric acid, then
add silver nitrate solution. Distinct cloudiness is produced, much
greater than with pure tap water.
(6) Addition of hydrochloric acid and barium chloride solution shows a
much greater quantity of sulphates than the same quantity of tap water.
(7) A quantity of the water evaporated in a porcelain dish over a
Bunsen’s flame gives a white residue, which speedily turns brown, with
a urinous odour.
(8) Ignite the ash and add some nitric acid to oxidise it more
completely. Then dissolve in distilled water, and add acid molybdate
solution. A yellow colour, followed by a precipitate, indicates high
phosphates and sewage pollution.
=The Complete Systematic Examination= is (_a_) physical, (_b_)
bacteriological, and (_c_) chemical. Of the =physical tests=, _colour_,
which should never be yellow or brown except for peaty water, is
important. _Taste_ is a somewhat uncertain guide, but any badly-tasting
water should be rejected. The _odour_ on heating to 80° F. in a closed
flask may indicate pollution. The degree of _hardness_ can be roughly
tested by rubbing between the hands. The absence of _turbidity_ is most
important, as suspended impurities are more dangerous than all others.
Printed matter should be legible through a column of 18 inches of water.
=Microscopally= the suspended matter in water which has been allowed to
settle should be examined. Particles of vegetable matter, _e.g._ fibres
of cotton, linen, cells of potato, or spiral cells of cabbage, are
important as indicating domestic impurities. Bits of wool, hair, wings
and legs of insects and epithelium may be discovered. The presence
of algæ, diatoms and desmids, or of water-fleas, cannot be held to
indicate pollution, as these are found in all running streams and in
many wells. The eggs and embryos of worms are much more serious.
=Bacteria= are almost invariably present in water. The majority of
these micro-organisms are harmless. But as they may number among them
the germs producing diseases like enteric fever and cholera, the
estimation of their number and particularly of any deviation from
the number usually present in a given water, and if possible the
detection of special disease-producing bacteria, are very important.
This method has been made more practicable since Koch’s method of
“plate cultivation” of bacteria was discovered. A small quantity of
the water to be examined (kept surrounded by ice until this test is
applied, to prevent multiplication of bacteria in the bottle), is
mixed with sterilised gelatine which has been melted over a water
bath. Then the mixture is spread in a thin layer on a glass plate and
allowed to solidify, having been covered to prevent atmospheric germs
from settling on the gelatine. The bacteria in the water thus become
fixed, each growing and forming “colonies” dotted over the plate.
These colonies can be recognised by their size and appearance, and by
sub-culturing according to recognised methods. The number of such
colonies, and the number of bacteria, from which, presumably, such
colonies sprang in 1 c.c. of filtered Thames water is usually much
below 100; in the water before filtration many thousands are present.
It has been suggested that no water should be regarded as wholesome
which contains more than 100 bacteria in each c.c.
This standard is, however, obviously arbitrary. Chalk water ought to
have a smaller number than this; river waters may have more, and yet
be wholesome. Everything depends on the character of the bacteria
found. The detection of the _Bacillus coli communis_, which is present
in sewage, and normally in the human intestine, is very suggestive of
contamination by sewage. The bacteriological method of examination of
water is still in its infancy.
CHEMICAL ANALYSIS.
(1) The =total solids= are ascertained by evaporating a given quantity
of the water to dryness, and weighing.
(2) =Determination of Chlorine= (see page 81).
(3) =Determination of Hardness= (see page 80).
(4) The =Determination of Nitrites= is based on the reddish-brown
colouration produced when an acid solution of metaphenylene diamine is
brought into contact with a weak solution of nitrous acid. 100 c.c.
of the water under examination are placed in a clean glass cylinder.
Add 1 c.c. of H₂SO₄ solution (1 in 3), then 2 c.c. of metaphenylene
diamine solution (5 grains in 1 litre of water with a little H₂SO₄
added). Stir well with a glass rod. If a colouration is produced at
once, a smaller quantity of water must be taken, and made up to 100
c.c. with pure distilled water. The quantity of nitrous acid present
is measured by introducing different fractions of a c.c. of the
standard sodium nitrate solution[3] into similar glass cylinders.
Each is then made up to 100 c.c. with distilled water, and the
metaphenylene diamine solution and acid added as before. The colour
develops slowly; time must, therefore, be allowed in matching.
(5) The =Determination of Nitrates= can be conveniently made by the
following method. When phenyl-hydrogen sulphate solution is poured
upon a nitrate, and sulphuric acid is formed, picric acid is formed:—
(C₆H₅)HSO₄ + 3 HNO₃ = C₆H₂(NO₂)₃OH + H₂SO₄ + 2 H₂O.
The addition of free ammonia in excess forms yellow ammonium picrate,
the intensity of the colour of which is an index of the picrate, and
of the nitrate from which it was produced. (_a_) Evaporate 25 c.c.
of the water under examination, and (_b_) 5 c.c. of standard KNO₃
solution (containing 1 part N in 100,000) to dryness in two porcelain
dishes over the water bath. Add 1 c.c. of phenyl-sulphate solution to
each of these as soon as cool, stir well with a glass rod, then add 1
c.c. distilled water to each dish and 3 drops of strong H₂SO₄. Next
add 25 c.c. of water to each dish, and after heating for five minutes
over the water bath, add solution of ammonia to each dish in excess.
A yellow colour is produced in proportion to the amount of nitrate
present. Transfer the liquids to glass cylinders, and dilute each to
100 c.c. Take 50 c.c. of the solution showing the least colour, and
dilute the other with distilled water, until it has the same tint.
Supposing the 100 c.c. of the sample required to be diluted to 150
c.c.—
Then the amount of N will be 150∕100 × 5∕25 = ·3 parts per 100,000.
If the two solutions (_a_) and (_b_) when diluted have the same tint,
then the
Amount of N in the sample = 5∕25 = ·2 parts per 100,000.
(6) =Determination of Organic Matter.= Frankland’s =combustion process=
involves the use of delicate and costly apparatus, and is seldom
employed. In this process the organic carbon is evolved as carbonic
acid, and the nitrogen as such.
=Wanklyn’s ammonia process= is based on the reduction of organic
matter to ammonia. Part of this ammonia, =free or saline ammonia=, is
simply combined with carbonic, nitric, or other acids, or is easily
derived from the urea of urine, CH₄N₂O + 2H₂O = 2(NH₄)₂CO₃. Another
part is only set free when the water is boiled with a strongly alkaline
solution of permanganate of potassium. This is called the =albuminoid
ammonia=.
In carrying out this method, a retort is taken, and after having been
washed out, first with a little sulphuric acid, and then with some of
the water to be analysed, 500 c.c. of the latter is put in, and the
retort is connected with a condenser, and distillation begun; 50 c.c.
of the distilled water is collected in a cylindrical glass tube called
a Nessler glass. To this 1½ c.c. of Nessler’s reagent (mercuric
iodide dissolved in a solution of potassic iodide and made alkaline by
potass) are added. A rich brown colour is produced, if any ammonia is
present in the distillate. The amount of ammonia in the distillate is
determined by exactly imitating its colour by adding a known quantity
of a standard solution of ammonium-chloride to 50 c.c. of ammonia-free
distilled water, and then Nesslerising as before. Each c.c. of the
dilute standard ammonium chloride solution is equivalent to ·00001
gramme of ammonia (NH₃).
If the first 50 c.c. of water distilled over gives only a slight
colouration with the Nessler solution, no more water needs to be
distilled over for free ammonia. If more is present, two more 50 c.c.’s
must be distilled over, and the amounts of the standard solution
required for imitating the test in each Nesslerised 50 c.c. added
together. Thus, if 2 c.c. were needed. This
= ·00002 grm. NH₃, which is contained in 500 c.c. of the water
= ·00002 × 200 = ·004 parts saline NH₃ in 100,000 of water.
The free ammonia having been distilled over, 50 c.c. of an alkaline
permanganate solution (containing 8 grammes KMnO₄ and 200 grammes
of NaOH in 1100 c.c. of distilled water, boiled until the bulk is
reduced to 1,000 c.c.) is poured into the retort, and distillation
is begun again. Three successive 50 c.c.’s of water are collected,
and then the distillation stopped. Each of these is Nesslerised, and
the tint imitated as before with standard ammonia solution. The three
amounts of ammonia thus found to be present are added together; and
when multiplied by 200, we obtain the amount of albuminoid ammonia in
100,000 parts of water. This test is universally employed by water
analysts along with the next test.
The amount of =Oxygen Absorbed= from permanganate of potassium is
regarded as an approximate test of the amount of organic matter in
water. Qualitatively this forms a favourite method of testing the
purity of water. Two glass cylinders are taken, one filled with
distilled water, one with the water to be tested. To each is added a
given small amount of an acid solution of permanganate of potassium.
The distilled water to which permanganate has been added will retain
its pink colour; while, if the water being tested is very impure,
it will speedily become decolourised. The rapidity and degree of
decolourisation are a rough test of the amount of impurity. A rapid
decolourisation proves the presence of organic matter having an animal
origin, or of sulphuretted hydrogen, iron, or nitrites. Sulphuretted
hydrogen is rarely present, and can be easily recognised by its smell;
iron or nitrites are readily distinguished by their appropriate tests.
In the absence of these, the rapid discolouration is an indication of
animal contamination.
=To Determine the Amount of Oxygen Absorbed=, two glass-stoppered
bottles, each holding about 350 c.c. are required. Into one, 250 c.c.
distilled water, and into the other the same amount of the water under
examination are placed. To each are then added 10 c.c. of standard
permanganate of potassium solution[4] and 10 c.c. of a standard pure
25 per cent sulphuric acid solution. The two bottles, after being
shaken, are placed in a water-bath at 27°C for four hours. At the
end of this time add a few drops of potassium iodide solution to each
bottle. The pink is now replaced by a yellow colour.[5] A standard
thiosulphate solution (Na₂S₂O₃, 5H₂O)[6] is placed in a burette. From
this the thiosulphate solution is run into the control bottle until the
yellow colour almost disappears. Now a few drops of starch solution
are added, and a blue colour is produced. The thiosulphate is then
added cautiously until all the blue colour disappears. The amount of
thiosulphate necessary for this is read off on the burette. The same
process is repeated with the bottle containing the sample of water. The
starch acts as an indicator. The amount of iodine liberated is an index
of the amount of permanganate in the water, which has not been used up
by its impurities. The amount of iodine liberated is measured by the
amount of thiosulphate required to decolourise the solution. Thus—
2 Na₂S₂O₃ + I₂ = 2 NaI + Na₂S₄O₆.
Suppose that 20 c.c. of thiosulphate solution were required to
decolourise the iodine liberated in 250 c.c. of a sample of water,
while the distilled water required 25 c.c. Then 25 c.c. thiosulphate
represents 10 c.c. of the permanganate solution = ·001 grains of
available oxygen.
25-20 = 5
As 25 c.c. = ·001 grm. O, 5 c.c. = 5∕25 of ·001 = ·0002 grm.
This is the amount of O absorbed by 250 c.c. of the sample.
Therefore „ „ „ 100,000 „ = ·08 grm.
It is usual to make a similar determination of the amount of oxygen
absorbed in fifteen minutes.
The =Interpretation of Results= of analysis is more difficult than
the analysis. A single analysis may be misleading, unless the source
of the water is known. Constancy in composition or analysis is almost
as important a criterion of purity as the actual character of the
constituents. A knowledge of the source is essential in interpreting
results of analysis, as the chemical composition of water varies with
its source. The following rules are only approximately correct, and
are subject to the above general considerations. The _total dissolved
solids_ in river-water are usually 10 to 30 parts in 100,000. Shallow
well-water may contain from 30 to 200 parts or even more, and deep
well-water from 20 to 70 parts.
_Saline Ammonia_ in water is commonly of animal origin, ammonia (NH₃)
being one of the first products of decomposition of nitrogenous animal
refuse. Upland surface water usually contains about ·002 parts per
100,000, but it may reach ·008 or more if the land over which the
water passes has been manured. Shallow well-water may be free from
ammonia, or this may be very excessive in amount. Deep well-water may
contain no ammonia or any amount up to ·1 per 100,000. Its presence is
suspicious if the albuminoid ammonia is above a trace, or if the oxygen
absorbed is appreciable in amount. Generally water is suspicious if
saline ammonia is up to ·01 per 100,000. _Albuminoid Ammonia_ indicates
the amount of organic nitrogenous matter present in the water. It
should not exceed ·005 parts per 100,000, while at the same time the
saline ammonia should not usually exceed ·01 per 100,000. For _Oxygen
consumed_ the following table of the weight of oxygen required for
100,000 parts of water is given by Clowes and Coleman:—
┌────────────────────┬─────────────────────┬─────────────────────────┐
│ │UPLAND SURFACE WATER.│WATER FROM OTHER SOURCES.│
│ _Water of_ ├─────────────────────┼─────────────────────────┤
│ _Great purity_ │Not exceeding ·1 │Not exceeding ·05 │
│ _Medium purity_ │From ·1 to ·3 │From ·05 to ·15 │
│ _Doubtful purity_│From ·3 to ·4 │From ·15 to ·20 │
│ _Impure_ │Exceeding ·4 │Exceeding ·20 │
└────────────────────┴─────────────────────┴─────────────────────────┘
The presence of more than 1 and still more so of 2 grains of _Chlorine_
per 100,000 of water is most suspicious, except in saline districts.
_Nitrites_ if present in an appreciable quantity indicate comparatively
recent contamination by sewage. In deep well-water they may be produced
by deoxidation of nitrates. _Nitrates_ in upland surface waters should
not be equivalent to more than ·03 of N. per 100,000; in shallow
well-waters the amount varies greatly; in deep well-waters it may be
excessive. As a rule it ought not to be equivalent to more than 5 parts
of N. per 100,000 of water; but the significance of nitrates depends
greatly on the source of the water and on the amount of the other
constituents present.
Chemical analysis alone cannot ascertain the safety of a given drinking
water. A minute amount of impurity inappreciable to analysis may be
competent to produce disease; while another water may be drunk with
impunity, which contains considerable organic matter. Chemical analysis
“can tell us of impurity and hazard, but not of purity or safety”
(Buchanan). An accurate opinion as to the character of a drinking
water can only be expressed when one knows the amount of each chief
constituent (as above), and whether these amounts deviate from the same
water at other times or from other waters in the vicinity.