a sample of water may be determined directly by dissolving the soluble
salts in distilled water, to which enough nitric acid is added to
preserve the solution slightly acid. After filtering and washing, silver
chlorid is added, little by little, with constant shaking until a
further addition of the reagent produces no further precipitate. The
beaker or flask should be placed in a dark place, on a shaking apparatus
which is kept in motion until the precipitate has entirely settled in a
granular state. The silver chlorid is then collected on a gooch, washed
free of all soluble matter, dried at 150° and weighed. If the
precipitate be ignited to incipient fusion, a porcelain gooch should be
used.
A more convenient method is to determine the chlorin directly in the
water, or, where the quantity is too minute, after proper concentration,
volumetrically by means of a titrated solution of silver nitrate, using
potassium chromate as indicator. As soon as the chlorin has all united
with the silver, any additional quantity of the silver nitrate will form
red silver chromate, the permanent appearance of which indicates the end
of the reaction. This process is especially applicable to water, which
in a neutral state contains no other acids capable of precipitating
silver. The chromate indicator does not work well in an acid solution.
=516. Solutions Employed.=—A quantity of pure silver nitrate, about five
grams, is dissolved in pure water and made up to a volume of one liter.
For determining the actual strength of the solution, 0.824 gram of pure
sodium chlorid is dissolved in water and the volume made up to half a
liter. Twenty-five cubic centimeters of this solution are placed in a
porcelain dish, and a few drops of the solution of potassium chromate
added. The silver nitrate solution is allowed to flow into the porcelain
dish from a burette graduated to tenths of a cubic centimeter. The red
color produced as each drop falls, disappears on stirring as long as
there is any undecomposed chlorid. Finally a point is reached when the
red color becomes permanent, a single drop in excess of the silver
nitrate being sufficient to impart a faint red tint to the contents of
the dish.
The solution of potassium chromate is prepared by dissolving five grams
of the salt in 100 cubic centimeters of water. Silver nitrate solution
is added until a permanent red precipitate is produced, which is removed
by filtration, and the filtrate is employed as the indicator as above
described. Water with any considerable quantity of chlorin can be
treated directly with the reagents; when the percentage of chlorin is
low, previous concentration to a convenient volume is advisable.
In waters containing bromids and iodids these halogens would be included
with the chlorin estimated as above. For agricultural purposes such
waters have little importance. In the case of soluble carbonates capable
of precipitating silver this action can be prevented by acidifying the
water with nitric acid and afterwards removing the excess of acid with
precipitated calcium carbonate. In this reaction McElroy recommends the
use of Congo paper, which is not affected by the carbon dioxid but is
turned blue as soon as an excess of nitric acid is added. After the
addition of the calcium carbonate the mixture should be boiled to expel
carbon dioxid.[343]
Irrigation waters from natural sources or derived from sewage rarely
contain enough chlorin to make their use objectionable. On the other
hand, when water is obtained for this purpose from artesian wells it may
often contain a quantity of chlorin which will eventually do more harm
to the arable soil than the water will do good.
=517. Carbon Dioxid.=—Free carbon dioxid in water has no significance in
respect of its use for irrigation purposes. Such waters, however, are
usually of a highly mineral nature and thus are justly open to suspicion
when used for farm animals and on the field. The presence of free carbon
dioxid as has already been pointed out in paragraph =42=, gives to
water, one of its chief sources of power as an agent for dissolving
rocks and ultimately forming soil. The estimation of the total free
carbon dioxid in a sample of water issuing from a spring or well is a
matter of some delicacy by reason of the tendency of this gas to escape
as soon as the water reaches the open air and is relieved from the
natural pressure to which it has been subjected. The actual quantity of
the gas remaining in solution at any given time is determined as
follows: 100 cubic centimeters of the water are placed in a flask with
three cubic centimeters of a saturated solution of calcium and two of
ammonium chlorid. To this mixture is added forty-five cubic centimeters
of a titrated solution of calcium hydroxid. The flask is stoppered, well
shaken, and set aside for twelve hours to allow the complete separation
of the calcium carbonate formed.
When the supernatant liquid is perfectly clear an aliquot part thereof,
from fifty to one hundred cubic centimeters, is removed and titrated
with decinormal acid with phenacetolin or lacmoid as an indicator. From
the quantity of calcium hydroxid remaining unprecipitated the amount
which has been converted into carbonate can be determined by difference.
The difference between the quantity of calcium hydroxid originally
present in the solution and that remaining after the above treatment
multiplied by the factor 0.0022 will give the weight of carbon dioxid
present in the water in a free state or in excess of that present as
normal carbonates.
UNUSUAL CONSTITUENTS OF SOIL.
=518. Boric Acid.=—Boron, while not regarded as an essential plant food,
is yet found quite uniformly in the ashes of a large number of plants.
It may, therefore, be of some interest to the agricultural analyst to
determine the amount of it which may be present in a soil extract or
mineral water. For this purpose the following method due to Gooch may be
employed.[344] To one liter of the water supposed to contain boric acid
add enough sodium carbonate to produce distinct alkalinity. After
evaporation to dryness acidify the residue with hydrochloric acid, apply
a piece of turmeric paper and dry at a moderate heat. The usual
brown-red tint will reveal the presence of boric acid.
The quantitive estimation of the acid is accomplished as follows: One or
more liters of the water rendered alkaline as above are evaporated to
dryness. With the aid of as small a quantity as possible of acetic acid
the dry residue is transferred to a distillation flask and condenser
arranged as shown in Fig. 92. About one gram of recently ignited pure
lime, cooled in a desiccator and weighed accurately, is introduced into
the flask at the bottom of the condenser and slaked by a few cubic
centimeters of water. When the flask is attached, the terminal tube of
the condensing apparatus should dip into the lime-water in the flask.
The heating-bath is partly filled with paraffin at a temperature of
about 120°. The paraffin-bath is raised so that the entire bulb of the
flask is immersed therein and the distillation continued until all the
liquid has been distilled. The bath is removed and after cooling
somewhat, ten cubic centimeters of methyl alcohol are introduced by
means of the stoppered funnel-tube and the process of distillation
repeated. This operation with methyl alcohol is repeated five times. The
boric acid passes off with the distillate and is found in the flask
below the condenser as calcium borate. The contents of the distillation
flask are evaporated to dryness and ignited conveniently in the same
crucible in which the lime was burned. The increase in weight represents
the quantity of boric anhydrid, B₂O₂ obtained.
[Illustration:
FIGURE 92. GOOCH’S APPARATUS FOR BORIC ACID.
]
=519. Method Of Moissan.=—The principle of the method of Gooch, which
has just been described, is applied by Moissan in a slightly modified
manner.[345]
In this method the generating flask is made smaller than in the Gooch
apparatus, and the funnel at the top is oval and provided with a
ground-glass stopper. It is closed at the bottom with a glass stop-cock,
and the slender funnel-tube enters through a rubber stopper and ends
about the middle of the bulb of the flask. The delivery-tube is longer
than in Fig. 91, and is bent upward at its middle part in the form of an
obtuse angle. The receiving flask is connected with the condenser by
means of a tube-shaped funnel, which prevents any regurgitation into the
generating flask. The receiving flask also has attached to it a
three-bulb potash absorption tube, through which all vapors escaping
from the receiving flask must pass. The bulbs contain a five per cent
solution of ammonia. The receiving flask should be placed in a
crystallizing dish and kept surrounded with ice-water.
The boron which is to be estimated should be in the form of boric acid.
This can readily be accomplished by treating the residue to be analyzed
with nitric acid in a sealed tube. The mixture is introduced into the
generating flask, washing with a little nitric acid, and evaporated to
dryness. The heat is removed, and, by means of the funnel, ten cubic
centimeters of methyl alcohol added, and distillation is renewed. This
operation with methyl alcohol is repeated four times, taking care to
distill to dryness in each case before the addition of a fresh quantity
of alcohol. Afterwards, there is introduced into the apparatus one cubic
centimeter each of distilled water and nitric acid and the distillation
again carried to dryness. The treatment with methyl alcohol, as
described above, is then repeated three times. To determine whether all
the boric acid have passed over, the receiving flask at the bottom of
the condenser is disconnected and a drop of the alcohol taken from the
end of the condensing tube by means of a filament of filter paper. On
burning, the flame should not show any trace of green. In case a green
color is observed, the distillation with nitric acid and methyl alcohol
must be repeated.
The ammonia in the potash bulbs serves to arrest any of the vapors
carrying boric acid which might escape from the receiving flask. The
contents of the bulbs are to be mixed with the liquid in the receiving
dish, and the whole poured onto a known weight of recently ignited
calcium oxid contained in a platinum dish, and the mixture briskly
stirred. If the liquid be very acid the platinum dish should be kept in
ice-water to prevent heating. After fifteen minutes the liquid usually
becomes alkaline, and it is then evaporated at a temperature below the
boiling-point of methyl alcohol (66°). The mass, after the methyl
alcohol has disappeared, is dried at a gradually increasing temperature,
and finally, the dish is ignited over a blast, at first covered and
afterwards open. The dish is covered and weighed and again ignited until
constant weight is obtained.
The lime used should be specially prepared by igniting calcium nitrate
incompletely, and reigniting a portion of this to constant weight just
before beginning each analysis. The calcium oxid is then obtained in a
perfectly fresh state. It should be employed in considerable excess, for
each half gram of boric acid at least eight grams of the lime. The
operation is tedious but the results are quite accurate.
SPECIAL TREATMENT OF MUCK SOILS.
=520. General Considerations.=—Deposits of muck which are to be used as
soil for cultural purposes, or marsh lands, containing large quantities
of organic matter, require a special treatment in addition to the
general principles of examination illustrated in the previous pages.
These soils, essentially of an organic origin, do not permit of the same
treatment either chemical or physical as is practiced with soils of a
mineral nature. For instance, it would be useless to attempt a silt
analysis with organic soils, and the extraction of them with
hydrochloric acid for the purpose of determining the materials passing
into solution would prove of little utility. The object of the
examination is not only to obtain knowledge of the ultimate constitution
of the sample, but also, and this is the practical point, to gain some
idea of its stores of plant food and of the proper steps necessary to
secure a supply of the deficient nutrients. The final analytical
processes for the estimation of the constituents of a muck or vegetable
soil are the same as those already given, but the preliminary treatment
is radically different.
=521. Sampling.=—First of all the geologic and meterologic conditions of
the muck formations must be determined as nearly as possible. It is fair
to presume that these formations are of comparatively recent origin, in
fact that they are still in progress. The geologic formation in the
vicinity of the deposit should be noted. Information should be given in
respect of the character of the water, whether running or stagnant,
fresh, salt, or brackish, and changes of level to which it is subject,
should be noted. It should be particularly stated whether the vegetable
growth contributing to the formation be subject to frost or freezing.
The character of the growth is to be carefully noted, and observation
made of any changes in vegetation due to drainage preparatory to
cultivation. It is to the original vegetation that the chief vegetable
accretions in the muck must be accredited. In all cases, for purposes of
comparison, some samples must be taken from parts of the field which
have not been under cultivation or fertilization. The original
properties of the muck can thus be determined and compared with the
portions which have been changed under cultivation. If the vegetation in
different parts of the field vary it is an indication that the muck is
not homogeneous, and in such cases all the different kinds must be
separately sampled. Any alluvial deposit should be carefully separated
from the muck found _in situ_, for the two layers are radically
different in nature.
The sampling should be made by digging a pit, if possible to the bottom
of the muck formation, and taking the samples at depths of one foot from
one or all of the sides. The samples from sections of even depth are to
be mixed and about five kilograms of the well-mixed sample preserved.
Blocks of the unbroken and unshattered material should also be taken
from each section for the purpose of determining permeability to water
and air. All living vegetable matter should be as fully as possible
removed before the sampling begins. The nature of the subsoil must be
observed, and it should be stated whether it be sand, clay, limestone,
etc. Fresh samples should be taken at various depths for the purpose of
determining the content of moisture in the manner described in paragraph
=65=. The tubes used are made sharp at the end to be inserted in the
soil, and so arranged as to cut cylinders of soil a trifle smaller than
their interior diameter. By this means the sample slips easily into its
place. The same care and judgment must be used in taking these samples
as are required in the case of common soils.
_Illustration._—Samples of muck soil taken at Runnymede, Florida.
(_a_) Formation. Littoral fresh-water lacustrine deposits, varying from
a few inches to four feet in depth, and from a few feet to half a mile
in width.
(_b_) Vegetation before drainage. Saw grass (_Cladium Mariscus_ or
effusum).
(_c_) Principal present vegetation (see pages 59–60).
(_d_) _Kinds of Soil._—The muck shows two distinct colors, black and
brown. The vegetation, however, seems to be the same in both cases. The
black muck has the appearance of being more thoroughly decomposed.
(_e_) _Geologic Formation._—This portion of the Florida peninsula is
covered generally with sand due to marine submergence during recent
geologic periods. The forest growth is pine. The drainage from the pine
land is towards the muck deposits. The pine land lies from four to ten
feet higher than the surface of the muck and is much less subject to
frost.
=522. Water Content.=—The capacity of a muck soil for retaining water is
very great. In a moist state these soils are heavy and apparently quite
firm. When dry they are light and fluffy and unsuited to hold the
rootlets of plants. Saturated to their greatest capacity they hold
considerably more than their own weight of water. Attention has already
been called to the danger of drying such samples at a high temperature.
As in most cases of drying exposure at the temperature of boiling water
until a constant weight is obtained is a perfectly safe way. It is hard
to say what comes off in addition to water at a higher temperature. All
that comes off even at the temperature of boiling water is not water.
The method of determination usually employed in this laboratory is the
following:
From four to five grams of the material are spread as evenly as possible
over the flat bottom of a circular aluminum dish, about seven
centimeters in diameter. The dish is exposed for three hours at the
temperature of boiling water and then kept for two hours in an air-bath
at 110°. At the end of this time constant weight is obtained. Additional
drying at 110° for five hours, usually gives no further loss of volatile
matter. The dish should be covered during weighing on account of the
hygroscopicity of the residue. When well sampled the dry matter thus
obtained serves as the basis of calculation for the general analytical
data.
_Results._—Samples of muck soil taken in brass tubes in March during the
dry season had the following contents of moisture:
Matter volatile at 110°, per cent.
Taken near the surface 61.60
„ one foot below the surface 84.35
„ two feet „ „ „ 81.52
It is thus seen that the normal content of moisture in such a soil
during the dry season, exclusive of the top layer, is about eighty per
cent.
=523. Organic Carbon and Hydrogen.=—The organic carbon and hydrogen in
muck soils are determined on the carefully dried sample by combustion
with copper oxid. This process gives not only the quantities of these
bodies combined as humus, but also those in a less advanced state of
decomposition and present as fatty bodies or resins. The method employed
is given on pages 319–20.
_Results._—The data obtained on a sample of muck soil from Florida are
as follow:
Per cent. carbon. Per cent. hydrogen.
One foot from surface 57.67 4.48
Two feet „ „ 47.07 5.15
Three „ „ „ 8.52 0.53
The last sample was largely mixed with sand, the muck at the point when
it was taken not being quite three feet deep.
=524. Total Volatile and Organic Matter and Ash.=—The ignition of the
sample should be very carefully conducted at the lowest possible
temperature. About five grams of the air-dried sample or double that
amount of the moist sample should be taken. In the latter case the
calculations should be made on the basis of the dry material. The
ignition should be continued with frequent stirring with a platinum wire
until all organic matter is destroyed. At the same time in a large dish
one or more kilograms of the sample should be ignited in order to secure
an ash for analysis. The ash should be quickly weighed to avoid
absorption of moisture.
=525. Sulfur.=—The sulfur present in muck is combined either in an
organic form or with iron. It may be estimated by the method of
Fleischer.[346]
From five to ten grams of the sample are ignited carefully in a hard
glass tube in a stream of air or better of oxygen. The sulfur compounds
escape as sulfuric or sulfurous acid.
The combustion is carried on in the apparatus shown in Fig. 93.
[Illustration:
FIGURE 93. APPARATUS FOR DETERMINING SULFUR.
]
The end of the tube A, next to B, is lightly stopped with a plug of
glass wool, the substance introduced and held in place by a second plug
of glass wool next to C. A is connected to the working flask C,
containing water, as is shown in the illustration. The chief object of
the flask is to control the rate of aspiration of the air or oxygen. A
is also connected with the bulb-tube B, as shown in the figure. B
contains potash-lye, free of sulfur. On the top of B is placed a drying
tube filled with glass pearls, moistened with potash-lye. This is
connected with the aspirator by a small bulb-tube bent at right angles,
as indicated. The bulb of this tube contains a little neutral litmus
solution, which must suffer no change of color during the progress of
this analysis. The tube, thus arranged, is placed in a combustion
furnace and gradually heated to redness, beginning with the part next to
B. A moderate stream of air or oxygen is passed through the tube during
the operation. Any product of the combustion collecting in the tube
before reaching B, is driven into B by careful heating. At the end of
the combustion the contents of B are acidified with hydrochloric acid,
and treated with bromin to convert the sulfurous into sulfuric acid. The
excess of bromin is afterwards removed by boiling, and the sulfuric acid
precipitated by barium chlorid and estimated in the usual way.
The total sulfuric acid having thus been determined, the sample is
extracted with water and the sulfuric acid estimated in the residue.
The sulfuric acid in a muck which is injurious to vegetation is
classified by Fleischer, as follows:
(1) Free sulfuric acid. (The residue which is obtained by calculation as
sulfates of the bases in the water extract.)
(2) The sulfuric acid contained as copperas (calculated from the iron
oxid content of the aqueous extract).
(3) Sulfuric acid arising from the oxidation of pyrites (calculated from
the sulfuric acid obtained by treatment of the water-extracted sample).
A better idea of the distribution of the sulfur in the sample can be
obtained by estimating it according to the method given in paragraph
=385=.
=526. Phosphoric Acid.=—The method for determining the phosphorus in
muck is given in paragraph =382=. The process given in =378= may also be
used.
The method of extraction with hydrochloric acid is wholly unreliable as
a means of determining the available phosphoric acid in muck.
There are some vegetable soils which contain so much iron and lime that
the whole of the acid ordinarily used would be consumed thereby. This
fact has been clearly pointed out by Wiklund in determining the
phosphoric acid in a large number of peaty soils.[347] His experiments,
were made with acid of only four per cent strength. In some cases,
however, it may be found useful to determine the quantity of phosphoric
acid which can be extracted with hydrochloric acid, and afterwards to
separate the humus and determine the content of phosphoric acid therein.
=527. Humus.=—In this laboratory the humus is estimated by the method of
Huston and McBride, as given in paragraph =312=. In samples so rich in
organic matter the method of Grandeau does not give as good results.
Often more than half the weight of the dry substance is soluble in
ammonia after treatment with acid. The nitrogen in the original sample
and the separated humus should be estimated by moist combustion with
sulfuric acid in the usual manner.
=528. The Mineral Contents of Humus.=—The material obtained by
precipitating the alkaline extract of a vegetable earth with an acid
does not consist alone of oxygen, carbon, hydrogen, and nitrogen. The
complex molecules which make up this mixture contain certain quantities
of iron, sulfur, and phosphorus in an organic state. These bodies are
left as inorganic compounds on ignition, provided there is enough of
base present to combine with all the acid elements. Much of the sulfur
and phosphorus, however, in these compounds might be lost by simple
ignition. In such cases moist oxidation of these bodies must be
practiced, or the gases of combustion passed over bodies capable of
absorbing the oxidized materials in order to detect and determine them.
The proper methods of accomplishing this have already been pointed out
for vegetable soils, and the same processes are applicable in the case
of extracted and precipitated humus.