Wilfred W. (Wilfred Welday) Scott.

Standard methods of chemical analysis; a manual of analytical methods and general reference for the analytical chemist and for the advanced student online

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accomplished by placing the carbon disuiphide in a small flask (A, Fig. 7) connected
by means of a glass tube (C) to a second flask (B), cork stoppers being used. The
vessels are immersed in beakers of water, the container with the reagent being placed
in hot water (60-80 C.) and the empty flask in cold water. The reagent quickly dis-
tills into the empty flask in pure form.

Properties of Bismuth Sulphide. Bi 2 S 3 , mol.wt., 512.21; sp.gr., 7-7.81;
decomposed by heat, solubility =0.0000l8g. per 100 cc. cold H 2 0; soluble in nitric
acid; brown rhombic crystals.

1 P. Jannasch, Zeit. anorg. Chem., 8, 302, 1895.


Determination of Bismuth as the Metal

Reduction with Potassium Cyanide. 1 Bismuth precipitated as the car-
bonate and ignited to the oxide according to the procedure given, is fused in
a weighed porcelain crucible with 5 times its weight of potassium cyanide over
a low flame. The cooled melt is extracted with water, pouring the extracts
through a filter that has been dried and weighed with the crucible. Bismuth
is left undissolved as metallic bismuth. After washing with water, alcohol,
and ether, the filter, with the metal and loosened pieces of porcelain glaze, is
dried at 100 C. together with the crucible. These are then weighed and the
increased weight taken as the amount of bismuth present in the sample.

Electrolytic Deposition of Bismuth

With samples containing less than 0.03 gram bismuth, the metal may be
satisfactorily deposited by electrolysis of its sulphuric acid solution, lead having
been removed previously by sulphuric acid by the standard procedure. The
solution contains about 6 cc. of strong sulphuric acid per 100 cc. This is
electrolyzed with a current of 0.6 to 0.7 ampere and about 2.7 to 3 volts
Further details of this method may be found in " Technical Methods of Ore
Analysis," by A. H. Low, page 54, VII Edition. John Wiley & Sons.


Determination of Bismuth by Precipitation as Oxalate and
Titration by Potassium Permanganate 2

Normal bismuth oxalate, produced by addition of oxalic acid to a nitric
acid solution of the element, boiled with successive portions of water, is trans-
formed to the basic oxalate. This may be titrated with potassium perman-
ganate in presence of sulphuric acid.

Procedure. Preparation of the Sample. One gram of the finely ground
sample is treated with 5 to 10 cc. of concentrated nitric acid and digested on
the steam bath and finally evaporated to dryness, the residue is taken up with
5 cc. of nitric acid (sp.gr. 1.42) +25 cc. of water, and diluted to 100 cc.

Precipitation of the Oxalate. About 5 grams of ammonium oxalate or
oxalic acid are added and the liquid boiled for about five minutes, the pre-
cipitate allowed to settle and the supernatant solution filtered off. The
precipitate is boiled twice with 50-cc. portions of water and the washings poured
through the same filter. If the filtrate still passes through acid, the washing
is continued until the acid is removed and the washing passing through the
filter is neutral. The bulk of the basic oxalate precipitate is placed in a beaker
and that remaining on the filter paper is dissolved by adding 2 to 5 cc. of hydro-
chloric acid, 1:1, the solution being added to the bulk of the precipitate.

1 Method by H. Rose, Pogg. Ann., 110, p. 425.

Vanino and Treubert (Ber.. 31 (1898), 1303), reduce bismuth by adding formal-
dehyde to its slightly acid solution and then making stiongly alkaline with 10%
NaOH solution and warming. N.B. Treadwell and Hall, Anal. Chem., p. 181, 3d Ed.

2 The method is rapid and is sufficiently accurate lor commercial work. Warwick
and Kvle (C. N.. 75, 3).

Miiir and Robbs, J. C. S., 41, 1.


This is now warmed until it goes into solution and the liquid is diluted to 250
cc. with hot water. Dilute ammonia is now added until the free acid is
neutralized ; the resulting precipitate is taken up with dilute sulphuric acid, 1 : 4,
added in slight excess. The resulting solution, warmed to 70, is titrated with
standarcl potassium permanganate.

One cc. KMn0 4 N/10 =0.0104 gram Bi.

NOTE. Lead, copper, arsenic, iron, zinc, and tellurium do not interfere. Hy-
drochloric acid should not be used to dissolve the sample, as it interferes with the
oxalate precipitation.

Cinchonine Potassium Iodide, Colorimetric Method *

This method is applicable for the determination of small amounts of bis-
muth, 0.00003 to 0.00015 gram, in ores and alloys. The procedure depends
upon the fact that bismuth nitrate produces a crimson or orange color when its
solution is added to a solution of cinchonine potassium iodide, the intensity
of the color depending upon the amount of bismuth in the resulting product.

Special Reagents. Cinchonine Potassium Iodide Solution. Ten grams of
cinchonine are dissolved by treating with the least amount of nitric acid that
is necessary to form a viscous mass and taking up with about 100 cc. of water.
The acid is added a drop at a time, as an excess must be avoided. Twenty
grams of potassium iodide are dissolved separately and cinchonine solution added.
The resulting mixture is diluted with water to 1000 cc. After allowing the
reagent to stand forty-eight hours, any precipitate formed is filtered off and
the clear product is ready for use. The reagent preserved in a glass-stoppered
bottle keeps indefinitely. It should be filtered free of suspended matter before use.

Standard Bismuth Solution. One gram of metallic bismuth is dissolved
in the least amount of dilute nitric acid (1:1) that is necessary to keep it in
solution and diluted to 1000 cc., in a graduated flask. One hundred cc. of this
solution is diluted to 1000 cc. One cc. of this diluted solution contains 0.0001
gram bismuth.

Procedure. Isolation of Bismuth. The solution is freed from lead by
H 2 S0 4 , and from arsenic, antimony, and tin by precipitation of the sulphides
and extraction with Na 2 S solution. The residual sulphides are dissolved in hot
dilute nitric acid, according to the standard methods of procedure. The free
nitric acid is nearly neutralized by the cautious addition of dilute ammonia,
the last portion being added drop by drop, until a faint cloudiness is evident,
and then 10 to 15 cc. of 10% ammonium carbonate are added with constant
stirring. The mixture is digested for about three hours on the steam bath,
the clear solution decanted through a small filter, the residue washed by de-
cantation once or twice with hot water containing ammonium carbonate and
then on the filter twice with pure hot water.

Colorimetric Comparison

The residue of bismuth basic carbonate is dissolved in the least amount of
dilute nitric acid necessary to effect solution and the filter washed free of bis-
muth with a little water containing a few drops of nitric acid. The solution
is made up to a definite volume, 50 cc. or 100 cc. according to the bulk of

1 Method of W. C. Ferguson.


precipitate dissolved. Two small beakers placed side by side may be used
for the color comparison, a sheet of white paper or tile being placed under the
beakers. Two 50-cc. Nessler tubes, however, are preferred. Three cc. of cin-
chonine solution are added to each container. From a burette the bismuth
nitrate sample is run into one of these containers in just sufficient quantity to
color the reagent a crimson or orange tint. The exact volume required to do
this is noted and the equivalent amount of sample used calculated. (If no
color is produced bismuth is absent.) The reagent in the adjacent beaker or
Nessler tube is diluted to 5 to 7 cc., and into this is run, from a burette, the
standard bismuth nitrate solution until the color exactly matches the sample.
From the cc. of the standard required the amount of bismuth in the sample
can readily be calculated.

Reaction. 3KI+C 19 H 22 N 2 OKI+Bi(N0 3 ) 3 = C 19 H 22 N 2 OKIBiI 3 +3KN0 3 .

Precautions. The sensitiveness of the method is lost if the depth of color is too
great. It is necessary, then, to add the sample to the cinchonine reagent in such
quantity only as will produce a light crimson or orange color.

Solutions in the comparison tubes or beakeis must not be overdiluted, since the
bismuth salt formed by the reaction of the cinchonine reagent is soluble in water
with the disappearance of color in too dilute solutions.

Comparison must be expeditiously made, as a precipitate is apt to form upon
standing, and iodine will sometimes separate.

The order of addition must be observed; e.g., the bismuth solution is added to
the cinchonine reagent, never the reverse.

Colorimetric Determination of Bismuth. Bismuth Iodide

Method !

Bismuth iodide gives an intense yellow, orange, or red color to its solu-
tion. The color is not destroyed by S0 2 , as is that of free iodine. The intensity
of the color varies as follows:

1 part of bismuth in 10,000 parts of water produces an orange-colored solution.
1 part of bismuth in 40,000 parts of water produces a light orange color.
1 part of bismuth in 100,000 parts of water produces a faint yellow color.

Reagents. Standard Bismuth Solution. One gram of bismuth is dissolved
in 3 cc. of strong nitric acid and with 2.8 cc. of water and made up to 100 cc.
with glycerine. Glycerine is added to keep the BiI 3 in solution. Glycerine
is not necessary for amounts of bismuth below 0.0075 gram per cc.

Potassium Iodide Solution. Five grams of potassium iodide dissolved in
5 cc. of water is diluted to 100 cc. with glycerine.

Procedure. The sample is dissolved with just sufficient nitric acid and
water necessary to cause solution, 10 cc. of glycerine and 10 cc. of potassium iodide
solution added and the sample diluted to 50 cc. Comparison is now made with
10 cc. of the standard bismuth solution to which has been added 10 cc. of
potassium iodide and 30 cc. of water. It is advisable to have the standard
stronger in bismuth than the sample and to draw out the standard from the
comparison cylinder until the two colors match.

. C. Thresh, Pharm. Jour., 641, 1880.



( amor P- s P-ff r - 3 - 45 ? m-P- 2200; b. p. sublimes.

I cr^sf . sp.^r. 2.55 ; m.p. 2500 ; 6.p. 3500 C ; oxide, B 2 O 3


Flame Test. Boric acid is displaced from its salts by nearly all acids,
including even carbonic acid. Upon ignition, however, it in turn drives out
other acids which are volatile at lower temperatures. A powdered borate,
previously calcined, is moistened with sulphuric acid and a portion placed on
the loop of a platinum wire is heated to expel the sulphuric acid, 1 then moistened
with glycerine and placed in the colorless flame; a green color will be imparted
to the flame. Copper salts should be removed with H 2 S and barium as BaSCX if
present, as these also color the flame green.

The flame test may be conveniently made by treating the powdered sample
in a test-tube with sulphuric acid and alcohol (preferably methyl alcohol). A
cork carrying a glass tube is inserted and the test-tube gently warmed. The
escaping gas will burn with a green flame.

The test may be made by igniting the mixture of powder, alcohol, and sul-
phuric acid in an open porcelain dish. The green color will be seen in presence
of a borate. The test is not as delicate as the one with the test-tube.

Borax Bead. Na 2 B 4 7 10H 2 O fused in a platinum loop, swells to several
times its original volume as the water of crystallization is being driven out,
then contracts to a clear molten bead. If the bead is dipped into a weak solution
of cobalt and plunged into the flame, until it again becomes molten, the bead
upon cooling will be colored blue.

Turmeric Test. A few drops of acetic acid are added together with 2
or 3 drops of an alcoholic turmeric solution to an alcoholic extract of the
sample, placed in a porcelain dish. The solution is diluted with water and then
evaporated to dryness on the water bath. 1/1000 milligram of boric acid will
produce a distinct color, 2/100 milligram will give a strong reddish-brown
colored residue, which becomes bluish-black when treated with a drop of sodium
hydroxide solution.


The determination of boron is required in the valuation of borax,
Na 2 B 4 (V10H 2 0; boracite, 4MgB 4 7 2MgOMgCl 2 ; borocalcite, CaB 4 7 -6H 2 0;
hydroboracite; boronatrocalcite, etc., the element being reported generally as
the oxide, B 2 3 . The determination is required for obtaining the true value of
commercial boric acid, in the analysis of fluxes and certain pigments. It is
determined as a food-preservative in milk, meat, canned goods, etc. The ele
ment is determined in certain alloys of nickel, cobalt, zinc, chromium, tungsten,
molybdenum and in the analysis of steel.

1 Silicates should be mixed with potassium fluoride and potassium acid sulphate,
KHSO 4 , then held in the flame.



Preparation and Solution of the Sample

It will be recalled that crystalline boron is scarcely attacked by acids or
alkaline solutions; the amorphous form, however, is soluble in concentrated
nitric and sulphuric acids. Both forms fused with potassium hydroxide are
converted to potassium metaborate. Boric acid is more readily soluble in pure
water than in hydrochloric, nitric, sulphuric, or acetic acids, but still more soluble
in tartaric acid (Herz, Chem. Zentr., 1903, 1, 312). It is soluble in alcohol and
volatile oils. Borax is insoluble in alcohol. With acids it becomes transposed
to boric acid and the sodium salt of the acid.

Boric Oxide in Silicates, Enamel, etc. About 0.5 gram of the finely ground
material is fused with five times its weight of sodium carbonate, the melt extracted
with water and the extracts, containing the sodium salt of boric acid, evap-
orated to small volume. The greater part of the excess sodium carbonate
is neutralized with hydrochloric acid and finally made acid with acetic acid
(litmus paper test = red). Boric oxide is now determined by the distillation
process according to the procedure given later in the chapter.

Boronatrocalcite, Borocalcite, Boracite, Calcium Borate. Ten grams of
the powdered material is placed in a flask with a reflux condenser and about 50
cc. of normal hydrochloric acid added and the mixture boiled for half an hour.
The contents of the flask, together with the washings, including those of the
reflux condenser (C0 2 -free water being used), are filtered into a 500-cc. flask
and made to volume with C0 2 -free water. Fifty cc. of this solution is titrated
with half-normal sodium hydroxide, using paranitrophenol indicator. When
a yellow color appears the hydrochloric acid has been neutralized. A second
50-cc. portion is now taken for analysis and the free hydrochloric acid neutral-
ized with sodium hydroxide, using the amount of caustic required in the trial
analysis (this time without an indicator). Boric acid is now determined by titra-
tion according to the procedure on p. 76.

Borax, Boric Acid. Ten grams of the material are dissolved in about
300 cc. of water (free from C0 2 ) and made to 500 cc. in a graduated flask, with
pure water. One hundred-cc. portions are taken for analysis and the solution
titrated, in presence of mannitol or glycerol, according to directions given under
the volumetric procedures.

Boric Acid in Mineral Water. Water containing more than 0.1 gram
boric acid per liter, about 200 cc. are evaporated to small volume, the precipitated
salts are filtered off and washed. Boric acid passes into the filtrate and may be
determined by the distillation method of Gooch given on p. 74.

With water containing traces of boric acid, 1 5 liters or more are evaporated to
about one-tenth the original volume the precipitate filtered off and washed with
hot water. The filtrate is evaporated down to a moist residue. If the residue is
small, it is acidified with acetic acid and the boric acid determined by distillation,
as stated on p. 74. If considerable residue is present, hydrochloric acid is added
to acid reaction, and then the mixture digested with absolute alcohol in a corked
flask for ten to fifteen hours, with occasional shaking. The solution is filtered, the
residue washed with 95% alcohol, the filtrate diluted with water, 10 cc. of 10%
sodium hydroxide solution added and the alcohol distilled off. A second alcoholic

1 Reference is made to Treadwell and Hall, Anal. Chem., 4th ed., p. 431-432 for those
desiring more explicit details of this method.


extraction is generally recommended. The final alkaline solution is taken to
dryness and gently ignited. The residue is extracted with water, made acid with
acetic acid and B 2 3 determined by distillation.

Carbonates. The material is treated with sufficient acid (M. 0. indicator)
to liberate all the C0 2 and react with the combined alkali of boric and carbonic
acid; it is boiled in a flask with reflux condenser to expel C0 2 , ten to fifteen
minutes, the solution exactly neutralized with sodium hydroxide, (M. 0.), and the
liberated boric acid titrated in presence of glycerol and phenolphthalein as usual.

Boric Acid in Milk, Butter, Meat and Other Foods

Milk. 1 One hundred cc. of milk is treated with 1 to 2 grams of sodium
hydroxide, and evaporated to dryness in a platinum dish. The residue is
thoroughly charred 2 by gently heating; at this stage care must be exercised or
loss of boric acid will result; 20 cc. of water are added, the sample heated and
hydrochloric acid added drop by drop until all but the carbon has dissolved.
The mixture is washed into a 100-cc. flask with as little water as possible, 0.5
gram calcium chloride added, then a few drops of phenolphthalein indicator,
then a 10% sodium hydroxide solution until a slight permanent pink color
is obtained and finally 25 cc. of lime water. (All P 2 5 is precipitated as calcium
phosphate.) The liquid is made to 100 cc., mixed thoroughly, and then filtered
through a dry filter. To 50 cc. of the filtrate, equivalent to 50 cc. of the milk
taken, normal sulphuric acid is added until the pink color disappears, then
methyl orange indicator is added, followed by more of the standard acid until
the yellow color changes to a faint pink. Carbon dioxide is expelled and the
liberated boric acid titrated in presence of glycerine, according to the procedure
given for evaluation of borax and boric acid, under " Volumetric Determination
of Boron."

Butter. 3 Twenty-five grams of butter are weighed out in a beaker and 25
cc. of a sugar sulphuric acid mixture added. (Mix =6 grams sugar of milk,
4 cc. normal sulphuric acid per 100 cc. of solution.) The beaker is placed in the
oven (100 C.) until the fat is melted and the mixture is thoroughly stirred.
When the aqueous solution has settled, 20 cc. are pipetted out, phenolphthalein
added, the solution brought to boiling and half-normal sodium hydroxide added
until a faint pink color is obtained. Ten cc. of neutral glycerine are added
and the titration carried on until a permanent pink color appears. The dif-
ference between the two titrations multiplied by the factor for equivalent boric
acid gives the weight of boric acid in the portion taken.

The determination is not affected by the phosphoric or butyric acid or by
the sugar of milk in the butter.

Meat. 4 Ten grams of the chopped meat are mixed in a mortar with 40 to
80 grams of anhydrous sodium sulphate, and dried in the water oven. The
mass is powdered, then placed in a flask and 100 cc. of methyl alcohol added
and allowed to stand for about twelve hours. The alcohol is distilled into a
flask and saved. Fifty cc. more of alcohol are added to the residue and this
again distilled into the first distillate. The distillates are made up to 150 cc., a

1 R. T. Thomson, Glasgow City Anal. Soc. Repts., 1895, p. 3.

2 The milk residue thoroughly charred will give a colorless solution upon extraction.

3 H. Droop Richmond and J. B. P. Harrison, Analyst, 27, 197.

4 C. Fresenius and G. Popp, Chem. Centr., 1897, 2, 69.



50-cc. portion diluted with 50 cc. of water and 50 cc. of neutral glycerine added
with phenolphthalein indicator, and the boric acid titrated with twentieth-
normal sodium hydroxide.

One cc. N/20 NaOH =0.0031 gram boric acid, H 3 B0 3 .

Boric acid in canned goods, sauces, cereals, etc., may be determined by
evaporation of the substance with sodium hydroxide and incineration as in
case of milk. The sodium hydroxide is neutralized and boric acid titrated as


The solubility of boron compounds prevents complete precipitation by any
of the known reagents, hence most of the gravimetric methods are indirect.

Distillation as Methyl Borate and Fixation by Lime 1

This excellent method, originally worked out by F. A. Gooch, l and later modi-
fied by Gooch and Jones, 2 depends upon the fact that the borates of alkaline
earths and alkalies give up their boron in the form of the volatile methyl borate
(b.p., 65 C.), when they are distilled with absolute methyl alcohol (acetone-
free). The methyl borate passed over lime in presence of water is completely

saponified, the liberated boric acid
combining with the lime to form
calcium borate, which mav be dried,
ignited, and weighed. The increase
of the weight of the lime represents
the B 2 3 in the sample.

2B(OCH 3 ) 3 +CaO+6H 2

= 6CH 3 OH+Ca(B0 2 ) 2 +3H 2 0.

Procedure. About 1 gram of pure
calcium oxide is ignited to constant
weight over a blast lamp and then
transferred to the dry, Erlenmeyer
receiving flask (Fig. 8). The crucible
in which the lime was heated and
weighed is set aside in a desiccator for
later use.

0.2 gram or less of the alkali borates,
obtained in solution by a procedure
given under "Preparation of the Sam-
ple," is treated with a few drops of litmus
(or lacmoid), solution and the free al-
kali neutralized with dilute HC1 solution
Fia. 8. Distillation of Methyl Borate. added drop by drop. A drop of dilute

sodium hydroxide solution is added and
then a few drops of acetic acid. The slightly acid solution is transferred to the

'Proc. Am. Acad. of Arts and Sciences, 22, 1G7-176 (1886). Anal. Chem.,
Treadwell-Hall, Vol. 2.
2 See note on u. 75.


pipette-shaped retort R, Fig. <, by means of the attached funnel F, washing
out the beaker and funnel with three 2- to 3-cc. portions of water. The stop-
cock of the funnel is closed, the apparatus is connected up as shown in the illus-
tration^the paraffine bath, heated to not over 140 C., placed in position and
the liquid in the retort distilled into the receiver containing the known amount
of lime. When all the liquid has distilled over, the paraffine bath is lowered,
the retort allowed to cool for a few minutes, 10 cc. of methyl alcohol (acetone-
free) added to the residue in R and the contents again distilled by replacing
the paraffine bath. The process is repeated three times with methyl alcohol.
The contents of the retort (which are now alkaline), are made distinctly acid
by addition of acetic acid, and three more distillations made with 100-cc. portions
of methyl alcohol, as before. The paraffine bath is now removed, the receiving
flask is stoppered, the contents thoroughly mixed by shaking, and set aside for
an hour or more for complete saponification of the methyl borate. The con-
tents are now poured into a large platinum dish and evaporated on the water
bath at a temperature below the boiling-point of the alcohol. (Loss of boric
acid will occur if the alcohol boils.) The adhering lime in the receiving flask
is dissolved by wetting its entire surface with a few drops of dilute nitric acid
(the flask being inclined and revolved to flow the acid over its sides). The
contents are transferred to the dish with a little water and the evaporation
repeated. No loss of boric acid will take place at this stage, the alcohol having
been removed during the first evaporation. The residue is gently heated to
destroy any calcium acetate that may have formed, the cooled borate and
lime are taken up with a little water and transferred to the crucible in which
the lime was heated and weighed. The material clinging to the dish is dis-
solved with a little nitric acid (or acetic acid), and washed into the crucible.
The contents of the crucible are evaporated to dryness on the water bath, then
heated very gently over a flame (the crucible being covered) and finally more
strongly. The heating is continued until a constant weight is obtained. The
increase of weight of the lime represents the amount of B 2 3 in the sample.

NOTES. Gopch and Jones worked out a procedure which utilizes sodium tung-

Online LibraryWilfred W. (Wilfred Welday) ScottStandard methods of chemical analysis; a manual of analytical methods and general reference for the analytical chemist and for the advanced student → online text (page 12 of 111)