POLITECHNIKA WARSZAWSKA
WYDZIAŁ INśYNIERII LĄDOWEJ
KATEDRA INśYNIERII
MATERIAŁÓW BUDOWLANYCH
WARSAW UNIVERSITY OF TECHNOLOGY
FACULTY OF CIVIL ENGINEERING
DIVISION OF BUILDING MATERIALS
ENGINEERING
al. Armii Ludowej 16, p. 551, 00-637 Warszawa, POLAND; tel.: (+48 22) 825-76-37, fax: (+48 22) 825-75-47, e-mail:[email protected]
Lech Czarnecki
Paweł Łukowski
Andrzej Garbacz
Bogumiła Chmielewska
Building chemistry – laboratory exercises
collaborative work under the chairmanship of Lech Czarnecki
4. ELEMENTS OF CHEMICAL ANALYSIS
Theoretical background
Practical task 1. Identification of chosen cations
Practical task 2. Identification of chosen anions and chemical compounds
Practical task 3. Determining the content of sodium hydroxide in aqueous solution
Laboratory of Building Chemistry, Division of Building Materials Engineering, WUT
4. ELEMENTS OF CHEMICAL ANALYSIS
THEORETICAL BACKGROUND
Use of chemical analysis in building engineering
Analysis of the composition of materials plays a vital role in every branch of industry;
with regard to building engineering, methods of analytical chemistry are used during quality
control of raw materials and building products, during analysis of products of corrosion of
various building materials, especially concrete, and when specifying the conditions of
working of a structure or its element (evaluation of environmental aggressiveness).
In most general terms, chemical analysis can de divided into qualitative and
quantitative analysis. Qualitative analysis deals with detection and identification of substances
(elements, ions or chemical compounds), i.e. determining the chemical qualitative
composition of materials, whereas quantitative analysis aims at determining the amount of
analysed substances and so at determining the quantitative composition of materials.
Methods employed in qualitative analysis can be divided into physical-chemical
(instrumental) and chemical methods, whereas in quantitative analysis we distinguish
instrumental methods and so-called classical methods, that is gravimetric and volumetric
methods (first of all titration).
Size of analytical sample
The size of the sample for analysis is the basis for division of analytical methods into
methods of macro, semi-micro, micro and ultramicro scale (Table 4.1).
Table 4.1
Division of analytical methods according to the amount of analysed sample
Name of method
macroanalysis
semi-microanalysis
microanalysis
ultramicroanalysis
Amount of sample
Mass, g
1-0.1
0.1-0.01
0.01-0.001
<0.001
Volume, cm3
10-1
1-0.1
0.1-0.01
0.01-0.001
These methods differ basically only in their scale and the technique of conducting the
analysis. Currently, the so-called trace analysis has gained significance. It deals with
determining very small amounts (traces) of substances, beginning with concentrations of
10-1 % to 10-10 – 10-12 % (submicro traces – contamination with radioactive substances
determined in building materials).
Methods of separating mixtures
The analysed material is usually a mixture of different substances. Therefore,
conducting determinations often requires separating the constituents of a sample. If one of the
constituents is soluble in an available solvent, it can be separated in this way and then
determined. This method is often encountered in analysis of building materials; as an example
may serve determining the composition of hardened concrete by determining soluble parts in
hydrochloric acid (see Chapter 7). If, for various reasons, we cannot use separation by simple
solution (lixiviation) of one of the constituents (e.g. more or even all constituents of the
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Laboratory of Building Chemistry, Division of Building Materials Engineering, WUT
analysed material go to the solution), after dissolving the sample it is necessary to use one of
the following methods of separating mixtures:
• precipitation – separating particular constituents of the solution in the form of a
precipitate,
• electrolysis – separating metals by electric current flow in the solution of the electrolyte;
metals of a higher potential – more noble ones – are reduced and educed on the cathode
and metals of a lower potential are oxidized and educed on the anode in the form of
oxides,
• chromatography – separating phases moving in a given system at different speeds, e.g.
paper chromatography uses differences in the speed of moving of different solution
constituents in filter paper under the influence of capillary forces,
• extraction – using different solubility of chemical compounds in two liquids that do not
mix, one of which is usually an aqueous solution and the other – an organic solvent;
substances in the aqueous solution stay in it or move to the organic phase, depending on
which medium is a better solvent for them.
• distillation – separating (when heating) more volatile (i.e. boiling at a lower temperature)
constituents from the solution.
In some cases, the influence of constituents that disturb the determination can be
removed by changing them into the form of a different chemical character (so-called
masking). Separating the constituents of the solution is then unnecessary.
Chemical methods of analysis
Qualitative analysis usually occurs in a few stages. These are:
• observation of the sample,
• preliminary tests,
• dissolving the sample,
• systematic analysis of cations and anions.
The appearance of the sample, beginning with its state of matter, may provide clues as
to its composition. Some substances have a characteristic colour, odour or form (e.g. minerals,
carbon, metal ores, mercury etc.).
Preliminary tests are simple experiments conducted before beginning the actual
systematic analysis. As an example may serve heating the sample and observing the possible
emission of a gas or melting of the sample or putting the sample in the flame of a burner and
observing its colour. Some elements, especially K, Rb, Cs, Fr and alkaline-earth metals, give
the flame a characteristic colour, which enables their identification.
Analytical reactions occur in the liquid phase, therefore it is necessary to change the
sample into a solution. Dissolving a sample of an inorganic substance begins from using
distilled water. Substances insoluble in clear water are dissolved subsequently in:
• diluted acids or bases,
• concentrated acids or bases,
• mixtures of acids.
In case of sparingly soluble compounds (e.g. some silicates) it is necessary to melt the
analysed substance earlier with special fluxes (e.g. sodium or potassium carbonate) in order to
change it into a more soluble salt.
Inorganic compounds in an aqueous solution undergo electrolytic dissociation, i.e.
ionic decomposition, e.g. NaCl → Na+ + Cl–. Using appropriate reagents, i.e. solutions
containing ions that react with detected ions, analytical reactions are conducted, the so-called
characteristic reactions that allow the identification of ions in the analysed solution. A
reaction may be regarded as characteristic if it results in:
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Laboratory of Building Chemistry, Division of Building Materials Engineering, WUT
• a change in the colour of the solution,
• precipitation,
• emission of a gas.
Characteristic reactions are written down in the ionic form, e.g.:
Ag+ + Cl– → AgCl ↓
As a result of the reaction of a silver cation with a chloride anion, a characteristic
white cheese-like precipitate of silver chloride is precipitated. This reaction may be used to
detect both silver cations and chloride anions.
For analytical purposes, ions have been divided into analytical groups. Ions belonging
to one group react in a characteristic way with one reagent, called a group reagent, which
enables their preliminary separation. Separating particular groups of ions from an analysed
solution with the help of group reagents is a preliminary stage of analysis that precedes the
actual identification. Then selective reagents are used, which react with a small number of
ions, and finally – specific reagents, which, in specific conditions, react only with one ion. In
the analysed solution, the determination of cations and determination of anions are conducted
separately.
Detection of cations and anions
In building materials and in the environment, in which they are used, there are many
cations, among which the most important are: Ca2+, Fe2+, Fe3+, Al3+, Cu2+, Na+, K+, NH4+,
Pb2+, Ba2+. Learning about characteristic reactions (Table 4.2) and identification of these
cations are the subject of practical task 1.
Table 4.2
Characteristic reactions of cations
Reagent
(NH4)2CO3
NaOH
NaOH
in air it oxidates
NaOH
KSCN
(potassium thiocyanate)
NaOH, NH4OH
excess of NaOH
excess of NH4OH
NaOH
NaOH
Reaction
calcium cation, Ca2+
Ca2+ + CO32– → CaCO3↓
Ca2+ + 2OH– → Ca(OH)2↓
iron (II) cation, Fe2+
Fe2+ + 2OH– → Fe(OH)2↓
4Fe(OH)2 + 2H2O + O2 → 4Fe(OH)3↓
iron (III) cation, Fe3+
Fe3+ + 3OH– → Fe(OH)3↓
Fe3+ + 3SCN– → Fe(SCN)3↓
aluminium cation, Al3+
Al + 3OH- → Al(OH)3↓
Al(OH)3↓ +NaOH → NaAlO2 + H2O
no reaction
3+
copper cation, Cu2+
Cu2+ + 2OH– → Cu(OH)2↓
ammonium cation, HN4+
+
NH4 + OH– → NH3↑ + H2O
Result
white precipitate
white precipitate
green precipitate
red brown precipitate
red brown precipitate
blood-red colour of the
solution
white precipitate
dissolving of precipitate
the precipitate does not
dissolve
blue precipitate
A gas of a characteristic
smell (ammonia)
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Laboratory of Building Chemistry, Division of Building Materials Engineering, WUT
HCl
KI
K2CrO4
H2SO4
K2CrO4
(NH4)2CO3
strong acids
lead cation, Pb2+
Pb + 2Cl → PbCl2↓
Pb2+ + 2I– → PbI2↓
Pb2+ + CrO42– → PbCrO4↓
barium cation, Ba2+
2+
Ba + SO42– → BaSO4↓
Ba2+ + CrO42– → BaCrO4↓
Ba2+ + CO32– → BaCO3↓
BaCO3↓ + 2H+ → Ba2+ + CO2↑+ H2O
2+
–
white precipitate
yellow precipitate
yellow precipitate
white precipitate
yellow precipitate
white precipitate
dissolving of precipitate
Among anions, the biggest role in construction play: SO42–, Cl–, CO32–, SiO32–, S2–.
Learning about characteristic reactions (Table 4.3) and identification of these anions is the
subject of practical task 2.
Table 4.3
Characteristic reactions of anions
Reagent
BaCl2
Pb(NO3)2
AgNO3
NH4OH
AgNO3
in air it decomposes
BaCl2
strong acids
AgNO3
BaCl2
HCl
AgNO3
HCl
Reaction
sulfate anion, SO42–
SO42– + Ba2+ → BaSO4↓
SO42– + Pb2+ → PbSO4↓
chloride anion, Cl–
Cl– + Ag+ → AgCl↓
AgCl↓ + 2 NH4OH →
→ Ag(NH3)2+ + Cl– +H2O
carbonate anion, CO32–
CO32– + Ag+→ Ag2CO3↓
Ag2CO3↓ → Ag2O↓ + CO2↑
CO32– + Ba2+→ BaCO3↓
BaCO3↓ +2H+ → Ba2+ + H2O +CO2↑
silicate anion, SiO32–
SiO32– + 2Ag+→ Ag2SiO3↓
SiO32– + Ba2+→ BaSiO3↓
SiO32– +2H+ → H2SiO3 → H2O + SiO2↓
sulfide precipitate, S2–
S2– + 2Ag+→ Ag2S↓
S2– +2H+ → H2S↑
Result
white precipitate
white precipitate
white precipitate
dissolving of the precipitate
white precipitate
brown precipitate
white precipitate
dissolving of the precipitate
bright yellow precipitate
white precipitate
white gelatinous precipitate
black precipitate
a gas of a characteristic
odour of rotten eggs
(hydrogen sulfide)
Gravimetric methods of quantitative analysis
Gravimetric determination usually consist in precipitating a determined constituent in
the form of sparingly soluble precipitate, which, after filtration, careful rinsing and drying, is
weighed directly or it is first changed into another compound of a strictly defined
composition. From the mass of the precipitate, the content of the determined constituent in the
analysed sample is calculated.
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Laboratory of Building Chemistry, Division of Building Materials Engineering, WUT
Another method of gravimetric determinations consists in removing the determined
constituent from a weighed sample and calculating its content from mass loss. A typical
example of this type of analysis is determining the amount of water in an analysed sample.
Volumetric analysis
Volumetric analysis includes, first of all, titration. The method and calculations used
in titration analysis have been discussed in Chapter 2.3.
The methods of titration analysis are usually divided on the basis of chemical reactions
used. The following can be distinguished here:
• acidimetry and alkalimetry, which uses neutralisation reactions (acid-base), e.g.:
HCl + NaOH → NaCl + H2O
H+ + OH– → H2O
(acid-base determination is the subject of practical task 3),
• compleximetry, which uses reactions of creating the so-called complex compounds of
complicated structure,
• precipitation methods, which use reactions in which sparingly soluble substances are
created (precipitation),
• redoximetry, using the oxidation-reduction (redox) reactions.
Gasometry is also included in volumetric analysis. In this method we use reactions, as
a result of which one of the constituents of the analysed sample is changed into a gaseous
compound. The measurement of the volume of the gas emitted during such a reaction enables
us to determine the content of the determined constituent in the sample. The gasometrical
method in building may be used e.g. to determine the content of undecomposed calcium
carbonate in lime (see Chapter 8) or to evaluate the effectiveness of working of corrosion
inhibitors in reinforcing steel (see Chapter 13).
Use of instrumental methods of chemical analysis
In instrumental methods we employ physical and physical-chemical phenomena
occurring in analysed substances under various external influences. The possibility of
attributing a particular characteristic to the analysed substance, to which the value of the
characteristic is typical, enables using the instrumental method for identification of this
substance (qualitative analysis). Determining the content of the substance (quantitative
analysis) is possible as long as there is a relation between the content (concentration) of the
substance and the value of the measured characteristic.
Among different types of available instrumental analysis, in relation to building
materials, we use mostly spectroscopic, electrochemical, thermal methods and microscopic
analysis.
Spectroscopic methods
In spectroscopic methods we examine spectra of electromagnetic radiation which
interacts with different elements of the structure of matter (Table 4.4). Identification of a
substance occurs by comparison of the obtained spectrum with standard spectra. Determining
the content of a given constituent is possible by determining the intensity of radiation
absorbed, emitted or dispersed (depending on the analytical method) by this constituent and
referring this value to an appropriate standard curve determined earlier, i.e. the relation
“intensity of radiation – content of constituent”.
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Laboratory of Building Chemistry, Division of Building Materials Engineering, WUT
Table 4.4
Characteristics of spectroscopic methods of analysis
The element of the
structure of the matter on
which electromagnetic
radiation acts
Used phenomena
Range of used
radiation
absorption of radiation VIS, UV
atom
emission of radiation
VIS, UV
absorption of radiation IR, VIS, UV
molecule
emission of radiation VIS, UV
diffraction (bending of X
element of crystal lattice
a beam of radiation)
X = X-radiation, wavelength 10-11 – 10-8 m
UV = ultraviolet radiation, wavelength 10-8 – 5⋅10-7 m
VIS = visible radiation, wavelength 5⋅10-7 – 10-8 m
IR = infrared radiation, wavelength 5⋅10-7 – 10-8 m
Examples of analytical
methods
atomic absorption
spectroscopy
emission spectrography,
flame photometry
molecular
spectrophotometry
fluorimetry
X-ray diffractometry
absorbance
In analysis of building materials, absorption spectrophotometry in the infrared range
(Fig. 4.1) is very often used. In this method we examine the relation between absorption of
radiation and wavelength (expressed usually with the so-called wave number that is the
reciprocal of wavelength).
wave number
Fig. 4.1. Absorption spectrum
in the infrared of clinker phases
of Portland cement
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Laboratory of Building Chemistry, Division of Building Materials Engineering, WUT
The ability of specific diffraction (deflection) of X rays on the planes of crystal lattice
(Fig. 4.2) is used in X-ray diffractometry. The phenomenon of diffraction is described with
the Bragg equation:
n λ = 2d (sinθ)
where:
n – order of interference,
λ – wavelength,
d – the distance of lattice planes in the crystal,
θ – the angle of ray reflection.
Fig. 4.2. Diffraction (deflection) of X-rays on the planes of crystal lattice; n – order of
reflection, λ – the X-ray wavelength, θ – angle of reflection, d – the distance between the
lattice planes in the crystal.
The values of the angle θ are characteristic to different crystalline substances, which
allows their identification. The Bragg equation enables also the determination of the distances
of lattice planes (d), which is significant for learning about the structure of the crystal.
Flame photometry, in which emission of radiation in the range visible to the analysed
substance occurs under the influence of burner flame, is especially useful for determination of
metals from the first and second group of the periodic table (K, Rb, Cs, Fr and alkaline-earth
metals); identification of these elements is possible by observing the colour of the flame to
which a sample is inserted (Table 4.5).
Table 4.5
Characteristic colouring of the flame by some elements
Element
sodium, Na
potassium, K
calcium, Ca
barium, Ba
copper, Cu
Colour of flame
yellow
bright violet
brick red
yellow-green
green
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Laboratory of Building Chemistry, Division of Building Materials Engineering, WUT
Electrochemical methods
In electrochemical methods we employ phenomena occurring during flow of electric
current through solutions of electrolytes. With reference to building materials, especially
important is the potentiodynamic method, which allows the determination of the protective
capacities of concrete towards reinforcing steel. It consists in determining the polarization
curve, i.e. the relation: density of electric current flowing in a steel rod placed in the analysed
concrete – the potential of the electrode (Fig. 4.3); on the basis of the course of the curve, we
determine (in accordance with the norm PN-86/B-01810) if the surface of the steel is passive.
density of current
passive zone
voltage
Fig. 4.3. The result of a potentiodynamic test – the polarization curve of a steel rod in
concrete cover: Ep – passivation potential, ED – potential of break-down
Thermal analysis
In thermal analysis we use the changes which substances undergo during their heating
and cooling. In issues concerning building chemistry we often use differential thermal
analysis – DTA. It consists in measuring the energetic effects that occur in the analysed
sample during its heating. Both the analysed sample and the standard sample, which does not
undergo any change in the conditions of the experiment, are heated simultaneously. The
difference between the temperatures of both samples is observed. If the analysed material
undergoes an exothermic change, its temperature is higher than the temperature of the
standard material; on the graph (Fig. 4.4) this difference is marked as the local maximum. In
case of an endothermic reaction, we observe a minimum on the graph. On the basis of
characteristic course of the DTA graph we can identify analysed substances.
In analysis of setting and hardening of binding agents, we often use calorimetry. In
this method we determine the amount of heat given off during a given process (Fig. 4.5).
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difference of temperatures
Laboratory of Building Chemistry, Division of Building Materials Engineering, WUT
–
0
+
temperature
Fig. 4.4. The curve of differential thermal analysis (DTA) of gypsum: 1 – dehydration of
dihydrite gypsum to hemihydrite gypsum, 2 – dehydration of hemihydrite gypsum to
anhydrite III, 3 – phase change of anhydrite III into anhydrite II, + endothermic change, –
exothermic change
time, h
Fig. 4.5. The speed of emission of energy (calorimetric curve) during setting of Portland
cement
Microscopic analysis
Microscopic examination allows us to learn about the internal structure of materials
and its changes under the influence of different factors. A traditional research tool is an
optical microscope; for basic examination of building materials we use e.g. polarization
microscope, which enables the magnification by 100 – 500 times. Such observations can also
help to determine the mineral composition of the material, allowing us to determine the
crystal habit – identification of crystal phases (e.g. varieties of hemihydrite gypsum).
Information about microstructure of materials can be obtained using electron microscopes
(scanning or scanning transmission electron microscopes), enabling magnification by up to
100,000 times. Nowadays microscopic analysis is usually combined with computer image
analysis, enabling us to draw also quantitative conclusions. It pertains especially to research
into the mechanisms of damage – cracking of materials and cooperation of different elements
of composite materials (e.g. cement – aggregate in concrete).
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Laboratory of Building Chemistry, Division of Building Materials Engineering, WUT
PRACTICAL TASK 1. IDENTIFICATION OF CHOSEN CATIONS
Equipment needed to complete the task:
test tubes,
test tube stand,
burner,
platinum wire or glass rod.
Used reagents:
aqueous solutions 0.1m:
Al2(SO4)3
FeSO4
KCl
NaCl
Pb(NO3)2
BaCl2
Fe2(SO4)3
K2CrO4
NaOH
dropping funnel,
CaCl2
HCl
KJ
(NH4)2CO3
CuSO4
H2SO4
KSCN
NH4OH
Completion of the task
The task begins with conducting all reactions characteristic to cations according to
Table 4.2. The reactions are conducted in test tubes; into a tube app. 0.5-1.0cm3 of solution
containing the analysed cation is poured and then the same volume of the reagent is added and
occurring changes are observed. The conducted reactions must be written down in the ionic
form, mentioning their effect and possible remarks.
For ions Cu2+, Ca2+, Ba2+, Na+, K+ flame tests need to be conducted; immerse a
platinum wire or a glass rod in the solution containing the analysed cation and then place it in
the burner flame and observe its colour, comparing with the information provided in Table
4.5.
After conducting all characteristic reactions we identify cations in 5 samples of
solutions of unknown composition, by conducting a preliminary division (according to Fig.
4.6) and then their identification on the basis of characteristic reactions and flame tests. All
conducted reactions and results of determinations must be written down.
SAMPLE
↓
+ HCl
precipitate
↓
Pb2+
↓
characteristic reactions
no precipitate
↓
+ NaOH
precipitate
↓
Ca2+, Cu2+, Al3+, Fe2+, Fe3+
too much reagent
dissolving of precipitate
↓
Al3+
↓
no effect
no precipitate
↓
Ba2+, Na+, K+, NH4+
organoleptic tests (odour)
characteristic odour (ammonia)
↓
Ca2+, Cu2+, Fe2+, Fe3+
NH4+
↓
↓
characteristic reactions and flame tests
no odour
Ba2+, Na+, K+
↓
Fig. 4.6. Pattern of cation analysis
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Laboratory of Building Chemistry, Division of Building Materials Engineering, WUT
PRACTICAL TASK 2. IDENTIFICATION OF CHOSEN ANIONS AND CHEMICAL
COMPOUNDS
Equipment needed to complete the task:
like in practical task 1.
Used reagents:
like in practical task 1 and
aqueous solutions 0.1m:
AgNO3
Na2SiO3
Ag2SO4
(NH4)2CO3
BaCl2
NH4OH
HCl
Pb(NO3)2
Completion of the task
The task begins with conducting all characteristic reactions for anions according to
Table 4.3. The reactions are conducted in test tubes. The conducted reactions must be written
down in the ionic form, mentioning their effect and possible remarks.
After conducting all the characteristic reactions, we identify anions in 5 samples of
solutions of unknown composition by conducting a preliminary division according to Fig. 4.7
and then their identification on the basis of characteristic reactions.
SAMPLE
↓
+ AgNO3
↓
↓
precipitate
↓
–
Cl , CO32–, SiO32–
↓
+ BaCl2
↓
↓
↓
precipitate
no precipitate
↓
↓
2–
2–
Cl–
CO3 , SiO3
↓
↓
characteristic reactions
↓
no precipitate
↓
SO42–
↓
characteristic reactions
Fig. 4.7. Pattern of anion analysis
Then we should identify chemical compounds (i.e. anions and cations) in two samples
of solutions of unknown composition. The analysis of cations is done analogically to task 1.
All conducted reactions and results of determination should be written down.
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Laboratory of Building Chemistry, Division of Building Materials Engineering, WUT
PRACTICAL TASK 3. DETERMINING THE CONTENT OF SODIUM HYDROXIDE
IN AQUEOUS SOLUTION
Equipment needed to complete the task:
250 cm3 conical flasks (3 pieces),
250 cm3 measuring flask,
dropper,
25 cm3 calibrated pipette,
burette.
Used reagents:
NaOH – aqueous solution (for analysis),
cresol red,
distilled water,
HCl – 0.1m solution.
Completion of the task
The 250 cm3 measuring flask with the sample of analysed solution is filled up with
distilled water up to the mark. Then with a pipette, 3 samples of the solution (of the volume
25 cm3 each) are taken and inserted in conical flasks. To each flask 3-4 drops of the indicator
– cresol red – are added and then it is titrated with the solution of hydrochloric acid of the
0.1m concentration; the colour of the solution should change from violet to yellow-orange
(the determination of the shade of colour is always subjective so titration should be continued
until the first observation of the change of colour). The technique of titration and rules of
calculations have been discussed in Chapter 2.3.
On the basis of the results of titration one should calculate the molar concentration of
NaOH in the titrated solution and then the mass of the hydroxide in the analysed sample.
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