L. KALINA et al.: CONDUCTOMETRY ANALYSIS – BENEFICIAL METHOD FOR DETERMINING ...
357–361
CONDUCTOMETRY ANALYSIS – BENEFICIAL METHOD FOR
DETERMINING THE CHEMICAL COMPOSITION OF ALKALINE
SILICATE SOLUTIONS
KONDUKTOMETRIJA – KORISTNA METODA ZA DOLO^ITEV
KEMIJSKE SESTAVE ALKALNIH SILIKATNIH RAZTOPIN
Luká{ Kalina
1*
, Vlastimil Bílek
1
, Michaela Flídrová
1
, David Markusík
1
,
Libor Topoláø
2
, Josef Fládr
3
1
Brno University of Technology, Faculty of Chemistry, Purkyòova 118, 612 00 Brno, Czech Republic
2
Brno University of Technology, Faculty of Civil Engineering, Veveøí 331, 602 00 Brno, Czech Republic
3
Czech Technical University in Prague, Faculty of Civil Engineering, Thákurova 2077, 166 29 Prague, Czech Republic
Prejem rokopisa – received: 2023-09-21; sprejem za objavo – accepted for publication: 2024-04-12
doi:10.17222/mit.2024.1005
The study deals with the methodology for determining the chemical composition of alkaline silicate solutions. Nowadays, the
most widely used method in industry is titration, using the color change of an acid-base indicator. This technique is very accu-
rate for the determination of the total alkalinity (i.e., Na2O, K2O, or Li2O). However, a problem arises in the determination of the
SiO2 content, since the color change of the methyl red indicator is very slow and therefore the equivalence point is unclear. The
main aim of this work is to present the benefits of conductometric titration, where the equivalence point is indicated by a sudden
change in the conductivity. The applicability of the method was verified with other analytical techniques, such as ICP-OES and
gravimetric analyses. Their results confirmed the values of the obtained silicate module (the molar ratio of SiO2 and alkaline ox-
ide) in the cases of sodium, potassium and lithium water glasses. Based on the obtained results, one can say that conductometry
is a very promising method providing an accurate, fast and instrumentally undemanding chemical characterization of alkaline
silicate solutions, usable even in a manufacturing process.
Keywords: conductometry, titration, chemical composition, alkaline silicate solutions
V ~lanku avtorji opisujejo {tudijo, ki se ukvarja z metodologijo dolo~evanja kemijske sestave alkalnih silikatnih raztopin.
Dandanes je titracija najbolj raz{irjena in uporabna metoda v industriji pri kateri ovrednotenje temelji na spremembi barve
kislega indikatorja. To je zelo natan~na metoda za dolo~itev celotne alkalnosti (naprimer Na2O, K2O, ali Li2O). Vendar problem
nastopi, ~e je potrebno dolo~iti koncentracijo SiO2, ker je sprememba barve metil oran`nega indikatorja zelo po~asna in je zato
to~ka enakosti zelo nejasna. Glavni namen tega dela je bil predstaviti prednosti konduktometri~ne titracije pri kateri se to~ka
enakosti dolo~i z skokovito spremembo prevodnosti raztopine. Uporabnost te metode so avtorji verificirali z uporabo drugih
analitskih tehnik, kot sta ICP-OES (angl.: Inductively Coupled Plasma – Optical Emission Spectrometry) in gravimetrija.
Eksperimentalni rezultati so pokazali medsebojno primerljivost vrednosti dobljenega silikatnega modula (molarnorazmerje med
SiO2in alkalnimi oksidi) v primeruNa, KinLi vodnih stekel. Na osnovi dobljenih rezultatov avtorji zagotavljajo, da je
konduktometrija obetajo~a metoda, ki zagotavlja natan~no, hitro in in{trumentalno nezahtevno kemijsko karakterizacijo alkalnih
silikatnih raztopin, ki je uporabna celo v proizvodnih procesih.
Klju~ne besede: analiza elektri~ne prevodnosti elektrolitov (konduktometrija), titracija, kemijska sastava, alkalne silikatne
raztopine
1 INTRODUCTION
Alkaline silicate solutions (water glasses) are impor-
tant raw materials for many applications. They are
widely used, e.g., in foundry, building, paper, and paint
industry.
1–3
The most mass-produced alkaline silicate is
sodium silicate, whose production in Europe exceeds
500,000 tons per year, followed by the production of po-
tassium silicate, which is at a level of approximately
21,000 tons per year, and a maximum of 1,000 tons is at-
tributed to the production of lithium silicate, which is a
specialized chemical, whose use is limited to the area of
coatings.
4
Great demands are placed on its production,
regarding its product quality, technological procedures,
or certain parameters that the input raw material must
meet. When processing water glasses to obtain a new
product with desired properties, it is crucial to know the
chemical composition of the water glass used. The analy-
ses can be performed with different methods, of which
the most commonly industrially used one is the titri-
metric method based on the color change of the indica-
tor.
5
This analysis includes a titration of a water glass
sample with a volumetric solution of hydrochloric acid
using methyl red as the indicator. The mentioned proce-
dure determines the M
2
O alkali content (i.e., Na
2
O, K
2
O,
or Li
2
O) very precisely, but the method is inaccurate for
the determination of the SiO
2
content. The main problem
is the color change of the indicator during the subsequent
titration, which is very gradual and, therefore, the deter-
mination of the equivalence point is ambiguous.
From this point of view, a conductometric analysis
based on titration with a volumetric solution of hydro-
Materiali in tehnologije / Materials and technology 58 (2024) 3, 357–361 357
UDK 544.623:543.242 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 58(3)357(2024)
*Corresponding author's e-mail:
kalina@fch.vut.cz (Luká{ Kalina)

chloric acid appears to be a very promising method,
which could solve the above-mentioned problem. The
method is based on the fact that in solutions of electro-
lytes the electric-current conduction is realized by the
movement of ions. Therefore, the conductivity can be re-
ferred to as ionic or electrolytic.
6
However, what is very
important for conductometric titrations is the fact that
different ions can have different conductivities, so there
can be changes in conductivities during the titration. The
specific conductivity of an electrolyte solution depends
not only on the type of ion, but also on its concentration.
The most concentrated solutions of titration agents are
used for the determination in order to avoid deformation
of the linear dependence by diluting the titrated solution.
The end of the titration is determined by the break in the
titration curve.
7
The aim of this article is to verify the applicability of
conductometry analysis for the purpose of fast and accu-
rate determination of the chemical compositions of vari-
ous types of alkaline silicate solutions on an industrial
scale. The measured results were compared with the cur-
rently used titrimetric method based on the color change
of the acid-base indicator and subsequently verified us-
ing the ICP-OES and gravimetric determinations.
2 EXPERIMENTAL PART
2.1 Materials
The sodium (Na-WG), potassium (K-WG) and lith-
ium (Li-WG) silicate solutions (water glasses) used for
the chemical analyses described below were supplied by
the SChem a.s. company. The silicate modules as well as
alkali and silica contents provided by the manufacturer
are summarized in Table 1.
Table 1: Chemical-composition data of water glasses according to the
supplier
w
M2O
(w/%) w
SiO2
(w/%) Ms
Na-WG 16.65 31.66 1.96
K-WG 25.50 28.38 1.75
Li-WG not specified not specified 2.64
2.2 Conductometric titration
The procedure itself consists of two titrations, one of
which determines the content of M
2
O and the other the
content of SiO
2
. When water glass is titrated with a stan-
dard volumetric solution of hydrochloric acid and the sil-
icate modulus (molar ratio of SiO
2
and alkaline oxide) of
the water glass is approximately 2, or higher, only reac-
tion (2) applies, so only one sudden change in the con-
ductivity appears in the obtained titration curve, which
corresponds to the total M
2
O. An example of an obtained
titration curve is shown in Figure 1, where the intersec-
tion point of two linear parts corresponds to the volume
of volumetric solution of HCl at the equivalence point –
V
HCl
in Equation (3).
SiO
3
2–
+H
+
 HSiO
3
–
(1)
HSiO
3
–
+H
+
 H
2
SiO
3
(2)
The determination process itself proceeded as fol-
lows. An amount of approximately1gto2gofawa -
ter-glass sample was weighed on an analytical balance,
and transferred to a 400 mL plastic beaker using distilled
water. It was filled up with water to a volume of approxi-
mately 200 mL. The resulting solution was placed on a
magnetic stirrer. The solution was titrated conducto-
metrically (Mettler Toledo conductometer In Lab 731)
w i t h1Mh y drochloric acid. Conductivity values were
recorded at each addition of 0.5 mL of volumetric HCl
solution. The percentage of M
2
O was calculated with
Equation (3):
w
cV
M
m
MO
HCl HCl
MO
s
22
=
⋅
⋅
1
2 1000
100
(3)
where w
M2O
represents the percentage weight amount of
M
2
O in the water-glass sample (w/%), c
HCl
is the con-
centration of the HCl measuring solution (mol·L
–1
),
V
HCl
is the consumption of volumetric solution of HCl
(mL), M
M2O
is the molar mass of M
2
O (g·mol
–1
), and m
s
is the weight of the water-glass sample (g).
For the determination of SiO
2
, before starting the
conductometric titration, a suitable excess of the corre-
sponding alkali metal hydroxide (LiOH, NaOH, or
KOH) was added to the water glass, which also brought
equation (1) into play; as a result, two breaks are visible
on the titration curve. The first break corresponds to the
titration of OH
-
and SiO
3
2–
, while the second one corre-
sponds to the titration of HSiO
3
–
. An example of the ob-
tained titration curve is shown in Figure 2, where the in-
tersection points of three linear parts correspond to the
volume of volumetric solution of HCl – V
1HCl
and V
2HCl
.
Subsequently, the percentage of SiO
2
was calculated
with Equation (4),
w
cVV
M
m
SiO
HCl
SiO
s
2
HCl HCl
2
=
⋅−
⋅⋅
()
21
1000
100
(4)
L. KALINA et al.: CONDUCTOMETRY ANALYSIS – BENEFICIAL METHOD FOR DETERMINING ...
358 Materiali in tehnologije / Materials and technology 58 (2024) 3, 357–361
Figure 1: Example of a measured conductometric curve for the deter-
mination of M
2
O

where w
SiO2
represents the percentage weight amount of
SiO
2
in the water-glass sample (w/%), c
HCl
is the con-
centration of the HCl measuring solution (mol·L
–1
), V
1
HCl
is the consumed volume of the HCl solution (mL)
corresponding to the titrated sum of OH
–
and SiO
3
2–
ions, V
2 HCl
is the consumed volume of the HCl solution
(mL) corresponding to the titrated amount of HSiO
3
–
,
M
SiO2
is the molar mass of SiO
2
(g·mol
–1
), and m
s
is the
weight of the water-glass sample (g).
2.3 Titrimetric method with an acid-base indicator
The principle of total alkalinity determination is the
titration with a standard volumetric solution of hydro-
chloric acid, in the presence of methyl red as the indica-
tor. Approximately1go ft h ew ater-glass sample was
weighed on the analytical balance, and transferred to a
plastic beaker with a volume of 400 mL. 50 mL of dis-
tilled water and 5 drops of the methyl red indicator were
added to the sample. Subsequently, the solution was ti-
trated with 1M HCl to the equivalence point, i.e., until a
red-violet coloration appeared. The percentage of M
2
O
was calculated with Equation (3), where w
M2O
represents
the percentage weight amount of Li
2
O, Na
2
OorK
2
Oin
the water-glass sample (w/%), c
HCl
is the concentration of
the HCl measuring solution (mol·L
–1
), V
HCl
is the con-
sumption of the volumetric solution of HCl (mL), M
M2O
is the molar mass of M
2
O (g·mol
-1
), and m
s
is the weight
of the water-glass sample (g).
The principle of the silica-content determination is in
an addition of sodium fluoride, which reacts with silicic
acid, forming an equivalent quantity of sodium hydrox-
ide (5) followed by neutralization with a solution of hy-
drochloric acid (6).
H
2
SiO
3
+6NaF+H
2
O  Na
2
SiF
6
+ 4NaOH (5)
NaOH + HCl NaCl + H
2
O (6)
100 mL of a 5-% sodium fluoride solution was added
to the solution created during the determination of total
alkalinity. The resulting solution was titrated with 1M
HCl to the equivalence point, i.e., to a red color. Subse-
quently, the percentage of SiO
2
was calculated with
Equation (7),
w
cVVM
m
SiO
HCl SiO
s
2
2
=
⋅−⋅ ()
10
40
(7)
where w
SiO2
represents the percentage weight amount of
SiO
2
in the water-glass sample (w/%), c
HCl
is the con-
centration of the HCl measuring solution (mol·L
–1
), V
1
is the consumed volume of the HCl solution (mL), V
0
is
the volume of the HCl solution (mL) added for the
blank test, M
SiO2
is the molar mass of SiO
2
(g·mol
–1
),
and m
s
is the weight of the water-glass sample (g).
2.4 Induction coupled plasma-optical emission spec-
troscopy (ICP-OES)
The high accuracy of the chemical determination of
sodium silicate solutions was achieved using ICP-OES
(Horiba Scientific Ultima 2). Exactly 0.10 mL of the wa-
ter-glass sample was added to a 100 mL plastic volumet-
ric flask using an automatic pipette. The flask was filled
with distilled water to its calibration mark. The prepared
solution was diluted 1000×. The sample was then sub-
jected to an ICP-OES determination by transport via a
peristaltic pump to a nebulizer and further carried by ar-
gon to a torch. The measured concentrations of elements
were obtained in mg·L
–1
. These values were converted to
molar concentrations of the given oxides, from which the
silicate modulus was calculated.
2.5 Gravimetric determination of SiO
2
For the gravimetric determination of SiO
2
, approxi-
mately1gofthew ater-glass sample was weighed and
transferred to a porcelain dish with 20 mL of distilled
water. The solution in the porcelain dish was covered
with a watch glass and 10 mL of concentrated HCl was
slowly added to it. The dish was then placed on a hot
plate, where the decomposition of the sample was com-
pleted at a temperature of 80 °C. Subsequently, the watch
glass was rinsed with distilled water and a few drops of
concentrated HNO
3
. The dish was left on the hot plate
until the solution was evaporated to dryness, then the
dish was placed in an oven preheated to 105 °C for 10
min to completely dehydrate the precipitated silicic acid.
After being removed from the oven, the dish was placed
again on the hot plate and moistened with 5 mL of con-
centrated HCl. After 3 min, 100 mL of hot water was
poured into the solution. The excluded silicic acid was
filtered through ashless filter paper with a high pore den-
sity. The filter was washed 3 times with 5 mL of hot 1-%
HCl solution. The paper filter with expelled silicic acid
was inserted into an annealed crucible and placed in a
triangle high above a Meker burner. The annealing pro-
cess was lasted until the contents of the crucible dried
and the filter paper was ashed. Then the crucible was
placed in a laboratory furnace, where it was annealed to
L. KALINA et al.: CONDUCTOMETRY ANALYSIS – BENEFICIAL METHOD FOR DETERMINING ...
Materiali in tehnologije / Materials and technology 58 (2024) 3, 357–361 359
Figure 2: Example of the measured conductometric curve for the de-
termination of SiO
2

a constant weight at 1100 °C; after cooling it down in a
desiccator, the crucible with silica was weighed to the
nearest 0.1 mg. The entire procedure was repeated 3
times. From the obtained values, the percentage of SiO
2
in the water-glass sample was calculated with Equa-
tion (8),
w
mm
m
SiO
s
2
=⋅
−
100
21
()
(8)
where w
SiO2
represents the percentage weight amount of
SiO
2
in the water-glass sample (w/%), m
1
is the weight
of an empty annealed porcelain crucible (g), m
2
is the
weight of the annealed sample and crucible (g), and m
s
is the weight of the water-glass sample (g).
3 RESULTS AND DISCUSSION
The titrimetric method using the color change of the
acid-base indicator was chosen as the most widely used
industrial analysis of alkaline silicate solutions for the
verification of the chemical composition declared by the
manufacturer. Total alkalinity determination for different
types of water glasses provided results without any sig-
nificant deviation (Table 2). The color change of the
methyl red indicator showed a sharp transition from yel-
low to red at the equivalence point. However, complica-
tions occurred during the silica content determination.
According to the work procedure described in the experi-
mental part, the neutralization process with the hydro-
chloric acid solution changes the indicator color again
L. KALINA et al.: CONDUCTOMETRY ANALYSIS – BENEFICIAL METHOD FOR DETERMINING ...
360 Materiali in tehnologije / Materials and technology 58 (2024) 3, 357–361
Figure 3: Color transition in the determination of the SiO
2
content by the titrimetric method
Table 2: Chemical compositions of used water glasses determined with the titrimetric method using the color change of the acid-base indicator
titrimetric method
(color change from yellow to red)
titrimetric method
(color change from yellow to orange)
Na-WG K-WG Li-WG Na-WG K-WG Li-WG
w
Na2O w
SiO2
w
K2O
w
SiO2
w
Li2O
w
SiO2
w
Na2O w
SiO2
w
K2O
w
SiO2
wLi2O
wSiO2
1 17.00 39.08 25.97 35.95 3.31 20.25 16.73 32.91 25.17 28.44 3.27 17.33
2 16.70 39.46 25.80 35.94 3.33 20.35 16.67 32.89 25.32 28.88 3.20 17.36
3 16.17 39.65 26.11 36.06 3.28 20.25 16.69 32.50 25.51 27.92 3.26 17.34
4 16.47 39.77 26.25 36.19 3.32 20.37 16.79 31.93 25.40 28.15 3.24 17.37
5 16.49 39.65 26.11 36.23 3.14 19.16 16.48 31.95 25.19 28.96 3.24 17.34
6 16.86 39.84 25.89 36.13 3.26 20.24 16.19 31.87 25.10 28.80 3.25 17.40
Avg. 16.62 39.57 26.02 36.08 3.27 20.10 16.59 32.34 25.28 28.53 3.24 17.35
SD 0.30 0.27 0.17 0.12 0.07 0.47 0.22 0.49 0.16 0.42 0.02 0.03
Ms 2.46 2.17 3.05 2.01 1.77 2.66
Ms (ICP-OES) 1.84 1.74 2.60 1.84 1.74 2.60
Table 3: Chemical compositions of used water glasses determined with the conductometric analysis
conductometric analysis
Na-WG K-WG Li-WG
w
Na2O w
SiO2
w
K2O
w
SiO2
w
Li2O
wSiO2
1 16.81 32.06 25.61 28.36 3.22 17.15
2 17.02 32.60 25.48 28.92 3.24 17.16
3 16.56 31.97 25.31 28.13 3.23 17.15
4 16.92 32.15 25.52 28.62 3.25 17.14
5 16.80 32.37 25.49 28.83 3.26 17.17
6 17.01 32.08 25.43 28.76 3.24 17.15
Avg. 16.85 32.20 25.47 28.60 3.24 17.15
SD 0.17 0.24 0.10 0.30 0.01 0.01
Ms 1.97 1.76 2.63
Ms (ICP-OES) 1.84 1.74 2.60

from yellow to red. However, as can be seen (Figure 3),
the color transition in this case is very unclear at differ-
ent stages of the titration. In the first phase, the equiva-
lence point is determined as a bright red-violet color-
ation, which corresponds approximately to the seventh
shade in Figure 3 (from left to right). The results of the
SiO
2
content showed a relatively small deviation (Table
2 – left side); nevertheless, the total silica content and
subsequently calculated silica modulus differed consider-
ably from the declared composition (Table 1). For this
reason, the approach to determining the equivalence
point was changed. It was found from several measure-
ments that the equivalence point belongs to an orange
shade from Figure 3 (the fourth from the left). Such de-
termination has already produced results corresponding
to the declared chemical composition as well as the re-
sults obtained from ICP-OES analyses.
The problematic determination of the silica content
gives reason to use another method of evaluation such as
conductometric analysis. Its results are included in Ta-
ble 3. Both the alkali metal and silica contents are al-
ways in good agreement with the values given by the
manufacturer. The volume consumptions of the HCl vol-
umetric solution were determined from the intersections
of linear parts of conductometric curves and not from
color transitions. This fact allows a more accurate mea-
surement in contrast to the titrimetric method using an
acid-based indicator where the accuracy may be affected
by subjective color perception. Moreover, measurement
deviations are very low, and the results are more consis-
tent with the results of ICP-OES.
The accuracy of the achieved results was also as-
sessed from gravimetric determinations of SiO
2
. The av-
erage values of three measurements were 32.07 w/% of
SiO
2
in sodium water glass, 28.34 w/% in potassium wa-
ter glass, and 17.16 w/% in lithium water glass. Com-
paring these results with those of the conductometric
analysis, marginal differences were observed. The verifi-
cation using the mentioned analyses proves that
conductometry provides meaningful results and can also
be used advantageously in industrial practice.
4 CONCLUSIONS
The presented method for determining the chemical
composition of the used water glasses, based on a
conductometric analysis, provides satisfactory and accu-
rate results, verified with other analytical methods. The
currently used titrimetric method using the color change
of the acid-base indicator had been shown to be mislead-
ing, especially in the determination of SiO
2
. Another rea-
son for its unsuitability lies in the use of sodium fluoride,
which is classified as a highly toxic substance. On the
contrary, in the determination of total alkalinity of alka-
line silicate solutions, this method provides representa-
tive results. For this reason, a combination of the
conductometry analysis and the titrimetric method based
on the color change of the acid-base indicator can also be
considered.
Acknowledgment
This outcome was achieved with the financial support
within the project GA22-04828S “Advanced lithium sili-
cate sealers: on the way to sustainable building materi-
als”.
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L. KALINA et al.: CONDUCTOMETRY ANALYSIS – BENEFICIAL METHOD FOR DETERMINING ...
Materiali in tehnologije / Materials and technology 58 (2024) 3, 357–361 361