Annals of West University of Timisoara
Series Chemistry 16 (1) (2007) 1-8
ANALYSES BY CONDUCTOMETRIC TITRATION OF
CHLORIDES FROM WASTEWATERS THAT CONTAINS
HEAVY METALS
L aur a B ULG AR I U , Bri n d us a RO B U, D o in a B IL B A , M . M AC O VE A N U
Technical University "Gh. Asachi" Iasi, Faculty of Chemical Engineering , Department
of Environmental Engineering and Management, D. Mangeron, 71A, Iasi, 700050,
ROMANIA
Received: 07 March 2007
Modified: 09 March 2007
Accepted: 15 March 2007
SUMMARY
The Romanian legislation trough its standards recommend to analyze the
chlorides from wastewaters using the method of standard Mohr titration. A very facile and
selective method is proposed for fast determination of chloride from wastewaters resulted
from galvanization processes, by conductometric titration using AgNO3. The method allows
to analyze the chlorides over the range 177.5 ÷ 17750 mg Cl-/l (5⋅10-3 ÷ 0.5 mol/l), with a
detection limit of 34.21 mg Cl-/l. The interferences caused by several ions such as: (Fe(II),
Fe(III), Pb(II), Zn(II), Cu(II), Co(II), Ni(II), SO42-) which can be present in wastewaters
were identified. The validation of conductometric titration method was done by recovery
test of chloride in artificial matrix and wastewaters resulted from a galvanization
installation. The experimental results show that the proposed method can be successfully
applied for the determination of chloride in metallurgical wastewaters.
Keywords: chlorides
galvanization.
analyses;
conductometric
titration;
wastewaters
from
INTRODUCTION
In galvanization processes the hydrochloric acid is frequently used as pickling
agent. Thus, from environmental point of view the determination of chloride content in
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BULGARIU L., ROBU B.
ET AL.
wastewaters becomes necessary to establish the quality of these waters that are discharged
into sewage [1, 2].
Conventionally, various methods such as ion-chromatography [3, 4], FIAspectrophotometry [5], colometry [6], spectrophotometry [7, 8] and potentiometry [9, 10]
are used for chlorides analysis. These methods have excellent analytical performances like
low detection limit and high sensitivity, but most of them are time and energy consuming or
complicated and expensive instruments are needed.
The Romanian legislation recommends trough its standards to analyze the
chlorides from waste waters applying the titration method. This is the reason that for
environmental analysis the chloride determination is recommended to be done using the
method of standard Mohr titration [11, 12]. The Mohr method is a simple and inexpensive
classical titration of chloride with AgNO3, in presence of K2CrO4 as standard.
Using the Mohr method can be analyzed the chlorides content from water and soil
samples, but the high accuracy of the experimental results is obtained only when the
analyzed samples are clear, with pH between 4 and 6, and the content of heavy metals are
relatively low. The presence of some heavy metals ions, such as Pb(II), Zn(II), Fe(II) etc.,
can consume the K2CrO4 (due to the formation of compounds with low solubility), and the
end point of titration cannot be observed in time. Considering all these facts, the Mohr
method is not suitable for chlorides analysis from wastewaters resulted from galvanization
processes, where the heavy metal ions are used in considerable amounts.
The purpose of this paper is to propose an alternative method to chlorides analyses
in presence of heavy metal ions in order to be included in Romanian standards as
recommended more suitable for chlorides analysis. The suitable method for that is the
method of conductometric titration.
MATERIALS AND METHODS
Stock solution of 1 mol/l potassium chloride (Fluka, Buchs, Switzerland) was
prepared by salt dissolving in distilled water and dilution to the 250 ml. Working solutions
of chloride were prepared by dilution from stock solution with distilled water (H.G.
188/2002, H.G. 352/2005).
The silver nitrate titrate solution (0.1 mol/l) was obtained by AgNO3 (Reactive
Bucharest, Romania) dissolving in distilled water, followed by solution standardization,
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CONDUCTOMETRIC TITRATION OF CHLORIDES FROM WASTEWATERS
according with titrimetric procedure [11].
All others reagents were analytical reagents degree and were used without
purification. The aqueous solutions were prepared with distilled water. All glassware was
shaken in sulphochromic mixture, for at least 24 hours and washed with distilled water.
The conductivity of working solutions was measured with a Radelkis OK-109
Conductometer (Budapest, Hungary), equipped with a conductivity cell of cell constant 1.47
cm-1. The conductivity was measured after the solution was homogenized, and the measured
value is stable in this case for 30 or 60 seconds. Solution of 0.1 mol/l AgNO3 (from a 10-ml
burette) was added, until at least five measured values after the equivalence points were
made. The exact equivalence volume is obtained from titration curve.
The selectivity coefficients were calculated as ratio between chloride concentration
and interfering ions concentration which gives a 5 % equivalence volume change in a
reference solution.
The recovery test was done using artificial matrix and wastewaters resulted from
galvanization process, without any preliminary treatment. The characterizations of artificial
matrix and wastewater sample are presented in Table I.
Table I. The composition of artificial matrix and wastewaters,
used for recovery test
Suspensions, mg/l
pH
Chloride, mg/l
Fe total, mg/l
Zn, mg/l
Pb, mg/l
Cr total, mg/l
Artificial matrix
Clear solution
3.26
1574
5.124
6.248
-
Wastewaters
1667.0
4.45
3905
1979
2.023
0
RESULTS AND DISCUSSION
Silver chloride has very low solubility in water and acid media. Thus, the addition
of AgNO3 to a solution which contains chloride results in formation of white precipitate of
AgCl, according with the well known reaction:
(A+ + Cl-) + (Ag+ + NO3-) AgCl + (A+ + NO3-)
(1)
where: A+ is a common counter-ion.
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BULGARIU L., ROBU B.
ET AL.
conductance
conductance
This reaction provides a basis for the determination of chloride by conductometric
titration. Conductometric curves (Figure 1) were obtained using KCl laboratory solution (a),
artificial matrix (b) and wastewater (c).
In all cases, it can be observed that under equivalence, the solutions conductivity
remains almost constant, due to the formation of AgCl, which a non dissociated precipitate
is. After equivalence, the solutions conductivity increase proportional with the AgNO3
volume added in excess. The AgNO3 volume at equivalence point is obtained graphically,
and is then used for the determination of chloride concentration, according with usual
analytical calculation procedures [12].
0
1
ve
vAgNO3, ml
2
0
3
1
ve
vAgNO3, ml
3
(b)
conductance
(a)
2
0
1
ve
vAgNO3, ml
2
3
(c)
Figure 1. Conductometric titration curves obtained after titration of: (a) 0.1 mol/l KCL; (b) 0.1 mol/l
chloride added to artificial matrix; (c) wastewater with 0.11 mol/l chloride content. All solutions were
titrated with aqueous AgNO3 0.1 mol/l. (ve – equivalence volume of AgNO3).
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CONDUCTOMETRIC TITRATION OF CHLORIDES FROM WASTEWATERS
The conductometric titration of chlorides with AgNO3 is a very facile and fast
method, and can be used for environmental analysis, without a preliminary treatment of
samples. The limit of detection was calculated as three times of standard deviation of six
replicate measurements, the precision (RSD %) as well as other analytical characteristics
[13, 14] of this method are summarized in Table II.
Towards to Mohr method, the conductometric titration has several experimental
advantages: (1) it can be analyzed samples with various pH, without affecting the accuracy
of experimental results; (2) it is not necessary a preliminary treatment of samples; the
presence of sediments doesn’t influence the conductometric titration of chlorides. This
avoids the necessity for leaching and it reduces the analysis time from 30 min to cca. 5 min
per sample; and (3) the presence of some heavy metal ions, which are frequently discharged
in wastewaters resulted from galvanization process, does not interfere in chloride
determination. To quantify the selectivity of this method, it was used the selectivity
coefficients defined as the ratio of chloride concentration (cCl-) and interfering ion
concentration (cj) which gives a 5 % equivalence volume change from the reference solution.
Table II. The analytical characteristics of the proposed method
Analytical parameter
177.5 ÷ 17750 mg Cl-/l
Studied concentration domain
Limit of detection (3σ)
34.21 mg Cl-/l
Limit of quantification (10σ)
114.04 mg Cl-/l
Variation coefficient
0.31
RDS %
1.08
σ - standard deviation (n=6, ca 23°C).
In this study, the interfering ions were added to an identical reference solution,
with a constant chloride content (1775 mg Cl-/l or 5⋅10-2 mol/l) until a 5 % change in
equivalence volume was obtained. The calculated values of the selectivity coefficients are
summarized in Table III.
The values of selectivity coefficients show that Fe(II), Fe(III), Zn(II), Pb(II),
Cu(II), Co(II), Ni(II), SO42- ions practically do not interfere, which means that this method
can be used for quantitative determination of chloride in metallurgic waste waters.
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BULGARIU L., ROBU B.
ET AL.
Table III. The selectivity coefficients values
Interfering, j
log aCl-,j
Interfering, j
log aCl-,j
Fe(II)
-2.936
Cu(II)
-3.262
Fe(III)
-2.868
Co(II)
-3.478
Pb(II)
-4.102
Ni(II)
-3.472
Zn(II)
-3.521
SO42-3.688
The aCl-,j values were determined in a 1775 mg Cl-/l reference solution and calculated for
a 5 % changing in equivalence volume (n=4, ca 23°C).
In order to use the selectivity advantages, the recovery test was done using an
artificial matrix and metallurgic waste water, from S.C. MITTAL STEEL S.A Iasi, Romania,
without a preliminary treatment. The composition of both, artificial matrix and metallurgic
waste water, used for recovery test are presented in Table I.
For to investigate the recovery of chloride volume of 20 ml from artificial matrix
and metallurgic waste water was transferred to each of the four 50 ml volumetric flasks, and
0.0 ml; 5.0 ml; 10.0 ml and 15.0 ml chloride standard solution (3550 mg Cl-/l) was added to
each flask. All the samples were analyzed by conductometric titration with AgNO3. The
average recovery percentages obtained for the chloride addition to artificial matrix and
metallurgic waste water are presented in Table IV.
Table IV. The recovery test
Cl- added,
mg/l
0.0
355
710
1065
Artificial matrix
Cl- found, mg/l
358.07
708.92
1067.52
% Recovery
100.86
99.84
100.23
Metallurgic waste water
Cl- found, mg/l
3906.20
4263.05
4615.60
4981.23
% Recovery
100.04
99. 89
100. 20
As Table IV show a good recovery of chloride was obtained for both artificial
matrix and metallurgic waste water samples, indicating that the components of these do not
interfere significantly with the chloride determination. Therefore, this method can be a viable
alternative for chloride determination in waters samples with a high content of heavy metal ions.
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CONDUCTOMETRIC TITRATION OF CHLORIDES FROM WASTEWATERS
CONCLUSION
The conductometric titration is a facile and fast method for analysis of chlorides from
wastewaters resulted from galvanization processes, and not only. The suitable detection limit
(34.21 mg Cl-/l), appropriate dynamic range (177.5 ÷ 17750 mg Cl-/l) and well selectivity
towards chloride at the presence of several heavy metal ions are parameters that have pointed out
the applicability of this method. In addition, using this method for chloride determination can be
analyzed samples with various pH and a preliminary treatment of these is not necessary.
The results of recovery test showed that this method can be a better alternative
device for the direct determination of chloride in metallurgic waste water.
Considering also the fact that the energy consuming, time and reactive can be
saved, this method is more suitable than classical method recommended by Romanian
standards to analyze chlorides from wastewaters that contains various amounts of heavy metals.
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