bae2015

Article
DOI: 10.1002/bkcs.10554
H.-S. Bae et al.
BULLETIN OF THE
KOREAN CHEMICAL SOCIETY
Argentometric Titration Apparatus with a Light Emitting Diode-based
Nephelometric Detection System
Hyun-Soo Bae,† Hoon Hwang,‡ and In-Yong Eom†,*
†
Department of Life Chemistry & Natural Science Research Institute, Catholic University of Daegu,
Gyeongsan 712-702, Republic of Korea. *E-mail: [email protected]
‡
Department of Chemistry, Kangwon National University, Chuncheon 192-1, Republic of Korea
Received July 17, 2015, Accepted July 29, 2015, Published online October 1, 2015
A newly revised apparatus for argentometric precipitate titrations was fabricated and evaluated. A light emitting diode was adapted as a solid-state light source and a syringe pump was used for manipulating titrant both
in stopped flow mode and continuous mode. The performance of the two modes was compared in terms of
accuracy and precision. The continuous flow mode is much faster and shows generally higher accuracy under
given experimental conditions. The patterns of precipitate titration curves were changed with different coagulant concentrations and addition flow rates of titrant. However, end point volumes were not affected by these
experimental factors tested here.
Keywords: Nephelometry, Argentometric titrimetry, Titration curve, Coagulant, Syringe pump
Introduction
Precipitation titration is one of the classical volumetric analysis techniques that measure the volume of titrant to determine
the concentration of an ionic analyte which forms insoluble
precipitates. This method is often used to determine halide
ions by precipitating them with aqueous silver (I) solution.
This technique is called argentometric titration which has been
widely used for halide ion quantification because of its simplicity and accuracy.
The end point of this titration process can be determined by
monitoring color change of indicator right after an equivalent
point passed. Mohr, Volhard, and Fajans methods are three
types of the argentometric titration based on the formation
of red substances at the end points.1 Potentiometry2,3 (i.e.,
adapting a silver ion selective electrode) is also widely used
to find an end point so as to determine an equivalent point.
Optical technique4 is another alternative to determine an
end point by measuring fluorescence, absorbance, or reflectance from the precipitates. Measuring the reflected and/or
scattered light from precipitate forming during a titration process is called nephelometry,5,6 which is known to be more precise for determining an end point for lower concentration of
halide ions because the reflected light change is very sensitive
to the very low level of reflective particle’s concentration.
Recently, pseudo nephelometry using light emitting diodes
(LEDs) was reported and used to determine halide ion
concentration.7 – 12 This LED-based nephelometric detection
system takes advantages from the nephelometry and LEDs
as a solid-state light source: nephelometry has a potential to
find an end point more clearly for the lower concentration
of analytes and LEDs are very stable in intensity fluctuation
so as to be suitable for developing an accurate and robust
Bull. Korean Chem. Soc. 2015, Vol. 36, 2725–2729
optical detection system with a minimized noise level. In this
system, the standard solution of sliver ions (i.e., titrant) was
gravimetrically added to an aqueous solution of halide ions
under a stopped flow mode. AgCl precipitate was formed by
adding a drop of titrant (one step) and light intensity reflected
from the precipitates was monitored simultaneously. The
intensity of reflected light increased as the Ag(I) ion added
to the aqueous solution of halide ions but it did not increase
after an end point because the precipitate did not form any
more. Therefore, the end point volume was determined by
counting the steps of titrant added just before the end point.
This system was simple but could determine the millimolar
concentrations of chloride ions accurately.9,10 The similar system was used to determine the binary mixture of I− and Br−,
simultaneously.10
We revisit this technique after revising the system by means
of a flow manipulation. The originally developed system was
operated gravimetrically only under stopped flow mode. So,
the accuracy for the system was truly depending on a volume
of a drop of a titrant. However, unfortunately the volume of a
drop could be varied (could be getting smaller) as the titration
process goes on due to the fact that the height of titrant inside a
burette is lowered so the pressure on the bottom of the burette
is decreased. This possible limitation was clearly solved by
replacing the burette with a step motored syringe pump. Using
the syringe pump enables the system operating under continuous mode, which is eventually more convenient and makes a
titration process faster. Also, the volume of a drop, unlike the
initially reported system,9,10,12 does not vary under the
stopped flow mode because the flow is regulated not gravimetrically but stepped-motor driven for the currently revised
apparatus. Reportedly, in nephelometry the reflectance
increased linearly and reached the maximum at the end point
© 2015 Korean Chemical Society, Seoul & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Wiley Online Library
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Article
ISSN (Print) 0253-2964
| (Online) 1229-5949
and then decreased after the end point passed due to the dilution effect.13 However, the titration curve was not like that
with our system. The reflected light intensity was not increased
linearly as the precipitates were formed before the end point.
The reflectance increment was exponentially increased nearly
just before the end point.
With a revised argentometric titration apparatus here, we
compared the stopped flow mode and continuous flow mode
in accuracy and precision. Using coagulant and different flow
rates of titrant affected the patterns of titration curves dramatically but the end point volumes were not affected by the use of
coagulant (see Results and Discussion).
Experimental
Silver nitrate (AgNO3; Junsei Chemical Co., Tokyo, Japan),
sodium chloride (NaCl; Duksan Pharmaceutical Co., Seoul,
South Korea), and sodium nitrate (NaNO3; Duksan Pharmaceutical Co.) were all reagent grade and used without any further purification. LED-based reflectance measurement system
was fabricated newly (see below for details) for this study. A
blue LED (nominal λmax = 455 nm) was purchased from
Nichia Co. (Tokushima, Japan).
The argentometric titration system (shown in Figure 1) was
modified from the previously reported system12 by replacing
the burette (containing titrant) with a step motored (48 000
steps) syringe pump (Versa 6; Kloehn, Las Vegas, NV,
USA). The pump aspirates the AgNO3 solution (ca. 0.1 M)
from the reservoir (SN) and then dispenses the titrant into
the halide solution (HS) in a quartz vessel (QV). The QV
was placed on a magnetic stirrer (MS) and the white stir bar
(SB, 1.5 mm long) agitates HS continuously to suspend AgCl
precipitates during the whole titration process. Aspiration and
dispense modes were controlled by the distribution valve
(DV) and KCOMM™ software provided from the manufacturer (Kloehn). The titrant was delivering through a
Figure 1. Schematic diagram of argentometric titration system
equipped with a LED reflectance detector: PP, PVC pipe; QV, quartz
vessel containing halide solution (HS); SB, stir bar; MS, magnetic
stirrer; LS, light source (i.e., blue LED powered by 5 V with a current
limiting resistance); LG, light guide; PD, photodiode; VR, variable
resistance (1 KΩ); OA, operational amplifier; DA, data acquisition
card; PC, desktop computer; SN, silver nitrate standard solution;
SP, syringe pump; DV, distribution valve, see text for details.
Bull. Korean Chem. Soc. 2015, Vol. 36, 2725–2729
BULLETIN OF THE
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polytetrafluoroethylene (PTFE) tubing (PT, 0.76 mm
i.d. Upchurch, Oak Harbor, WA, USA) of which end-tip
(stainless steel tubing, approximately 100 μm i.d.) was
immersed in the titrand in order to prevent a drop formation
on the end-tip. The syringe pump controls the flow rates very
precisely. A light source (LS) is a blue LED powered by 5 dc
volt with a current limiting resistance (VR, 1 KΩ potentiometer). The source light traverses a light guide (LG, 4.5 mm
i.d. glass tube wrapped with a PTFE tape which taped again
with a black electric tape, see more details described in Ref.
12) and illuminates a spot (ca. 10 mm diameter) on the
QV’s outer wall. A small silicone photodiode (PD; Goldmine,
Scottsdale, AZ, USA) converts the source light reflected from
the quartz’s outer surface into a voltage (conversion rate 1 volt/
μA), which further amplified (circuit shown in Figure 2)
through an operational amplifier (OA, TL082-cp; Texas
Instrument, Dallas, TX, USA). The reflected light intensity
was finally recorded (Ch1 signal in Figure 1) on a desktop
computer (PC) through a digital input and output card (DA,
12; Measurement Computing, Norton, MA, USA) for further
data processing. The black PVC pipe (PP, 50 mm i.d. and 60
mm o.d.) and the black lid enclosed the QV and other optical
components to accomplish light proof system.
Titrant (AgNO3) was added into titrand (aqueous NaCl
solution) in two ways; a stopped flow mode and a continuous
flow mode. Note that only the stopped flow mode was used in
the previously reported titration system12 because of difficulty
of getting precise flow controls under such a gravimetric flow
force. The use of syringe pump here made it possible to perform a titration under the continuous flow mode. Due to the
two different addition modes of titrant, end points could be
determined in two ways. For the stopped flow mode, a precisely allocated volume (i.e., one step volume) of titrant added
once and then waited for 2 min to monitor the reflectance
change before the next addition of the equally allocated volume of titrant. In this case, end point volumes were determined
by counting the number of additions of titrant (i.e., number of
steps of titrant addition). To count the number of additions, an
Figure 2. Circuitry of current-to-voltage converter and voltage
amplifier using an operational amplifier. PD: photodiode. See text
for details.
© 2015 Korean Chemical Society, Seoul & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.bkcs.wiley-vch.de
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Argentometric Titration Apparatus with a LED-based Nephelometry
BULLETIN OF THE
KOREAN CHEMICAL SOCIETY
Figure 3. End point determination method and examples of titration curve (titration of chloride ion with silver (I) ion) obtained under the stopped
flow mode (a) and the continuous flow mode (b). For the stopped flow mode (a), 21 steps can be used to determine the end point volume because
the reflected light intensity does not increase after those steps of addition. See text for details.
instant +5 volt signal was directly generated from the syringe
pump (Ch2 in Figure 1) for each and every addition. Under the
continuous flow mode, the titrant was added into the titrand
continuously. The flow rate of the Ag+ solution was generally
0.075 mL/min otherwise it was specified. At this case, the +5
volt signal (Ch2) was generated once at the initial moment of
the titration to mark a start point. An end point, volume for the
continuous mode was determined by multiplying the titrant
addition flow rate and the time at which the intensity of the
reflected light did not increase any more. Figure 3 shows
examples of titration curve obtained under the stopped flow
mode (a) and the continuous flow mode (b) and end point
determination method from those curves. It is worth to note
that end point volumes under the continuous mode could be
determined more clearly by differentiating a titration curve
because the intensity of reflected light increased steeply at near
the end point but this method cannot be always applied if noise
is too high to differentiate a titration curve.
Because understanding a shape of titration curve is very crucial to determine the end point with higher accuracy, we were
trying to reveal what kind of factors affected the shape of titration curves: the use of different concentrations of coagulant
and different flow rates of the titrant addition. Coagulant is
often used to aggregate fine particles so to turn them into bigger precipitates in a gravimetric analysis. In the nephelometry,
patterns of titration curve were greatly affected by the presence
of coagulant because the reflection is affected by the size of
particles. Here, titration curve patterns were monitored by
changing the concentration of NaNO3 (as a coagulant) while
the concentrations of chloride ions (2.01 mM) and AgNO3
(0.101 M) titrant were fixed. The concentration of NaNO3
Bull. Korean Chem. Soc. 2015, Vol. 36, 2725–2729
added into the titrand increased by the factor of 0, 5, 10, 25,
and 50 times higher compared with the moles of chloride ions
(10.05 μmol) in the titrand.
During the titration of precipitation of Cl− by Ag+, AgCl
nuclei are first formed and then each AgCl nucleus grows
by adding more Ag+ into the Cl− solution. Therefore, the addition flow rate of titrant governs the formation rate of nucleus
creation and the speed of particle growth throughout the whole
process of the titration. It is well-known that the speed of precipitant addition should be as slow as possible to get bigger and
better homogenized size distribution of precipitates in gravimetric analysis: slower the flow rate of titrant, bigger the size
of AgCl precipitates, and vice versa. Different flow rates of
titrant change the pattern of the reflected light, which results
in a different shape of titration curves also. Therefore, the titration curve patterns were monitored at different flow rates
(0.075, 0.150, 0.225, 0.300, 0.375, and 0.600 mL/min) of
titrant (0.103 M AgNO3). The volume of titrand (2.00 mM
NaCl) for this study was 5.00 mL.
Results and Discussion
Both modes of the stopped flow and the continuous flow were
evaluated in terms of performance accuracy. Under the
stopped flow mode, accuracy test was performed by decreasing the allocated volume (i.e., one step volume of titrant,
0.100 M of AgNO3) from 2.00 μL to 0.400 μL while the volume of titrand (0.199 mM of NaCl) was fixed at 10.0 mL.
Under the continuous flow mode, the syringe pump delivered
the titrant (0.101 M of AgNO3) continuously to different sample volumes (5.0, 10.0, and 15.0 mL) of titrand (2.01 mM of
© 2015 Korean Chemical Society, Seoul & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Argentometric Titration Apparatus with a LED-based Nephelometry
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Table 1. Comparison of two flow modes in performance accuracy and precision.
Flow modes ([Ag+]prepared)
Stopped (0.100 M)
Continuous (0.101
M)
a
One step volume
Sample volume
End point volume, mL (%RSD)
[Cl−]experimental, mM ([Cl−]prepared)
Error (%)
0.0240 (0.0)
0.0220 (0.0)
0.0211 (1.8)
0.0206 (4.1)
0.102 (0.6)
0.202 (0.3)
0.296 (0.2)
0.240 (0.199)
0.220 (0.199)
0.211 (0.199)
0.206 (0.199)
2.06 (2.01)
2.04 (2.01)
2.00 (2.01)
20.6
10.6
6.0
3.5
2.5
1.5
−0.5
2.00 μL
1.00 μL
0.667 μL
0.400 μL
5.00 mL
10.00 mL
15.00 mL
a
Error = 100 × {([Cl−]experimental − [Cl−]prepared) / [Cl−]prepared}.
Figure 4. Changed patterns of titration curves with different molar ratio of NaNO3 to NaCl (a) different addition flow rates (mL/min) of titrant (b).
See text for details.
NaCl). These accuracy study results are summarized in
Table 1. As expected, decreasing the allocated titrant volume
improved accuracy (lowered % error) because smaller volume
of a drop minimizes analysis error in the stopped flow mode.
Generally speaking, minimizing the influence of a volume of a
titrant is one of the main keys to improve both accuracy and
precision in the volumetric titration technique. Due to the same
scope of this view, it is reasonable to show higher accuracy as
increasing the sample volume under the continuous flow
mode. Based on these accuracy studies, we could say that
the continuous flow mode has a shorter analysis time from tens
of minutes to a few minutes and higher accuracy compared to
the stopped flow mode under these experimental conditions.
However, it should be noted here that the stopped flow mode
showed clearer end points for two components titration (i.e., I−
and Br− mixture10) while the continuous flow mode failed at
this time to differentiate each component’s concentration.
Table 1 also proves that the proposed argentometric titration
apparatus shows acceptable reproducibility (less than 1%
RSD) for the continuous flow mode.
As stated in experimental section, the use of coagulant and
changing the addition rate of titrant (i.e., flow rate) to the
titrand affected the size of AgCl precipitates formed, which
Bull. Korean Chem. Soc. 2015, Vol. 36, 2725–2729
in turn changed the patterns of titration curves (Figure 4).
To be a solid titration technique, the proposed method should
find end point volumes successfully even in any case of titration conditions. For different molar ratio of NaNO3 to NaCl
(Figure 4(a)), end point volumes were determined and summarized in Table 2. For different addition flow rates (mL/min)
of titrant (Figure 4(b)), end point volume for each flow rate was
determined after converted time interval to added titrant
(AgNO3) volume and those volumes are also summarized in
Table 2. As shown in the table, even though pattern of titration
curves are affected by the different molar ratio of coagulant
and addition flow rates, end point volumes are quite similar
for each case. Note that similar patterns were observed for
titration of NaBr and NaI with coagulant addition (data and figures not shown here). Therefore, these results prove that the
proposed system is suitable for performing argentometric titrimetric techniques with such a low concentration of halide ion
in single digit of millimolar (mM) concentration. Actually, we
checked possibility of titration of lower concentrations (0.20
mM and 0.020 mM) of chloride ions with 0.100 M AgNO3 by
monitoring whether AgCl precipitates are formed or not by
raw eyes. White precipitates were clearly found by raw eyes
for 0.20 mM but not for 0.020 mM of NaCl, which means that
© 2015 Korean Chemical Society, Seoul & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Argentometric Titration Apparatus with a LED-based Nephelometry
Table 2. End point volumes determined at different molar ratios of
NaNO3 to NaCl and different addition flow rates of titrant.
Molar ratios
End point
Flow rate
volume (mL) (mL/min)
0
5
10
25
50
0.103
0.103
0.101
0.102
0.102
Average (%RSD)
0.102 (0.8)
[Cl−]experimental,
mM (error, %)
2.06 (2.7)a
a
b
0.075
0.150
0.225
0.300
0.375
0.600
Average (%
RSD)
[Cl−]experimental,
mM (error, %)
End point
volume
(mL)
0.106
0.0980
0.102
0.101
0.106
0.0940
0.101 (4.6)
continuous flow mode and about 20–30 min of titration times
for the stopped flow mode. Even though the patterns of titration curves were affected by the use of coagulant and different
addition flow rates of titrant, end point volumes were not
affected by those factors. With such a result, the proposed
method can be successfully used for pedagogical purpose.
A further study is in progress to use the proposed system as
a differential titration platform with two or three halide ion
mixture.
Acknowledgments. This work was supported by research
grants from the Catholic University of Daegu in 2012
(20121046).
2.08 (4.2)b
Error = 100 × {(2.064 – 2.01) / 2.01}.
Error = 100 × {(2.084 – 2.00) / 2.00}.
0.20 mM could be the lowest concentration that the proposed
system can perform the argentometric titration. However, note
that the halide concentration should be lowered if the higher
concentration of AgNO3 is used and/or more sensitive light
transducer such as a photomultiplier tube (PMT) is adapted
to the system.
Conclusion
A new argentometric system was developed and evaluated to
perform argentometric precipitate titrations. The system
adapted a LED as a solid-state light source and a syringe pump
for the manipulation of titrant both in stopped flow mode and
continuous mode. Both modes showed high precision and
accuracy with a few minutes of performance times for the
Bull. Korean Chem. Soc. 2015, Vol. 36, 2725–2729
BULLETIN OF THE
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