Monitoring acid-base, precipitation, complexation and redox titrations by a capacitively coupled contactless conductivity detector
Graphical abstract
A developed capacitively coupled contactless conductivity detector is used to monitor the process of chemical reaction in real time. Plus delivering titrant with a peristaltic pump, simple and automatic micro-titrations are realized, by using disposable glass reaction cells. All the four routine kinds of titrations, i.e. acid-base, precipitation, complexation and redox, can be performed easily, free of the exchange of working electrode.
Introduction
Because of its intrinsic attractive characteristics, e.g. precision, accuracy, simplicity and cost-effectiveness, titrimetry is popular in chemical laboratories and in the control of various industrial processes [1], [2], [3]. Traditional titrimetric analyses are based on the type of chemical reaction involved [4] and mainly include acid-base [5], [6], [7], redox [8], precipitation [3], [4], [9] and complexation [10], [11]. It is critical to accurately identify the equivalence point to know the exact amount of target constituents. Optical and electrochemical methods are commonly used to show the equivalence point. The former is generally based on a rapid and visible colour change at the endpoint [10], and plays a fundamental role in the development of titrimetry. In particularly, those methods employing automated photometric or spectrophotometric instrumentations are popular [5], [6], [7], [12]. There are two key advantages by employing optical instrumentations: (1) satisfactory accuracy [11] and sensitivity [6]; and (2) risk-free contamination of head stage by any component in the solution. However, these systems also have some challenges: (1) one or more types of indicators are needed [5], [6], [7], [12]; (2) there is indicator error, which results from the difference between the endpoint and the equivalence point; (3) the existing indicators are rarely completely selective [10]; and (4) they require transparent solutions and vessels.
Potentiometry can be used to monitor titrations via a reference electrode and an indicator electrode that responds selectively to the species of interest (e.g. glass electrode [13] and ion-selective electrode [8]). Meanwhile, conductivity will change during the process of many titrations resulting from the change of ionic strength and/or mobility. Thus, the conductometry is also an alternative method for monitoring titration [14], particularly useful when separate endpoints exist for each component [3]. For the physical property of solution conductivity is well established, in principle conductometry can be used for many purposes [4], [13], e.g. acid-base [14], redox [4], precipitation [2] and complexation [15]. In general, electrochemical methods can be easily automated, shorten the analysis time, and are precise and sensitive. In contrast to optics-based devices, photo-electric conversion is not needed for electrochemical devices [5]. Thus, both potentiometric [2], [8], [9], [16], [17] and conductometric [3], [11], [13], [18], [19] modes are excellent alternates to monitor titration. However, these electrochemical methods have not yet been perfected despite many important breakthroughs. During the performance, the tips of the working electrode and reference electrode must be immersed in the titration solution rather than a non-invasive test [4]. This causes a few bothersome consequences: (1) electrode deterioration is unavoidable [2], and results in erratic measurements that decrease the accuracy; (2) electrodes and reaction cell must be cleaned before each measurement to minimize the memory effect [9]; and (3) it’s hard to realize a micro-titration [7].
To resolve the problems faced by common electrochemical methods, contactless conductivity detection has been utilized. It was a very popular in the 50s and 60s of the last century, and was once extensively used [20], [21], [22], [23], with a name of high frequency titration or oscillometry. However, until the presence of modern C4D, which was supported with computer and special software, micro- and automatic titration became reality. For example, in 2010, Saito et al. reported that total dissociable hydrogen in sample solution could be detected by employing a capillary electrophoresis combined C4D [24]; in 2014, da Costa et al. reported that a kind of C4D for capillary electrophoresis could be used for detecting the endpoint of acid-base titration [25]. However, to the best of our knowledge, the employment of C4D for developing versatile titration platforms has not been reported up to now.
We have previously used a developed C4D to monitor DNA amplification [26] and the reaction process of ciprofloxacin hydrochloride and silver nitrate [27] in real time. In this paper we showed the feasibility of monitoring the process of all the four routine kinds of titrations by employing the modern C4D. Our goal was not to fully optimize each titration mode, but to determine the validity of the approach.
Section snippets
Materials
Silver nitrate and disodium ethylenediaminetetraacetate dihydrate were purchased from Aladdin Chemical Reagent Co., Ltd. (Shanghai, China). Ciprofloxacin hydrochloride standard reagent (C17H18ClFN3O3, CAS: 86393-32-0) was kindly supplied by National Standard Material Research Center (Beijing, China). Hydrochloric acid, sodium hydroxide, magnesium sulfate anhydrous magnesium sulfate, ammonium ferrous sulfate hexahydrate, potassium dichromate (attention – toxic and carcinogenic substance) and
Results
We characterized the properties of the monitoring system, i.e. the C4D, in comparison with a commercial conductometer at room temperature, using a series of KCl solutions at different concentrations. As shown in Fig. 2A, the apparent conductivity of the solution increases as a function of concentration over the range of 0.5 mM–0.5 M. From 1.0 to 100.0 mM, the relationship is nearly linear (R2 = 0.9971), suggesting that the C4D can report the conductive property of the solution similar to that
Discussion
The C4D uses a conductivity-based detector where the electrodes are not in direct contact with the measured solution [32]. As showed in Fig. 1 and Fig. S1, two cylindrical copper tubes acted as the electrodes and were separated by a gap to form the detection cell. In the equivalent circuitry for the cell, the two electrodes formed capacitors (CW) with the inside of the reaction cell, which were connected by a resistor (Rref) formed by the electrolyte solution. C0 was the stray capacitance,
Conclusions
Using the developed C4D as “eye” and “recorder”, all the four routine kinds of titrations can be performed with the proposed micro-titration system. Endpoints are identified easily from the peak of the V-shaped titration curves. The quantitative determination can be carried out based on the linear relationships between the elapsed time and the initial concentration of titrand, rather than being calculated from the consumption of titrant. The approach has the nature advantages of electrochemical
Acknowledgments
This work was supported by Central Public-interest Scientific Institution Basal Research Fund, CAFS (2016RC-BR02) and Qingdao National Laboratory for Marine Science and Technology (2015ASKJ02-05).
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