The Characterization of a Custom-Built ... - ACS Publications

In the Laboratory

The Characterization of a Custom-Built Coulometric Karl Fischer Titration Apparatus Victoria C. Dominguez, Cole R. McDonald, Matt Johnson, Doug Schunk, Rod Kreuter, and Dan Sykes* Department of Chemistry and Forensic Science Program, The Pennsylvania State University, University Park, Pennsylvania 16802 *[email protected] Benjamin T. Wigton and Balwant S. Chohan School of Science, Engineering, Technology, The Pennsylvania State University, Harrisburg, Pennsylvania 17057

Coulometric Karl Fischer (KF) titration is a widely used analytical technique for the quantification of water in various substances including laboratory solvents, transformer oils, pharmaceuticals, cosmetics, and food products. In principle, the water content of a sample is quantified based on the number of electrons transferred during titration. The number of electrons transferred is determined by the time necessary to reach the end point of an electrolytic titration of water with iodine under constant-current conditions, amount of e - ¼

I t F

ð1Þ

where I is the constant current at which the titration is taking place, t is the time necessary to reach the end point, and F is Faraday's constant. This method of titration is carried out within a bipotentiometric cell that contains a pair of generator and detector electrodes. The KF reaction is based on the Bunsen reaction and occurs within a liquid medium containing an alcohol (ROH), a base (B), SO2, and iodide. At the generator electrodes, the iodine (I2) titrant is anodically produced from iodide (I-) with a corresponding reduction of Hþ to hydrogen gas: 2I - f I2 þ 2e 2Hþ þ 2e - f H2 ðgÞ

ð2Þ

The alcohol and SO2 react to form an alkylsulfite. When H2O is present in the system, the alkylsulfite is oxidized by the iodine to an alkylsulfate. Under these conditions, 2 mol of electrons corresponds to 1 mol of H2O when H2O and I2 are consumed (1) in a 1:1 ratio: ROH þ SO2 þ B f BHþ þ ROSO2 H2 O þ I2 þ ROSO2 - þ 2B f ROSO3 - þ 2BHþ I - ð3Þ The end point of the titration occurs when all of the water is consumed by I2; the presence of excess I2 is then detected voltametrically by the platinum (Pt) detector electrode. One advantage of this coulometric technique is that it does not require standardization of reagents. Another advantage is

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that the same reagent system can be used repeatedly. The differences in sample properties, their solubility, and reactivity can influence the conditions and course of the KF reaction. Strong acids and strong bases, for instance, must be neutralized prior to titration. Methanol is the standard alcohol in solution; however, the use of non-methanol solvents such as methoxy2-propanol or 2-(2-ethoxyethoxy)ethanol can help prevent interfering side reactions, such as those involving aldehydes and ketone condensations. Traditionally pyridine was used as the base, but because of its unpleasant odor and requirement for special handling it has been replaced by less-volatile (and more basic) pyridine-free bases, such as imidazole and diethanolamine. These non-pyridine bases also allow for a more stable equivalence point and faster reaction rates. There are two cell types available for KF analysis: diaphragm cells and diaphragmless cells. Both cell types contain a pair of generator and detector electrodes. The generator electrodes of a diaphragm cell are separated from each other by a barrier such as an ion-permeable membrane, glass frit, or a ceramic plug. The anodic compartment of a diaphragm cell contains the generating Pt electrode and the sample of interest contained in an anolyte solution.1 The cathodic compartment consists of the generating Pt electrode immersed in a catholyte solution. The purpose of the diaphragm is to prevent cathodic reduction of the I2 titrant produced at the anode (2). Diaphragmless cells do not contain a physical barrier separating the generator electrodes. Both generating electrodes are therefore immersed in a single solution, and the reagents serve as both anolyte and catholyte. To prevent the reduction of I2 at the cathode, the surface area at the anode is maximized. Also, the current at which the titration is carried out must be great enough to produce a steady flow of gaseous H2 at the cathode such that little surface area (3) is available for I2 reduction. There are considerable advantages to both cell types. Diaphragm cells decrease the potential for I2 reduction at the cathode, as well as prevent I2 from engaging in side reactions with products such as thiosulfate and hydrogen sulfide that can form at a nonisolated cathode. Alternatively, diaphragmless cells significantly lower reagent and maintenance costs while slightly increasing the degree of uncertainty in the analysis due to cathodic reduction (3) of I2 and potential side reactions. Commercial KF titrators enable accurate water content analyses, but are often costly (>$5000) and impractical for educational purposes. Although most analytical chemistry curricula at the

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r 2010 American Chemical Society and Division of Chemical Education, Inc. pubs.acs.org/jchemeduc Vol. 87 No. 9 September 2010 10.1021/ed9000156 Published on Web 07/08/2010

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Journal of Chemical Education

987

In the Laboratory Table 1. A General Overview of Components and Costs To Build the KF Titrator components Pt-wire: 110 mm @ $0.75/cm

Figure 1. The custom-built KF titrator and experimental setup. The cell consists of a 50 mL beaker with a rubber stopper through which the electrodes are inserted and placed in the solution.

Figure 2. The generator electrode (on the left) consists of two Pt electrodes, one coiled to increase the surface area for I2 production and decrease the potential for significant reduction at the generator cathode. The detector electrode consists of two straight Pt wires.

undergraduate level teach students the principles of KF titration, few courses are able to incorporate it into a laboratory setting. The aim of the work presented in this article was to develop a low-cost, custom-built KF titration apparatus that facilitates the practical application of KF analysis within standard undergraduate analytical chemistry laboratory courses. Experimental Section Instrumental Design The titration apparatus utilizes a galvanically isolated bipotentiometric diaphragmless cell (Figure 1). The electrode compartments consist of 0.5 mm diameter Pt electrodes inside isolated glass tubes. The exposed surface of the generator anode is coiled to increase the surface area for I2 production (Figure 2). This feature enables greater titration efficiency and decreases the possibility for significant reduction at the generator cathode. 988


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Vol. 87 No. 9 September 2010

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cost/$ 82.50

glass tubing: 280 mm @ $0.010/mm

2.80

beaker, 50 mL

5.00

panel meter

7.50

LCD display

14.50

electronics

70.00

box

20.00

total

202.30

Likewise, the exposed surface area of the generator cathode is decreased to minimize I2 reduction as well as to decrease the possibility of undesired side reactions with species present in the solvent solution. The electrodes are placed in a 50 mL threenecked round-bottom flask, a 50 mL beaker, or a customdesigned round-bottomed 50 mL tube or beaker. The electronics portion of the KF device is comprised of two main components. The first is the constant current control, timing, and human interface. The second is the detection electronics which determine the reaction end point. The constant-current control, timing, and human interface is built around a PIC microprocessor and a standard 4-line 20-character LCD display. The current can be programmed for 10 mA, 20 mA, 50 mA, 100 mA, 200 mA, and 400 mA. The compliance of the constant-current generator is 35 V. A start-stop button enables simple control of titrant delivery and also controls the timing function. Either automatic or manual control of the reaction end point can be used. The detection electronics then measure the voltage between the two electrodes. The user chooses the threshold voltage level to determine the reaction end point by means of a simple potentiometer. Because the control and detection circuits must be galvanically isolated, an optocoupler is used to send the end point information from the detection electronics to the microprocessor. A three and a half digit LCD panel meter provides the display for the detection electronics. The upper limit for the detector is 2 V. Prior to titration, this voltage must be high enough to force cathodic reduction of a species present in the anolyte solution. At the equivalence point of the titration, excess I2 is present in the system and is reduced back to I- at the cathode of the detector. This results in a rapid voltage drop across the detector circuit. At the desired threshold voltage, the light-emitting diode signals the photodiode to shut down the generator and time-keeping device. The adjustable threshold voltage setting as well as an internal time-keeping device enables effective end point determination. A generalized list of components and their costs is given in Table 1. A complete list of all components and materials is provided in the supporting information. To build the instrument as shown, some machine work on the housing and face plate is required that adds a small additional cost (not reflected in Table 1). Chemicals Hydranal Coulomat AG solution (Fluka, Prussia PA), formamide, methanol, nanopure water, Rusk Sensories Calm body lotion, Aveeno Active Naturals Positively Smooth moisturizing lotion, and Hershey's Twizzlers red-colored licorice were used.

pubs.acs.org/jchemeduc

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r 2010 American Chemical Society and Division of Chemical Education, Inc.

In the Laboratory

Procedure For each experiment, the constant-current generator was set to 100 mA and the threshold, or trip level, was set to 200 mV. The Hydranal Coulomat AG solution is the KF reagent used in the experiments. The solution contains the alcohol, base, sulfur dioxide, and iodide necessary for the Bunsen reaction. The solution is hygroscopic, and the residual water within the solvent must be removed. The initial run time will vary depending on the age of the solution and length of exposure to ambient air conditions during manufacture and use. This blank run should be repeated approximately five times, with a recovery time of 2 min between each run to obtain a consistent blank reaction time. A recovery time of 2 min between injections should be used for all subsequent injections. The detector adjusts the voltage necessary to maintain a constant 10 μA current. In the absence of the I2 titrant, the voltage must be sufficient to reduce the KF reagent (i.e., methanol), which requires 500-600 mV. Once the sample is introduced and the I2 coulometrically generated, the voltage will remain approximately constant until the titrant completely consumes the water from the sample or standard. As the concentration of the excess I2 increases, the detector voltage rapidly drops. The voltage drop is not instantaneous; blank times are typically 10 to 25 s and depend on the volume of the KF reagent, electrode surface areas, generator current, and threshold or trip level. After the blank runs, a calibration curve was constructed by adding five different volumes of nanopure water to the KF reagent. Four replicate runs were performed at each volume of water. Two different brands of hand lotion, Aveeno and Rusk, were prepared using a procedure similar to that reported by Mabrouk and Castriotta (4). Approximately 0.4 g of lotion and 10.00 mL of methanol were mixed in a small weigh boat. The solution is not completely homogeneous as there are components in hand lotions that are not soluble in methanol; however, thorough mixing allowed most of the moisture from the lotion to be extracted. The lotion sample was then filtered using a 0.22 μm pore-size syringe filter, and 0.100 mL was injected into the titration cell. Experimental run times were adjusted by subtracting the average run time of five methanol blanks to exclude the presence of trace quantities of water in the methanol. Three replicate runs were performed for each lotion. The red-colored licorice strips were shaved into thin slivers using a paring knife. Then a sample, approximately 0.08 g, was dissolved in 10 mL of a 1:1 mixture of formamide and methanol. After 10 min, the sample solution was filtered using a 0.22 μm pore-size syringe filter and 0.500 mL injected into the titration cell. Experimental run times were adjusted by subtracting the average run time of three solvent (1:1 mix of formamide and methanol) blanks to exclude the presence of trace quantities of water in the solvent. Hazards Appropriate precautions need to be taken when constructing the apparatus, especially with hot soldering irons. Instructors and students need to take appropriate precautions when dealing with open live electrical circuits. Students are required to wear safety goggles during machine and solder work. All personnel involved in sample preparation should wear gloves and safety goggles as protection from the organic chemicals


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Table 2. Experimental Data for the Calibration Standards injection vol/μL

ttotal/s

tcorrected/sa

tavg/s

std dev

1

162

142

138

1.6

155

135

156

136

159

139

244

224

225

2.7

239

219

246

226

252

232

348

328

337

3.9

366

346

361

341

354

334 454

3.3

550

4.1

2

3

4

5

468

448

483

463

473

453

471

451

565

545

569

549

575

555

570

550

a

Determined by subtracting the blank time from the total run time. The average blank time was 20 s.

used in this experiment. Methanol is highly flammable and is toxic by inhalation, in contact with skin, and if swallowed. Imidazole and Hydranal Coulomat AG are harmful by inhalation, in contact with skin, and if swallowed. Sulfur dioxide is irritating to eyes and the respiratory tract. Iodine is a poison and may be fatal if swallowed; avoid breathing the vapor, and use with adequate ventilation. If chloroform is used as a cosolvent, then one should note that it is harmful by inhalation and ingestion and can be fatal. Inhalation of the vapor may cause headache, nausea, vomiting, and dizziness. Prolonged skin contact may result in dermatitis. The liquid is readily absorbed through the skin. The KF experiment requires students to use and inject samples using syringes; it is important to handle and dispose of syringe needles appropriately. Needles should be capped when not in use. Needles should be disposed of in special sharp plastic boxes. Results The results for the nanopure water standards are provided in Table 2. A calibration curve, which plots volume of nanopure water versus experimental run time, is shown in Figure 3. From the linear least-squares fit to the data, a minimum detectable volume (MDV) of 0.26 μL and the lower limit of quantitation (LOQ) of 0.85 μL were calculated.2 The linear range of response for the custom-built KF apparatus falls between 0.85 and 5.00 μL; additional experiments are necessary to ascertain if the linear range extends beyond this upper limit. The typical limit of detection (LOD) reported for commercial instruments is 0.1% w/w. We prefer to report a MDV as opposed to a LOD. We have obtained accurate and reproducible water concentrations below 0.1% w/w in olive oil by simply using sample injection volumes


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989

In the Laboratory

contributes to the large standard deviation in the average value. The uncertainty may also be a function of incomplete dissolution and sample size that may be overcome by adding cosolvents such as chloroform or 1-octanol to enhance sample solubility, by increasing the quantity of sample, or by increasing the injection volume. Additional studies were performed on the determination of water content in olive oil formulations. All obtained moisture content values fell within the range of 0.06-0.09% w/w. These values compare favorably with reported (5) literature values; however, because these values fall below the typical 0.1% w/w LOD of commercial instruments, they have not been reported in this article. Figure 3. A calibration curve expressing the relationship between volume of nanopure water and the coulometric titration time. The size of the data point markers is larger than the associated standard errors in the mean average values. The equation of linear least-squares fit is y = 105.2x þ 25.2, where r2 = 0.997. Table 3. Experimental Data for the Hand Lotions and Licorice Sample triala

ttotal/s

tcorrected/sb

H2O vol/μLc

1

382

341

3.00

76.0

2

373

332

2.92

73.9

3

386

345

3.04

77.0

H2O (%)

Aveeno/(0.3950 g)

Rusk/(0.3925 g) 1

352

311

2.72

69.3

2

348

307

2.68

68.3

3

360

319

2.79

71.1

1

489

78

0.502

12.2

2

523

105

0.759

18.5

3

493

73

0.454

11.1

Licorice/(0.0820 g)

a

All runs performed using a constant current of 100 mA. Injection volumes were 0.100 mL for samples and 0.500 mL for the red licorice. b The corrected time by was determined by subtracting the blank time from the total titration time. The blank time for the methanol extraction solvent was 41 s. The blank time for the formamide-methanol extraction solvent was 411 s. c Calculated from linear least-squares line in Figure 3.

that ensure the water content of the sample is above the MDV (see the student laboratory exercise in the supporting information). The results for the two hand lotion samples and the red licorice are given in Table 3. Using the calibration curve in Figure 3, the moisture contents in Aveeno and Rusk were calculated to be 75.6% ( 1.6% w/w and 69.6% ( 1.4% w/w, respectively.3 Water is listed as the largest ingredient in the hand lotions, but the manufacturers do not list concentrations. Therefore, it is not possible to evaluate the accuracy of the results. However, our calculated moisture contents compare favorably with those reported (4) for other hand lotions. The moisture content of red-colored licorice was calculated to be 13.9% ( 4.0% w/w, which is similar to the manufacturer's stated value of 16.5% w/w. The calculated volume of water in each of the red licorice trials falls below our stated LOQ, which 990


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Discussion Although commercial instruments are becoming increasingly more sophisticated, user interfaces are becoming simpler and more streamlined, leading many of us to believe the primary educational, or professional, objective is to know how to “insert” a sample and learn the instrument software. Instruments have become such “black boxes” that often students never appreciate or consider the practical limitations that influence instrument design and data acquisition and, therefore, never question the significance or quality of the data. The custom-built KF apparatus came about from an initiative within the instrumental analysis courses to require students to build a functional instrument as part of a semester-long class research project. The students learn that it is possible, with just a basic understanding of electronics and instrumentdesign theory, to obtain high-quality data from an instrument built with their own hands within a few hours. The only substantial differences between a commercial instrument and their custom-built instrument are the software and specialty components (“bells and whistles”) and sensitivity; the basic design and methods for probing chemical systems are identical. Instruments such as the KF apparatus, built by instrumentalanalysis students, are then used in lower-level general and analytical chemistry courses by their younger peers. As their younger peers advance through the curriculum, their knowledge of the instrumentation and their ability to use the instrumentation in more sophisticated ways increases until, eventually, they are involved in the construction of the instrumentation to be used by their younger peers. The use of student-built instrumentation promotes and enhances student competency in the sciences and engineering; it is a natural conduit for cocurricular experiences and fosters student ownership of their program's curriculum. The additional benefit of such instruments is that the features of low price, low maintenance, and low operating costs allow for the deployment of multiple units. The KF instrument described here has been used in a general chemistry and a quantitative analysis course. The instrument is easy to use, and all of the experiments reported here can be comfortably performed in one 4 h laboratory session. The most significant challenge has been students' inability to properly and reproducibly use a micropipettor. This experimental aspect was also highlighted as the source of frustration in the student response to the KF experiment. Students on the whole were satisfied with the instrument and commented positively on the use of real-life samples and methods. We have shown it is possible to introduce KF titrations into the chemistry instructional laboratory at a fraction of the cost of


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In the Laboratory

purchasing a commercial instrument. Currently, we use nine of the instruments in our quantitative analysis lab, all built for a total cost of less than $2000.

2. The MDV is defined as 3sy/m and the LOQ as 10sy/m, where sy is the residual standard deviation and m is the slope of the calibration plot. 3. Subscripted numbers represent the first insignificant digit in the value.

Acknowledgment We would like to thank the Penn State Schreyer Institute for Teaching Excellence, and the SEECoSProgram Summer Experience in the Eberly College of Science) for financial support. The SeeCoS program is supported by the Upward Bound Math and Science Center (UBMS) at Penn State and a USDoE TRIO grant. The authors are thankful to the entire staff of the Electronics Research Instrumentation Facility and Russ Rogers of the glass blowing shop at Penn State. The entire project is based on student effort and feedback, and we are grateful for the time and patience that each of our student participants has shown as we have developed and tested the instruments. The authors also thank the editors and reviewers for their assistance. Notes 1. The anodic compartment also contains the detector electrodes.


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Literature Cited 1. Harris, D. C. Quantitative Chemical Analysis, 7th ed.; W. H. Freeman and Company: New York, 2007. 2. Lanz, M.; De Caro, C. A.; Ruegg, K.; De Agostini, A. J. Food Chem. 2006, 96, 431–435. 3. Cedergren, A.; Jonsson, S. Anal. Chem. 1997, 69, 3100–3108. 4. Mabrouk, P. A.; Castriotta, K. J. Chem. Educ. 2001, 78, 1385–1386. 5. Jones, J. C.; Mc Brown, J.; Cargill, E. M.; McElligot, J. M. H.; Melvin, S.; Oseruvwoja, I. S. Int. J. Oil Gas Coal Technol. 2009, 2, 83–88.

Supporting Information Available Student handouts for the two experiments: determination of water in olive oil and in hand lotion and licorice; instructor notes containing further details on the KF instrument and the lab experiments; complete list of all components and materials. This material is available via the Internet at http://pubs.acs.org.


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