Article pubs.acs.org/jchemeduc
An Alternative Approach for Preparing and Standardizing Some Common Aqueous Reagents Used in an Undergraduate Laboratory Samuel Melaku and Rajeev B. Dabke* Department of Chemistry, Columbus State University, Columbus, Georgia 31907, United States S Supporting Information *
ABSTRACT: A guide for instructors and laboratory assistants to prepare some common aqueous reagents used in an undergraduate laboratory is presented. Dilute reagents consisting of H+(aq), I3−(aq), Ce4+(aq), and Ag+(aq) were prepared by electrolytic oxidation of respective precursors. Electrolysis was promptly stopped when potassium hydrogen phthalate (KHP) was completely neutralized by OH−(aq) resulting from an electrolytic reduction of water in the cathode compartment. The neutralization of KHP was monitored by a visual color change of an indicator. The amount of reagent produced in the anode compartment was directly determined from the mass of KHP added to the cathode compartment. The concentrations of reagents determined from the mass of KHP and from an independent volumetric titration were in agreement. The electrolysis cell, powered by an 18 V dc power supply, eliminated the requirement of a coulometer. Setup of an electrolysis cell and experimental details for preparing reagents are presented. KEYWORDS: General Public, First-Year Undergraduate/General, Analytical Chemistry, Laboratory Instruction, Hands-On Learning/Manipulatives, Acids/Bases, Electrolytic/Galvanic Cells/Potentials
A
the quantitative analysis of moisture.5 However, using a constant current source or monitoring the charge passing through the cell is critically important in a coulometric titration.6−9 This rigorous requirement may restrict coulometric methods, particularly for preparing and standardizing reagents in an undergraduate laboratory. We present an alternative approach for instructors and laboratory assistants for preparing and standardizing some common reagents used in an undergraduate laboratory. The electrolysis method presented here offers all the advantages of the coulometric method, and additionally, it simplifies the circuitry by eliminating the use of a coulometer. Reagents were prepared by an electrolytic oxidation of their precursors. Concurrently OH−(aq) ions were produced as an electrolytic reduction product of water. A known mass of potassium hydrogen phthalate (KHP) was added to the cathode compartment. Electrolytically produced OH−(aq) neutralized the KHP, and the end point of this neutralization was determined from a visual color change of phenolphthalein indicator from colorless to pink. Electrolysis was promptly stopped, and the amount of OH−(aq) was determined from the mass of KHP. The amount of reagent produced at the anode was then determined from the amount of OH−(aq) produced at the cathode. This paper presents the preparation and standardization of dilute solutions of H+(aq), I3−(aq), Ce4+(aq), and Ag+(aq) using KHP as a primary standard.
variety of reagents are prepared for volumetric analysis in an undergraduate laboratory. If a reagent is available in the pure form, its molarity can be directly determined from its mass. However, if a reagent is not available in the pure form or if its mass alters due to its hygroscopic nature, the reagent must be standardized prior to use in a titration. Often standardization of reagents requires ordering, storing, and preparing additional reagents. However, several practical difficulties are associated with the standardizations and primary standard substances: (a) An ideal primary standard substance is difficult to obtain, and a compromise between ideal requirements is usually necessary;1,2 (b) hydrated salts cannot be efficiently dried, but are used for standardization purpose; (c) sodium oxalate is used as a primary standard in a redox reaction, although reactions involving oxalates need elevated temperatures; (d) though arsenic(III) oxide is a primary standard, titration of Ce4+(aq) with arsenic(III) oxide needs a catalytic support of osmium tetroxide; (e) iodine solution is standardized with standard sodium thiosulfate, yet weighing sodium thiosulfate involves some uncertainty due to its efflorescent nature necessitating standardization with potassium iodate; and (f) determination of silver nitrate using sodium chloride (Mohr titration) necessitates indicator blank correction. These practical considerations make standardization of various reagents less straightforward. Reagent standardization is not required in a coulometric titration. In a coulometric titration the amount of reagent produced is directly determined from the charge passing through an electrolysis cell.3 Coulometric titration offers practical advantage over volumetric titration,4 e.g., coulometric Karl Fischer titrator is a sensitive and precise analytical tool for © 2014 American Chemical Society and Division of Chemical Education, Inc.
Published: July 28, 2014 1451
dx.doi.org/10.1021/ed5001763 | J. Chem. Educ. 2014, 91, 1451−1454
Journal of Chemical Education
■
Article
MATERIALS AND ELECTROLYTIC CELL All solutions were freshly prepared in deionized water. Chemicals and electrode materials were purchased from Pine Instrument Company, Fisher Scientific, Alfa Aesar, or Aldrich. KNO3(aq) was prepared in preboiled, then cooled deionized water, and stored in a stoppered flask to minimize the presence of dissolved CO2. Two 180 mL tall beakers were used as cathode and anode compartments. The schematic of the cell is shown in Figure 1, and the contents of the cell are summarized
Reduction:
2H 2O(l) + 2e− → 2OH−(aq) + H 2(g) (1) +
Electrolytic Production of H (aq)
A desired mass of KHP was added to the cathode compartment. KHP is electrolytically nonreducible.10 Electrolytically produced OH−(aq) neutralized KHP(aq) in an acid−base reaction (eq 2).
Simultaneously, in the anode compartment, electrolysis of water produced H+(aq) (eq 3). Oxidation:
H 2O(l) → 2H+(aq) + 1/2O2 (g) + 2e− (3)
Since the same current powered the electrolysis in both compartments, the mass of KHP neutralized in the cathode compartment facilitated the determination of the amount of H+(aq) concurrently produced in the anode compartment. Fluctuations in the current passing through the cell were not relevant, and the circuit eliminates the use of a constant current source. To assess the applicability of the method, the quantity of electrolytically produced H +(aq) was independently determined by a volumetric titration against aqueous sodium carbonate using methyl orange indicator.1 The method presented in this paper was extended to the electrolytic preparation of I3−(aq), Ce4+(aq), and Ag+(aq). These reagents were prepared by an oxidation of I−(aq), Ce3+(aq), and Ag(s) reagent precursors, respectively. The neutralization of KHP was used as a common counter reaction to determine the amounts of reagents produced at the anode. Relevant electrolytic oxidations in the anode compartment are described below.
Figure 1. Schematic diagram of the electrolysis cell: platinum cathode and anode (a and b, respectively) and agar-KNO3 salt bridge (c). Magnetic stir bars are also shown in the compartments.
in Table 1. Seventy milliliter portions of 0.2 M KNO3(aq) were placed in each compartment. The desired mass of dry KHP and 2 drops of 0.2% phenolphthalein indicator were added to the cathode compartment, and the contents of both compartments were magnetically stirred. Silver rod used for producing Ag+(aq) was polished with a sand paper. Electrolytically produced Ag+(aq) was protected from light during electrolysis and volumetric analysis. Detailed information on the preparation and standardization is included in the Supporting Information.
Electrolytic Production of I3−(aq)
I3−(aq) was produced by an electrolytic oxidation of KI(aq) (eqs 4a and 4b).11 An excess amount of KI(aq) was placed in the anode compartment. 2I−(aq) → I 2(aq) + 2e−
■
(4a)
and
EXPERIMENTAL OVERVIEW A preparative electrolysis cell (Figure 1) was powered by an 18 V dc power source. Coiled platinum wires were used as cathode and anode and the compartments, were connected by a Ushaped glass tube containing an agar−KNO3 mixture. In the cathode compartment, water (in the presence of KNO3(aq) as an electrolyte) electrolyzes to produce OH−(aq) (eq 1).
I 2(aq) + I−(aq) → I3−(aq)
(4b)
Electrolytic Production of Ce4+(aq)
Ce4+(aq) was produced by an electrolytic oxidation of Ce3+(aq) (eq 5).3 An excess amount of Ce3+(aq) in sulfuric acid was placed in the anode compartment.
Table 1. Summary of Reagents Produced and Cell Contents Reagent Produced H+(aq) I3−(aq) Ce4+(aq) Ag+(aq)
Anode Platinum wire (length 29 cm, diameter 0.05 cm, coiled) Platinum wire (length 29 cm, diameter 0.05 cm, coiled) Platinum wire (length 29 cm, diameter 0.05 cm, coiled) Silver rod (approximately 5.5 cm length dipped in electrolyte, diameter 0.32 cm)
Anode Compartment Contents 70 mL of 0.2 M KNO3(aq) 70 mL containing 0.2 M acetate/ acetic acid buffer and 0.2 M KI(aq) 0.15 M Ce3+(aq) in 3 M H2SO4(aq) 70 mL of 0.2 M KNO3(aq)
1452
Cathode
Cathode Compartment Contents
Platinum wire (length 18 cm, 0.07 cm, coiled) Platinum wire (length 18 cm, 0.07 cm, coiled) Platinum wire (length 18 cm, 0.07 cm, coiled) Platinum wire (length 18 cm, 0.07 cm, coiled)
Desired mass of KHP, 70 mL of 0.2 M KNO3(aq), 2 drops of 0.2% phenolphthalein Desired mass of KHP, 70 mL of 0.2 M KNO3(aq), 2 drops of 0.2% phenolphthalein Desired mass of KHP, 70 mL of 0.2 M KNO3(aq), 2 drops of 0.2% phenolphthalein Desired mass of KHP, 70 mL of 0.2 M KNO3(aq), 2 drops of 0.2% phenolphthalein
dx.doi.org/10.1021/ed5001763 | J. Chem. Educ. 2014, 91, 1451−1454
Journal of Chemical Education
Article
Table 2. Results of the Electrolytic Preparation of Reagentsa Reagent Produced in the Anode Compartment
a
Stoichiometry of KHP and the reagent produced, Respectively (see Eqs 1−5)
Mass of KHP added to the cathode Compartment (g)
Reagent Concentration Estimated from the Mass of KHP(aq) (mM)
H+(aq)
1:1
I3−(aq)
2:1
Ce4+(aq)
1:1
Ag+(aq)
1:1
0.1005 0.1503 0.2015 0.1001 0.1502 0.2005 0.1007 0.1500 0.2003 0.1000 0.1500 0.2004
4.92 7.36 9.87 2.45 3.68 4.91 4.93 7.34 9.81 4.90 7.34 9.81
Reagent Concentration Determined by Volumetric Titration (in mM) (Standard Deviation, n = 3) 4.98 7.15 9.25 2.47 3.59 4.81 4.76 7.17 9.81 4.83 7.18 9.57
(±2.89 (±1.33 (±8.08 (±4.93 (±1.39 (±2.00 (±2.08 (0) (0) (±1.04 (±8.08 (±5.77
× × × × × × ×
10−2) 10−1) 10−2) 10−2) 10−1) 10−4) 10−2)
× 10−1) × 10−2) × 10−2)
An average of three trials of volumetric titration is reported.
Ce3 +(aq) → Ce 4 +(aq) + e−
required to meet the mole relation presented in eq 2 and the proportionately larger amounts of H+(aq) were produced in the anode compartment. The amount of H+(aq) produced at the anode was estimated from the mass of KHP added to the cathode compartment. The electrolytically prepared H+(aq) was volumetrically titrated against Na2CO3(aq). The molarity of H+(aq) determined by volumetric titration agreed with the molarity estimated from the mass of KHP. The agreement between molarities determined from the mass of KHP and volumetric titrations was consistent for electrolytically prepared I3−(aq), Ce4+(aq), and Ag+(aq). The direct current flowing through the electrolysis cell was in a range of 50−80 mA. Considering this current range and approximate geometric areas of the platinum electrodes, the anode and the cathode current densities were 11−17 and 12−20 mA/cm2, respectively. This current density range confirms the optimal electrolysis efficiency of the oxidation of Ce3+(aq).13 Electrolytic oxidation of water leading to oxygen evolution is a possible side reaction in the anode compartment. However, in the presence of a large excess of Ce3+(aq), thermodynamically favorable electrolytic oxidation of Ce3+(aq) to Ce4+(aq) occurs as a preferred reaction.3 Similarly the oxidation of I−(aq) to I3−(aq) occurs as a preferred electrolytic reaction in the presence of excess I−(aq) in the electrolyte. In order to minimize the air oxidation of potassium iodide,14 solid potassium iodide was added to the half-cell prior to the electrolysis. In an independent coulometric experiment, the charge passing through the cell was monitored and the amount of electrolytically produced OH−(aq) was separately determined by a volumetric titration. The amount of OH−(aq) determined by volumetric titration agreed with the quantity of charge passing through the cell, confirming an optimal cathode current efficiency and negligible interference of electrolytic side reactions competing with the reaction stated in eq 1. A blank trial was performed to ensure the negligible interference of impurities in the electrolyte contributing to the neutralization of KHP. A blank trial was run without adding any KHP to the cathode compartment. As the electrolysis began, indicator showed an immediate color change. A similar blank trial was performed to ensure the negligible interference of active ions present in the anode compartment electrolyte. In this trial, an electrolyte (without the reagent precursors of I3−(aq) and Ce4+(aq)) was placed in the anode compartment. Electrolysis was promptly stopped when electrolytically produced OH−(aq)
(5)
Electrolytic Production of Ag+(aq)
Ag+(aq) was produced by an electrolytic oxidation of Ag in KNO3(aq) solution. A silver wire was used as an anode (eq 6).12 Ag(s) → Ag +(aq) + e−
(6)
Volumetric Titration Method
Each reagent prepared by the proposed electrolytic method was independently analyzed by a volumetric titration method. A comparison of the mass of KHP added to the cathode compartment, estimated concentration of the reagent produced in the anode compartment based on the mass of KHP, and the concentration of reagent determined by volumetric titrations are presented in Table 2. Consistency of the concentrations of a variety of reagents determined from the mass of KHP and from the volumetric titrations illustrates the efficacy of the electrolytic approach presented in the paper.
■
HAZARDS Work presented is intended for instructors and laboratory assistants as an instructional guide for preparing reagents. Approved safety goggles and proper laboratory attire must be used in a laboratory. Laboratory chemicals including potassium nitrate, potassium iodide, potassium hydrogen phthalate, potassium acetate, iron(II) sulfate heptahydrate, and cerium(III) sulfate can cause eye and skin irritation. Potassium chromate and potassium dichromate are toxic. Sodium oxalate is corrosive to body tissues. These chemicals must be handled with proper safety. Sulfuric acid can cause severe burns. Reagents involving acids must be prepared in a fume hood. H2(g) is produced at the cathode at an approximate rate of 0.38−0.61 mL per minute in a 50−80 mA current range. Hydrogen gas is flammable, and a fume hood or a cabinet with proper exhaust system must be used for electrolysis.
■
RESULTS AND DISCUSSION In view of ensuring the constancy of the electrolysis, each reagent was prepared in three concentrations. These concentrations were achieved by adding different masses of KHP to the cathode compartment (Table 2). As the mass of KHP was increased from 0.1 to 0.2 g, larger amounts of OH−(aq) were 1453
dx.doi.org/10.1021/ed5001763 | J. Chem. Educ. 2014, 91, 1451−1454
Journal of Chemical Education
■
neutralized the desired mass of KHP in the cathode compartment. The titration of the anode compartment electrolyte indicated the absence of the active reagents in the electrolyte. In a 60−65 mA current range, electrolytic production of 9.8 mM H+(aq) took about 25 min. Other power sources capable of delivering the direct current for the electrolysis can be used. Electrodes with smaller surface areas and a power source capable of delivering smaller amounts of direct current compromise the electrolysis time. Salient features of the electrolytic preparation and standardization method presented in this paper are as follows: • preparation of dilute acid solutions without purchasing, handling, and storing of concentrated acids; • preparation and standardization of a reagent in a single step; • use of only one primary standard substance for preparing and standardizing a variety or reagents; • the advantages of coulometric methods and additionally simplification of the electrolysis circuitry by eliminating the use of a constant current source or monitoring of an electrical charge. Undesirable features of the electrolytic preparation and standardization method presented in this paper are as follows: • use of expensive precious metal electrodes (however, electrodes are reusable); • suitability for the preparation and standardization of relatively small volumes of reagents; • reagent containing other electrolyte ions in addition to the active ions (however, the presence of electrolyte ions does not interfere with the active ions, e.g., acetate/acetic acid buffer in the electrolytically prepared I3−(aq) and excess Ce 3+ (aq) in the electrolytically prepared Ce4+(aq)).
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■ ■
ACKNOWLEDGMENTS We thank James O. Schreck for helpful comments and suggestions. REFERENCES
(1) Jeffery, G. H.; Bassett, J.; Mendham, J.; Denney, R. C. Vogel’s Textbook of Quantitative Chemical Analysis, 5th ed.; Longman: Essex, 1994; pp 257−309. (2) Harvey, D. Modern Analytical Chemistry; McGraw Hill: Boston, MA, 2000; pp 106−108. (3) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, 2nd ed.; Wiley: New York, 2001; pp 430−435. (4) Sawyer, D. T.; Sobkowiak, A.; Roberts, J. L. Electrochemistry for Chemists, 2nd ed.; Wiley: New York, 1995; pp 139−160. (5) Mabrouk, P. A.; Castriotta, K. Moisture Analysis in Lotion by Karl Fischer Coulometry. An Experiment for Introductory Analytical Chemistry. J. Chem. Educ. 2001, 78 (10), 1385−1386. (6) Swim, J.; Earps, E.; Reed, L. M.; Paul, D. Constant-Current Coulometric Titration of Hydrochloric Acid. J. Chem. Educ. 1996, 73 (7), 679−683. (7) Lowinsohn, D.; Bertotti, M. Coulometric Titrations in Wine Samples: Studies on the Determination of S(IV) and the Formation of Adducts. J. Chem. Educ. 2002, 79, 103−105. (8) Dabke, R. B.; Gebeyehu, Z.; Petermann, M.; Johnson, N., Jr.; Patel, K. Analysis of Household Products: Coulometric Titration Experiment in the Undergraduate Laboratory. Chem. Educ. 2011, 16, 160−163. (9) Dabke, R. B.; Gebeyehu, Z.; Thor, R. Coulometric Analysis Experiment for the Undergraduate Chemistry Laboratory. J. Chem. Educ. 2011, 88, 1707−1710. (10) Recknagel, S.; Breitenbach, M.; Pautz, J.; Luck, D. Purity of potassium hydrogen phthalate, determination with precision coulometric and volumetric titration−A comparison. Anal. Chim. Acta 2007, 599, 256−263. (11) Jeffery, G. H.; Bassett, J.; Mendham, J.; Denney, R. C. Vogel’s Textbook of Quantitative Chemical Analysis, 5th ed.; Longman: Essex, 1994; p 541. (12) Milner, G. W. C.; Phillips, G. Coulometry in analytical Chemistry; Pergamon Press: Long Island City, NY, 1967; p 107. (13) Lingane, J. J. Electroanalytical Chemistry, 2nd ed.; Interscience: New York, 1958; pp 493−495. (14) Jeffery, G. H.; Bassett, J.; Mendham, J.; Denney, R. C. Vogel’s Textbook of Quantitative Chemical Analysis, 5th ed.; Longman: Essex, 1994; p 386.
■
CONCLUSION Instructions for preparing and standardizing some common reagents used in an undergraduate laboratory are presented. The quantity of reagents estimated from the mass of KHP agrees with quantities determined from the volumetric results, over a range of concentration. The approach facilitates the standardization of four different reagents with a single primary standard. The approach can be extended for preparing other reagents resulting from the electrolytic oxidation (e.g., Mn(III)(aq), Cl2(aq), and Br2(aq)). Though the technique presented here has practical constraints, it is useful in relatively small laboratories where ordering and storing of reagent chemicals and concentrated acids have limitations. Large size platinum electrodes used in this study are expensive. However, longer electrolysis times or preparing relatively smaller volumes of reagents can compensate for small size electrodes. The pedagogy of electrolytic and redox chemical reactions presented in the paper can be incorporated for teaching electrochemical processes and stoichiometry.
■
Article
ASSOCIATED CONTENT
* Supporting Information S
A complete description of electrolysis cell and volumetric titrations. This material is available via the Internet at http:// pubs.acs.org. 1454
dx.doi.org/10.1021/ed5001763 | J. Chem. Educ. 2014, 91, 1451−1454
