Direct Electrometric Methods - American Chemical Society

Procedure pubs.acs.org/doi/book/10.1021/acsreagents

Direct Electrometric Methods Part 2, Analytical Procedures and General Directions eISBN: 9780841230460 Tom Tyner Chair, ACS Committee on Analytical Reagents James Francis Secretary, ACS Committee on Analytical Reagents

Downloaded by CORNELL UNIV on May 9, 2017 | http://pubs.acs.org Publication Date (Web): February 28, 2017 | doi: 10.1021/acsreagents.2005

ABSTRACT The ACS Committee on Analytical Reagents defines direct electometric methods including pH Potentiometry and Polarographic Analysis. Under Polarographic Analysis, various techniques are defined including direct current (dc), differential pulse (dp), square wave (sqw), and stripping voltammetry.

PH POTENTIOMETRY pH Range This pH requirement is intended to limit the amount of free acid or alkali that is allowed in a reagent. Sodium chloride, for example, has a requirement that the pH of a 5% solution should be from 5.0 to 9.0 at 25.0 °C. This requirement limits the free hydrochloric acid or sodium hydroxide to about 0.001%. The effect of excess acid or base on the pH of a 5% solution of salts of strong acids and bases is given in Table 2-6. Table 2-6. Effect of Excess Acid or Base on the pH of 5% Solution of Salts of Strong Acids and Bases

Excess Acid/Base

0.01%, pH

0.001%, pH

0.0001%, pH

HCl

3.9

4.9

5.9

H2SO4

4.0

5.0

6.0

HNO3

4.1

5.1

6.1

HBr

4.2

5.2

6.2

HClO4

4.3

5.3

6.3

KOH

9.9

8.9

7.9

NaOH

10.1

9.1

8.1

The pH of the solution is determined by means of any suitable pH meter equipped with a glass electrode (indicating electrode) and a reference half-cell, usually a silver/silver chloride electrode (Ag/AgCl). The combination electrode is best for general purpose applications. The meter and electrodes are standardized at pH 4.00 at 25.0 °C with 0.05 M NIST potassium hydrogen phthalate and at pH 9.18 at 25.0 °C with 0.01 M NIST sodium borate decahydrate. Because the pH values are reported to only 0.1 pH unit, the meter and electrodes are considered to be in satisfactory working order if they show an error of less than 0.05 pH unit in the pH range from 4.00 to 9.18 (use pH from NIST-certified values). For more accurate measurement of a test solution, the meter and electrodes should be standardized against a buffer standard solution whose pH is close to the pH of the test solution. For reference purposes, Table 2-7 presents a list of acid–base indicators with their visual transition intervals.

© 2017 American Chemical Society

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ACS Reagent Chemicals ACS Reagent Chemicals; American Chemical Society: Washington, DC, 2017.

DOI:10.1021/acsreagents.2005 ACS Reagent Chemicals, Part 2

ACS Reagent Chemicals

Procedure

pubs.acs.org/doi/book/10.1021/acsreagents


Table 2-7. Acid-Base Indicators, by Visual Transition Interval

Visual Transition Interval (pH)

Indicator

Color Change

0.1–2.0

Malachite green hydrochloride

Yellow to blue-green

0.1–2.3

Methyl green

Yellow to blue-green

0.2–1.0

Picric acid

Colorless to yellow

0.2–1.8

Cresol red

Red to yellow

0.8–2.6

Crystal violet

Green to violet

1.0–3.1

Basic fuchsin

Purple to red

1.2–2.8

m-Cresol purple

Red to yellow

1.2–2.8

Thymol blue

Red to yellow

1.2–2.8

Thymol blue, sodium salt

Red to yellow

1.4–2.6

Orange IV

Red to yellow

1.5–3.2

Methyl violet 2B

Blue to violet

3.0–4.6

Bromphenol blue

Yellow to blue

3.0–4.6

Bromphenol blue, sodium salt

Yellow to blue

3.0–5.2

Congo red

Blue to red

3.2–4.4

Methyl orange, sodium salt

Red to yellow

4.0–5.4

Bromocresol green

Yellow to blue

4.0–5.4

Bromocresol green, sodium salt

Yellow to blue

4.0–6.0

Alizarin red S

Yellow to blue

4.2–6.2

Methyl red hydrochloride

Pink to yellow

4.2–6.2

Methyl red, sodium salt

Pink to yellow

4.8–6.4

Chlorophenol red

Yellow to red

5.2–6.8

Bromocresol purple, sodium salt

Yellow to purple

5.8–7.2

Alizarin

Yellow to red

6.0–7.0

Nitrazine yellow

Yellow to blue

6.0–7.5

Bromthymol blue

Yellow to blue

6.0–7.6

Bromthymol blue, sodium salt

Yellow to blue

6.0–8.0

Rosolic acid

Yellow to red

6.6–8.6

m-Nitrophenol

Colorless to yellow

6.8–8.0

Neutral red

Red to yellow

6.8–8.2

Phenol red

Yellow to red

6.8–8.2

Phenol red, sodium salt

Yellow to red

7.0–8.8

Cresol red

Yellow to red

7.4–9.0

m-Cresol purple

Yellow to purple

8.0–9.2

Thymol blue

Yellow to blue

8.0–9.2

Thymol blue, sodium salt

Yellow to blue

8.0–10.0

Phenolphthalein

Colorless to red

8.6–10.0

Thymolphthalein

Colorless to blue

10.2–13.0

Nile blue A (sulfate)

Blue to red

10.5–12.5

Malachite green hydrochloride

Blue to colorless

11.0–13.0

Alizarin

Red to purple


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Procedure


Table 2-7. Acid-Base Indicators, by Visual Transition Interval (continued)

Visual Transition Interval (pH)

Indicator

Color Change

11.0–13.0

Azo violet

Yellow to violet

11.4–13.0

5,5′-Indigodisulfonic acid, disodium salt

Blue to yellow

12.0–14.0

Acid fuchsin

Red to colorless

12.0–14.0

Clayton yellow

Yellow to red


Procedure for pH of a 5% Solution at 25.0 °C Dissolve 5.0 g of the sample in 100 mL of carbon dioxide-free water while protecting the solution from absorption of carbon dioxide from the atmosphere. The pH measurement is made as previously described. For reference purposes, Table 2-8 reports the pH of a 5% solution of the pure reagent chemical. This value was determined experimentally for each salt by dissolving 10.0 g of the reagent in approximately 200 mL of water, and then making the solution slightly acid, and titrating with standard alkali. Another similar solution was prepared, made slightly alkaline, and titrated with standard acid. Graphs were constructed for each titration by plotting pH vs milliliters of titrating solution. The average of the two end points so determined is reported as the pH of a 5% solution containing no free acid or alkali. Table 2-8. pH of a 5% Solution of Reagents at 25.0 °C

Salt

pH

Ammonium acetate

7.0

Ammonium bromide

4.9

Ammonium chloride

4.7

Ammonium nitrate

4.8

Ammonium phosphate, monobasic

4.1

Ammonium sulfate

5.2

Ammonium thiocyanate

4.9

Barium chloride, dihydrate

6.9

Barium nitrate

6.9

Calcium chloride, dihydrate

6.6

Calcium nitrate, tetrahydrate

6.6

Lithium perchlorate

7.0

Magnesium nitrate, hexahydrate

6.6

Magnesium sulfate, heptahydrate

6.8

Manganese chloride, tetrahydrate

5.4

Potassium acetate

9.0

Potassium bromate

7.0

Potassium bromide

7.0

Potassium chloride

7.0

Potassium chromate

9.3

Potassium iodate

7.0

Potassium iodide

7.0

Potassium nitrate

7.0

Potassium phosphate, monobasic

4.2

Potassium sodium tartrate, tetrahydrate

8.4


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Procedure


Table 2-8. pH of a 5% Solution of Reagents at 25.0 °C (continued)


Salt

pH

Potassium sulfate

7.3

Potassium thiocyanate

7.0

Sodium acetate, anhydrous

8.9

Sodium acetate trihydrate

8.9

Sodium bromide

7.0

Sodium chloride

7.0

Sodium molybdate dihydrate

8.1

Sodium nitrate

7.0

Sodium phosphate, dibasic, heptahydrate

9.0

Sodium phosphate, monobasic, monohydrate

4.2

Sodium pyrophosphate, decahydrate

10.4

Sodium sulfate

7.2

Sodium tartrate dihydrate

8.4

Sodium thiosulfate, pentahydrate

7.9

Zinc sulfate heptahydrate

5.4

pH of Other Concentrations Directions for the preparation of solutions at concentrations of other than 5% are given in individual monographs. The pH measurements are made as previously described.

pH of Buffer Standard Solutions For reagents that are suitable for use as pH standards, concentrations are stated in the individual monographs. They are expressed in the same units (molar) as the NIST reference materials to which they are compared. The meter and electrodes must be capable of a precision of 0.005 pH unit at the specified pH for satisfactory results.

POLAROGRAPHIC ANALYSIS Polarography offers a rapid and sensitive method of determining a number of ions or compounds that are electrolytically reducible or oxidizable, usually at a dropping mercury electrode, in a solution with appropriate electrical conductivity. Commercial instruments are designed to perform various current measuring techniques, such as direct current (dc), differential pulse (dp), or square wave (sqw). Stripping voltammetry at the hanging mercury drop electrode, which permits higher sensitivity and greater resolution, is also available in modern instrumentation. Note: The use of polarography for analysis of carbonyls and ammonium in several acids has been eliminated in the Eleventh Edition.

Basics Electroanalytical techniques that are measuring current/voltage curves are summarized under the term “voltammetry”. The curves are registered at a working electrode that is immersed in the solution to be analyzed. The potential of the working electrode is uniformly altered from a start to an end potential following a predefined scheme (current measuring technique), whereas the current flowing between the working electrode and the counter/auxiliary electrode is recorded.


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Procedure


The working electrode is traditionally a dropping mercury electrode. In this case, the method is called polarography according to the recommendations of the International Union for Pure and Applied Chemistry (IUPAC). When a stationary electrode, for example, a hanging mercury drop electrode or rotating disk electrode, is used, the method is called voltammetry. If no substance is present in the measuring solution that could be reduced or oxidized in the scanned potential range, the recorded current remains low (residual current). If reducible or oxidizable substances are present, the current increases as soon as reduction or oxidation of the electroactive species takes place on the surface of the working electrode. The potential at which reduction or oxidation occurs can be used for qualitative information. It is specific for a certain substance and can be used to identify the substance. The current that is measured is proportional to the concentration of the substance and is used to determine the concentration.

Cell Setup: Electrode Types


Modern polarographic setups contain three electrodes: working (indicating) electrode, reference electrode, and counter or auxiliary electrode. The analyte is reduced or oxidized on the surface of the working electrode. It is usually a mercury electrode that can be operated in different modes: dropping mercury electrode (DME) or hanging mercury drop electrode (HMDE). Built-in electromagnetic or pneumatic valves permit controlling the mercury flow and allow choosing the electrode type from the controlling software. Alternatively, solid-state electrodes from precious metals or carbon can be used as working electrodes. Setups with stationary electrodes and separate stirrers as well as rotating disk electrodes are also available. The current measured in the experiment flows between the working electrode and the counter electrode, which is a stationary electrode made of inert material, usually platinum or carbon. The potential applied to the working electrode is applied with respect to the reference electrode, which is an electrode of the second kind, typically a Ag/AgCl half-cell with a potassium chloride reference electrolyte. In older setups, Hg/Hg2Cl2 reference electrodes, called saturated calomel electrodes (SCE), were also used. The use of a current-free reference electrode in combination with an electronic circuit, called a potentiostat, compensates for the potential drop that results when a current flows between the working and the counter electrode.

Measuring Solution: Supporting Electrolyte The sample to be analyzed is usually diluted with a supporting electrolyte that may contain a buffer to adjust the pH and provide defined ionic activity in the measuring solution. It may also contain complexing agents to improve resolution and minimize interferences.

Current Measuring Techniques Direct Current (dc) This is the classic current measuring technique (Figure 2-2). The potential scan is a linear ramp from the start to the end potential in classic instrumentation. Modern digitally controlled instruments use a staircase ramp in which the potential steps are synchronized with the mechanically induced mercury drop fall. On a dropping mercury electrode, sigmoidal curves are obtained. Due to their limited sensitivity and resolution capability, dc techniques are not commonly used.


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Procedure



Figure 2-2. Typical dc polarographic curve Pulse Techniques (dp, sqw) When potential pulses are superimposed to the dc staircase ramp, the techniques are called pulse techniques (Figure 2-3). This includes the differential pulse technique (dp) and square wave technique (sqw). In these techniques, the current is measured before the pulse application and at the end of the pulse. The difference of the two currents is calculated and displayed. The resulting peak-shaped curves allow reliable evaluation. Pulse techniques have significantly higher sensitivity than dc techniques and show better signal separation. Today, they are the prominent techniques used.

Figure 2-3. Differential pulse polarogram Cyclic Voltammetry (CV): Linear Sweep Voltammetry In cyclic voltammetry, the working electrode potential is ramped linearly versus time, similar to linear sweep voltammetry. CV is the most effective and versatile electroanalytical technique available for the mechanistic study of redox systems. It enables the electrode potential to be rapidly scanned in search of redox couples. Calibration The standard addition technique is most convenient when small numbers of samples are to be analyzed and is useful when the analyte is present in a complicated matrix and no ideal blank is available (Figure 2-4). It is the default method for polarography and voltammetry. The external calibration technique is most convenient when large numbers of similar samples are to be analyzed. However, it is not commonly used in polarography, as matrix effects are not compensated and errors may occur.


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Procedure



Figure 2-4. Calibration curve using the standard addition technique Stripping Voltammetry For the determination of certain analytes, predominantly transition metals, stripping voltammetry is used. Stripping voltammetry is a two-step technique. The first step is the deposition step in which the analyte is deposited on the working electrode by reduction or adsorption while applying a constant potential for a certain time. In the second step, the determination or stripping step, the preconcentrated analyte is oxidized or reduced in a conventional voltammetric scan. The in situ preconcentration enhances the signal by a minimum of two or three orders of magnitude and extends the application range to the determination of trace concentrations in the µg/L or ng/L range. Different types of stripping voltammetry can be distinguished by the mechanism of deposition involved and the electrochemical reaction during the stripping step. The most common types are described below. Anodic Stripping Voltammetry (ASV) The analyte is deposited by reduction on the electrode surface. The stripping step is a reoxidation of the analyte, which is an anodic process. One of the most common examples is the determination of trace levels of lead:

Adsorptive Stripping Voltammetry (AdSV) The analyte is deposited on the electrode surface in an adsorptive process. Metal ion complexes are the most common example. The stripping step may be an oxidation reaction but is usually a reduction reaction. One of the most common examples is the determination of trace levels of nickel after addition of dimethylglyoxime (DMG) to form a complex that can be adsorbed on the electrode surface:

Specific Polarographic Procedures Polarographic Procedure for Ammonium Transfer the specified amount of sample to each of two 25 mL volumetric flasks containing 5 mL of water. Neutralize, if necessary, with the appropriate quantity of ammonia-free 6 N sodium hydroxide (prepared by boiling and cooling 6 N


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Procedure


sodium hydroxide solution). To one of the flasks, add the indicated quantity of ammonium ion. To each, add 5 mL of acetate buffer (to establish pH 4) and 5 mL of ammonia-free formaldehyde (both described below). Heat on a hot plate (~100 °C) for 5 min with occasional shaking, cool, and dilute to volume with water. Transfer a suitable portion to the cell, deoxygenate for 5 min with inert gases such as nitrogen or argon, and record the polarogram from –0.60 to –1.10 V vs SCE. The following instrumental settings have proved to be satisfactory: drop time, 1 s; scan rate, 5 mV/s; sensitivity, 0.5 µA full scale; and modulation amplitude, 50 mV. The peak for the ammonium–formaldehyde derivative occurs at about –0.85 V, but its position is dependent on pH. The peak for the sample should not exceed one-half of the peak for the sample plus standard.

S o d i u m A c e t a t e B u ff ffeer. Dissolve 10.4 g of sodium acetate trihydrate in 200 mL of water, and add 32 mL of glacial acetic acid.


A m m o n i a - F r e e F o r m a l d e h y d e . Add 50 g of a cation-exchange resin in hydrogen form (e.g., Dowex Marathon C, 50–100 mesh) to a 250 mL bottle of 37% formaldehyde solution. Place the bottle in a shaker for 1 h or let it stand for several hours, and mix occasionally. Polarographic Procedure for Bromate and Iodate Add to the polarographic cell an appropriate volume of solution prepared as described in the specification. Deoxygenate for 5 min with inert gases such as nitrogen or argon, and record the polarogram from –0.6 to –1.8 V vs SCE. The following conditions are satisfactory: drop time, 1 s; scan rate, 5 mV/s; sensitivity, 0.5 µA full scale; and modulation amplitude, 25 mV. The peak occurs at about –1.25 V for iodate and –1.6 V for bromate. If the ionic strength is not sufficiently high, the bromate wave becomes more negative, and the cathodic wave may interfere. In this case, calcium chloride may be added, and a reagent blank correction may be applied. The peak for the sample should not exceed one-half of the peak for the sample plus standard. Procedure for Lead and Cadmium in Zinc Compounds Using Anodic Stripping Voltammetry Required Reagents • Acetic Acid, Glacial • Ammonium Hydroxide, 28.0–30.0 % • Nitric Acid, 68–70 % • Water, Ultratrace • 1g/L Pb standard stock solution • 1g/L Cd standard stock solution Reagents of best purity available should be used; trace analysis quality is strongly recommended. Required Solutions • Supporting electrolyte solution: 0.1 M acetic acid + 0.1 M ammonium hydroxide • Sample solution: 1 g/L zinc sample or zinc compound sample (e.g., ZnCl2) dissolved in hydrochloric acid • Standard solution: 10 mg/L Pb2+ + 10 mg/L Cd2+ in 0.01 M nitric acid

Determination of Lead and Cadmium Using Anodic Stripping Voltammetry Measuring solution: 20 mL supporting electrolyte solution + 0.02 mL sample solution The solution is placed into the polarographic cell and degassed with nitrogen for 5 min to remove interfering oxygen. The voltammogram is recorded using the following parameters: • Working electrode: Hanging mercury drop electrode (HMDE)


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Procedure


• Reference electrode: Ag/AgCl/3 M KCl • Auxiliary/counter electrode: Platinum or glassy carbon rod • Deposition time: 60 s (0–90 s) • Deposition potential: –0.8 V • Start potential: –0.8 V • Stop potential: –0.1 V • Differential pulse technique (50 mV pulse amplitude) • dc potential step height: 2–6 mV • Scan rate: 10–60 mV/s Downloaded by CORNELL UNIV on May 9, 2017 | http://pubs.acs.org Publication Date (Web): February 28, 2017 | doi: 10.1021/acsreagents.2005

• Peak potential Pb2+: approximately –0.38 V (vs Ag/AgCl/3 M KCl) The concentration of lead and cadmium are determined using two additions of 0.02 mL of the standard solution described above. Conditions may have to be optimized depending on the instrument/electrode used and the concentration in the sample, that is, volume of sample solution, deposition time, standard addition volume, and concentration. The square wave technique may be used alternatively. Polarographic Procedure for Nitrate Dissolve the prescribed weight of sample in 20 mL of mixed reagent solution (described below), and dilute with water to 25 mL. Prepare a standard with the specified quantity of nitrite and 20 mL of reagent solution also diluted to 25 mL. Place a suitable volume of either solution in the polarographic cell, deoxygenate for 5 min with nitrogen, and record the polarogram from –0.30 to –0.80 V vs Ag/AgCl electrode. The following conditions are satisfactory: drop time, 0.5 s; scan rate, 10 mV/s; sensitivity, 0.2 µA full scale; and modulation amplitude, 50 mV. Read the peak height at approximately –0.52 V. The peak for the sample should not be greater than that for the standard.

M i x e d Re a g e n t S o l u t i o n . Prepare by adding 5 mL of solution A, 10 mL of solution B, and 10 mL of solution C to 100 mL of water in an amber bottle. A fresh mixture should be prepared each week. S o l u t i o n A . Dissolve 0.044 g of diphenylamine in 40 mL of methanol, and dilute to 100 mL with water. S o l u t i o n B . Use 0.1 N potassium thiocyanate. S o l u t i o n C . Dilute 7.0 mL of 60% perchloric acid to 250 mL with water.


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Direct Electrometric Methods - American Chemical Society

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