Titrimetric Methods - ACS Reagent Chemicals (ACS Publications)

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

Titrimetric 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 NEW YORK UNIV on May 18, 2017 | http://pubs.acs.org Publication Date (Web): February 28, 2017 | doi: 10.1021/acsreagents.2002

ABSTRACT In titrimetry, materials or groups of materials are quantified by measuring the volume of a reagent solution with known concentrations of a substance, the titrimetric solution. The titrimetric solution is used for a defined, complete chemical conversion with the materials that are to be measured. Adding a reagent until one can recognize the end point of the reaction is known as titration. The reagent is called the titrant, and the material to be tested is the sample (or analyte). The chemical conditions that are required as a prerequisite for each titrimetric determination are a defined course of the reaction between the sample and the titrant and the ability to recognize the equivalence point (or titration end point). (This paragraph is adapted, with permission, from Schwedt, 1997.)

POTENTIOMETRIC TITRATIONS Potentiometric titrations are titrations in which the equivalence point is determined from the rate of change of the potential difference between two electrodes. The potential difference can be measured by any reliable millivoltmeter, including a pH meter that can be read either in pH units or millivolts. (These titrations can also be made by using commercially available automatic titrators, which either plot the complete titration or act to close an electrically operated burette valve exactly at the equivalence point.) The electrode pair, which consists of an indicating electrode and a reference electrode, depends on the type of titration and is indicated in the individual specifications. Relatively large increments of the titrant may be added until the equivalence point is approached, usually within 1 mL. In the vicinity of the end point, small equal increments (0.1 mL, for example) are added; after allowing sufficient time for the indicator electrode to reach a constant potential, the voltage and volume of titrant are recorded. The titration is continued for several increments of titrant beyond the end point. For acid–base titrations, the pH is recorded instead of the potential difference but is treated the same as potential difference for the determination of the end point.

First Derivative Method for Determination of Equivalence Point When the titration is symmetrical about the end point (same rate of change of potential before and after the end point), the position of the end point may be found by a graphical plot of ΔE/ΔV vs V, where E is the potential and V is the volume of titrant. The end point corresponds to the maximum value of ΔE/ΔV. (This is a plot of the first derivative, dE/dV, of E = f(V) vs the volume of titrant and is referred to as the first derivative method.)

Second Derivative Method for Determination of Equivalence Point The end point can be determined by the use of the second derivative, d2E/dV2, of E = f(V) and is the point where d2E/dV2 becomes zero. This point can be determined mathematically, assuming there is no significant difference between the average slope ΔE/ΔV and the true slope dE/dV. The end point lies in the increment where the ΔE is the greatest. The amount of titrant to be added to the burette reading at the beginning of the interval is found by multiplying the volume of the increment by the factor in which the numerator is the last +Δ2E value and the denominator is the sum of the last +Δ2E

© 2017 American Chemical Society

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

DOI:10.1021/acsreagents.2002 ACS Reagent Chemicals, Part 2

ACS Reagent Chemicals

Procedure

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


value and the first –Δ2E value, disregarding the sign. When the volume increments are identical, the method can be reduced to the use of the first and second differences. This simplification is employed in a typical example shown in Table 2-2.

ION-EXCHANGE COLUMN ASSAYS A 100 mL capacity class B burette with a Teflon TFE or PTFE stopcock or a commercially available column having a 1.5–2 cm bore may be used as a column. For fluoride assays, an acrylic–polyethylene burette having Teflon stopcocks is preferred. All other assays may be carried out in a glass column of the same dimensions. Note: All water used in the assay must be free of carbon dioxide and ammonia (18 MΩ water from a purification system is suitable).

Column Preparation Cation-exchange resin, Dowex HCR-W2 H+ form 16–40 mesh or Amberlite IR-120 or equivalent, is suitable for assays. About 75 mL of cation-exchange resin is placed in a liter-sized container and filled with water, stirred, and let to settle for 1–2 min. After settling, water is gently decanted, and the washing is repeated at least four to five times. In a suitable column, a cotton ball is inserted above the stopcock to prevent resin from plugging the stopcock and to achieve an even flow of the eluate. Following the last washing of the resin and decanting of the wash, the slurry is poured into the column with the stopcock open. The water level is always kept above the resin. Ion-exchange resin in the H+ form should be washed with water until about 50 mL of eluate requires about 0.05 mL of 0.1 N sodium hydroxide to neutralize using phenolphthalein as an indicator. An ion-exchange resin, in the Na+ form, should be regenerated by adding approximately 100 mL of 4 N hydrochloric acid through the column and then being washed until 0.05 mL of 0.1 N sodium hydroxide is required to neutralize about 50 mL of eluate using phenolphthalein as an indicator solution. About three or four assays may be carried out, but drastically lower assays indicate that a new ion-exchange resin is required. Resins may be recharged by repeating the treatment with 4 N hydrochloric acid.

Method for Ion-Exchange Column Assays Prepare the sample as directed in the individual reagent tests. Pass the sample solution through a cation-exchange column with water at a rate of about 5 mL/min, and collect about 250 mL of the eluate in a 500 mL titration flask. Wash the resin in the column at a rate of about 10 mL/min into the same titration flask, and titrate with 0.1 N sodium hydroxide. Continue the elution until 50 mL of the eluate requires no further titration.


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WATER BY THE KARL FISCHER METHOD The water content of most of the organic reagents discussed in this book is determined by the Karl Fischer method (for more on Karl Fischer titration, see Schilt, 1991, and Scholz, 1984). This method involves the titration of the sample in methanol or any suitable solvent, for example, pyridine, formamide, or petroleum ether, with the Karl Fischer reagent. This reagent consists of iodine, sulfur dioxide, an amine, and a solvent in which the iodine and the sulfur dioxide are rapidly and quantitatively consumed by the water in the sample. The end point is detected either visually from the color change caused by free iodine or electrometrically. The latter method is preferred in most cases and necessary in the case of colored solutions.


Two different techniques can be used to add the iodine needed for the reaction. In the volumetric Karl Fischer titration, the Karl Fischer reagent is added by means of a volumetric burette. With this technique, it is necessary to standardize the reagent often, and the amount of reagent added must be measured accurately. This accuracy is especially important in determining small amounts of water. A small error in delivering this required amount of reagent can lead to a large error in the determined water content. The development of commercial coulometric instrumentation has enabled the alternative coulometric titration method to be introduced into the analytical laboratory. Coulometry is an electrochemical process involving the generation of various materials in direct proportion to their equivalent weights. As applied to the determination of water by the Karl Fischer method, the iodine needed in the reaction is generated in situ (inside the actual titration vessel) from an iodide-containing solution. As in the volumetric Karl Fischer method, the excess iodine beyond the end point is usually determined by an electrometric technique. The amount of iodine generated for the reaction is determined accurately and is related to the amount of water present in the sample. The coulometric Karl Fischer titration is a more sensitive procedure (i.e., it can determine smaller amounts of water). Because of this, exclusion of extraneous sources of moisture is absolutely necessary. No standardization of reagent is required because this is an absolute method depending only on electrochemical laws and accurate measurement of electrical current. The method is generally recommended when the samples are liquid and have a low water content.

Volumetric Procedure The apparatus described here and the Karl Fischer reagent described in [Part 3: Reagents, Buffers, and Indicators; Solutions and Mixtures; Karl Fischer Volumetric Reagent] are suitable for the determination of water by this method. There are, however, a number of commercial titration instruments and Karl Fischer reagents available. These instruments and reagents may be used in conjunction with their manufacturers’ instructions. Apparatus Enclosed Titration System. The Karl Fischer reagent is highly sensitive to water, and exposure to atmospheric moisture must be avoided. The reagent is best stored in a reservoir connected to an automatic burette with all exits to the atmosphere protected with a suitable desiccant, for example, indicating-type silica gel or anhydrous calcium chloride. The closed titration flask, with provision for insertion of platinum–platinum electrodes and insertion of the sample through a septum or removable closure, must be connected to the burette in such a way that atmospheric moisture cannot enter through the joint. Because of the necessity for a closed titration system, stirring is best accomplished magnetically.

Polarizing Unit for Use with Electrometric End Point. A simple polarizing unit consists of a 1.5 V dry battery connected across a radio-type potentiometer of about 100 Ω. (One platinum electrode is connected to one end of the potentiometer; the other platinum electrode is connected through a microammeter to the movable control of the potentiometer.) By adjustment of the movable contact, the initial polarizing current can be set to the desired value of 100 µA through a 2000 Ω precision resistor. This current passes through the platinum–platinum electrodes, which are immersed in the solution to be titrated. Reagents Preparation of Reagent. If a commercial reagent is not being used, prepare the reagent according to Mitchell and Smith, 1984.

Commercial Reagents. Stabilized Karl Fischer reagents for volumetric water determinations are available from several suppliers. Pyridine and pyridine-free reagents can be used for water determination. For ketones and aldehydes, specially formulated reagents are available and should be used. Pyridine and pyridine-free reagents have defined water capacities, which should be considered when these reagents are used.


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Standardization of Karl Fischer Reagents The Karl Fischer reagent gradually deteriorates. It should be standardized within one hour before use or daily if it is in continuous use. The pyridine-free reagents are more stable, and titrant–solvent systems exist that are free from deterioration. However, a daily check may be necessary. A Karl Fischer solvent system that uses indicating-type silica gel is recommended.

Method 1. Place 25–50 mL of methanol in the titration flask, and titrate with Karl Fischer reagent to the electrometric end point. The deflection should be maintained for at least 30 s. This is a blank titration on the water contained in the methanol and the titration flask. Record the volume. Add 0.05 mL of water, accurately weighed, and again titrate with the Karl Fischer reagent to the end point. Calculate the strength of the reagent in milligrams of water per milliliter of Karl Fischer reagent from the weight of water added and the net volume of Karl Fischer reagent used.


Method 2. Use the same general procedure but add a weighed amount of reagent sodium tartrate dihydrate instead of water, and calculate the Karl Fischer titration factor (expressed in milligrams of water per milliliter) by the following:

Sodium tartrate does not dissolve completely in methanol and thus can cause premature, transient end points near the true end point. Hence, the titration must be performed slowly, and the suspension must be well stirred when near the final end point. Volumetric Procedure for Samples Using Karl Fischer Reagent, Method 1 Titrate 25–50 mL of methanol or other suitable solvent, as specified, to an end point with the Karl Fischer reagent, as in the standardization. Unstopper the titration flask, and rapidly add an accurately weighed portion of the solid or liquid sample to be tested. (For liquids, a weight burette or syringe is convenient or, when the density is known, a measured volume of sample may be introduced with a syringe or pipette.) Quickly restopper the flask, and titrate again with the Karl Fischer reagent to an end point. The percentage of water in the sample is calculated from the known strength of the Karl Fischer reagent, net volume used, and sample weight.

Coulometric Procedure Apparatus Follow the instrument manufacturer’s instructions for specific operation procedures. Reagents Pyridine and pyridine-free coulometric reagents have defined water capacities. Several suppliers offer solutions that are used for ensuring the performance of coulometric instruments. These usually are nonhygroscopic mixtures of solvents with a known water content. Reagent water may be used if the small amount required is weighed accurately. Coulometric Procedure for Samples Using Karl Fischer Reagent, Method 2 Titrate the sample cell to dryness. Activate the sample titration procedure, and add the specified amount of sample with a syringe (if liquid) or as a weighed solid. Determine the weight of the liquid sample by weighing the syringe before and after injection or by calculating from the volume injected and known sample density. The instrument will indicate the end of the titration, and the amount of water in the sample is automatically calculated.


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Titrimetric Methods - ACS Reagent Chemicals (ACS Publications)

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