Standardization of Titrant

The titrant, TnBAH solution, requires standardization at least twice daily when in use over an 8-h period. Duplicate samples of primary standard grade benzoic acid are weighed into the titration vessel and 60 ml of DMF (N,N -dimethylformamide) is added. The glass and TnBACl-filled calomel electrodes 

---

Selection and Standardization of Titrants EDTA is a versatile titrant that can be used for the analysis of virtually all metal ions. Although EDTA is the most commonly employed titrant for complexation titrations involving metal ions, it cannot be used for the direct analysis of anions or neutral ligands. In the latter case, standard solutions of Ag+ or Hg + are used as the titrant. 

Example Standardization of Titrant Followed by Analysis of Unknown... 

The accuracy of a standardization depends on the quality of the reagents and glassware used to prepare standards. For example, in an acid-base titration, the amount of analyte is related to the absolute amount of titrant used in the analysis by the stoichiometry of the chemical reaction between the analyte and the titrant. The amount of titrant used is the product of the signal (which is the volume of titrant) and the titrant s concentration. Thus, the accuracy of a titrimetric analysis can be no better than the accuracy to which the titrant s concentration is known. 

Although many quantitative applications of acid-base titrimetry have been replaced by other analytical methods, there are several important applications that continue to be listed as standard methods. In this section we review the general application of acid-base titrimetry to the analysis of inorganic and organic compounds, with an emphasis on selected applications in environmental and clinical analysis. First, however, we discuss the selection and standardization of acidic and basic titrants. 

Selecting and Standardizing a Titrant Most common acid-base titrants are not readily available as primary standards and must be standardized before they can be used in a quantitative analysis. Standardization is accomplished by titrating a known amount of an appropriate acidic or basic primary standard. 

Selected Primary Standards for the Standardization of Strong Acid and Strong Base Titrants... 

Primary Standard Standardization of Acidic Titrants Titration Reaction Comment... 

Procedure. Select a volume of sample requiring less than 15 mL of titrant to keep the analysis time under 5 min and, if necessary, dilute the sample to 50 mL with distilled water. Adjust the pH by adding 1-2 mL of a pH 10 buffer containing a small amount of Mg +-EDTA. Add 1-2 drops of indicator, and titrate with a standard solution of EDTA until the red-to-blue end point is reached. 

It is possible to titrate two substances by the same titrant provided that the standard potentials of the substances being titrated, and their oxidation or reduction products, differ by about 0.2 V. Stepwise titration curves are obtained in the titration of mixtures or of substances having several oxidation states. Thus the titration of a solution containing Cr(VI), Fe(III) and V(V) by an acid titanium(III) chloride solution is an example of such a mixture in the first step Cr(VI) is reduced to Cr(III) and V(V) to V(IV) in the second step Fe(III) is reduced to Fe(II) in the third step V(IV) is reduced to V(III) chromium is evaluated by difference of the volumes of titrant used in the first and third steps. Another example is the titration of a mixture of Fe(II) and V(IV) sulphates with Ce(IV) sulphate in dilute sulphuric acid in the first step Fe(II) is oxidised to Fe(III) and in the second jump V(IV) is oxidised to V(V) the latter change is accelerated by heating the solution after oxidation of the Fe(II) ion is complete. The titration of a substance having several oxidation states is exemplified by the stepwise reduction by acid chromium(II) chloride of Cu(II) ion to the Cu(I) state and then to the metal. 

Procedure. Charge the titration cell (Fig. 17.24) with 10.00 mL of the copper ion solution, 20 mL of the acetate buffer (pH = 2.2), and about 120mL of water. Position the cell in the spectrophotometer and set the wavelength scale at 745 nm. Adjust the slit width so that the reading on the absorbance scale is zero. Stir the solution and titrate with the standard EDTA record the absorbance every 0.50 mL until the value is about 0.20 and subsequently every 0.20 mL. Continue the titration until about 1.0 mL after the end point the latter occurs when the absorbance readings become fairly constant. Plot absorbance against mL of titrant added the intersection of the two straight lines (see Fig. 17.23 C) is the end point. 

A weighed amount of sample is dissolved in a mixture of propanone and ethanoic acid and titrated potentiometrically with standard lead nitrate solution, using glass and platinum electrodes in combination with a ferro-ferricyanide redox indicator system consisting of 1 mg lead ferrocyanide and 0.5 ml 10% potassium ferricyanide solution. The endpoint of the titration is located by graphical extrapolation of two branches of the titration plot. A standard solution of sodium sulfate is titrated in the same way and the sodium sulfate content is calculated from the amounts of titrant used for sample and standard. (d) Water. Two methods are currently available for the determination of water. 

The application of electroanalysis in non-aqueous media to a certain analytical problem requires a well considered selection of the solvent together with a suitable electroanalytical method, which can be carried out on the basis of the solvent classes mentioned in Table 4.3 and of the related theories. The steps to be taken include the preparation of the solvent and the apparatus for the electroanalytical method proper, together with other chemicals, especially when the method includes titration. Much detailed information on the purification of the solvents and on the preparation of titrants and primary standards can be found in the references cited in Section 4.1 and in various commercial brochures1,84,85 and books17,86-89 we shall therefore confine ourselves to some remarks on points of major importance. 

This wording may be considered as duplication, because one can hardly think of continuous titration without automation however, the intention is simply to stress its character as an alternative to automated discontinuous titrations. The principle of continuous titration can be illustrated best by Fig. 5.151 it applies to a steady stream of sample (C). Now, let us assume at first that the analyte concentration is on specification, i.e., it agrees with the analyte concentration of the standard (B). If, when one mixes the titrant (A) with the sample stream (C), the mass flow (equiv./s) of titrant precisely matches the mass flow of analyte, then the resulting mixture is on set-point. However, when the analyte concentration fluctuates, the fluctuations are registered by the sensor it is clear that the continuous measurement by mixing A and C is only occasionally interrupted by alternatively mixing A and B in order to check the titrant for its constancy. 

Typically, acid soils are titrated with a sodium or calcium hydroxide [NaOH or Ca(OH)2] solution and basic soils with hydrochloric acid (HC1), and pH changes are most commonly followed using a pH meter. Carbonates in basic soils release C02 during treatment with HC1, thus making the titration more difficult. For this reason, carbonates are often determined by other methods. It is important to keep in mind that basic solutions react with carbon dioxide in air and form insoluble carbonates. This means that either the basic titrant is standardized each day before use or the solution is protected from exposure to carbon dioxide in air. Specific descriptions of titrant preparation, primary standards, and the use of indicators and pH meters in titrations can be found in Harris [1] and in Skoog et al. [2],... 

Titration of soil pH is an old method that is not widely used today. Basically, an acid soil suspension is prepared and titrated with a standardized base, often sodium hydroxide, although various basic calcium compounds such as calcium oxide (CaO) and calcium hydroxide [Ca(OH)2] can also be used. Because of the dark color of many soils, they are often titrated using a pH meter as the indicator of the end point. A setup for the titration of soil is shown in Figure 10.1. Titration is slow in that it takes some time after the addition of titrant for some semblance of equilibrium to be reached. Once this happens, a reading can be made or simply another addition of titrant made. 

The titrant is standardized by titrating a known amount of water with the titrant and calculating the titer of the titrant, the weight of water consumed by 1 mL of titrant. 

Generally, most non-aqueous solvents possess greater coeffcients of expansion as compared to water, which is why small differences in temperature may afford significant and appreciable errors that can be eliminated by the application of appropriate correction factors. Hence, it is always advisable to carry out standardization and titration preferably at the same temperature. In a situation where these temperature parameters cannot be achieved, the volume of titrant may be corrected by the application of the following formula ... 

Standardization of 0.1 M Tetrabutylammonium Hydroxide To 10 ml of dimethylformamide add 0.05 ml of a 0.3 % w/v solution of thymol blue in methanol and titrate with the tetrabutylammonium hydroxide solution until a pure blue colour is produced. Immediately add 0.2 g of benzoic acid, stir to effect solution and titrate with the tetrabutylammonium hydroxide solution until the pure blue colour is restored. Protect the solution from atmospheric C02 throughout the titration. The volume of titrant used in the second titration represents the amount of tetrabutylammonium hydroxide required. Each ml of 0.1 M tetrabutylammonium hydroxide Vs is equivalent to 12.21 mg of C7Hg02. 

An analytical technique based upon the constant-current electrochemical generation of a redox-active titrant that can quantitatively react with a redox-active analyte. This technique takes advantage of the high precision with which the instrumentation controls electric current and time to produce standardized solutions of titrant. 

Any electromechanical device that utilizes an automated feedback servomotor to regulate the addition of titrant (a standardized solution of acid or base within a syringe) into a reaction vessel or sample to maintain pH. The rate at which the syringe expels its contents allows one to determine the rate of a chemical reaction producing or consuming protons. There are many such enzyme-catalyzed reactions whose kinetics can be examined with a pH Stat. For maximal sensitivity, one must use weakly buffered solutions. In his classical kinetic investigation of DNA bond scission by DNase, Thomas measured the rate of base addition in a pH Stat. The number of bonds cleaved was linear with time, and this was indicative of random scission. 

Standardization of a Titrant For wet chemistry analytical methods, a titration is often used and the titrant, or the solution to which an unknown sample is compared, must be standardized. This can be done by comparing it with another standard. The important thing here is that the standard with which it is compared is ultimately traced to a SRM. The procedure utilizes volumetric glassware heavily, and thus the analyst must be assured that these are properly calibrated, as discussed above. Auto-titrators can be used (Figure 5.4). In this case, the automated equipment can be calibrated against manual equipment, i.e., volume readings obtained with the auto-titrator must match the volume readings obtained with a calibrated buret for the same sample. If they do not match (within accepted limits), the auto-titrator must be taken out of service and repaired, just like the defective balance. 

Karl Fischer Titrators These titrators measure moisture (water) in a variety of samples. The titrant s concentration is usually expressed as titer, or grams of water consumed per milliliter of titrant. Standardization involves a certified primary standard (a material containing a known amount of water). This standard is purchased in ampules and is accompanied by a test certificate indicating traceability to a reference material. In addition, the titrator should be calibrated for the titrant volume measurement. The moisture can be measued by weight loss upon drying and checked against the Karl Fischer results. 

When disodium tartrate dihydrate is used, samples ranging from 50 to 120 mg are dissolved in methanol, and the concentration of the KF reagent is determined. Based on volume of titrant used, the weight of the sample and the percent water in the disodium tartrate dihydrate (15.66% w/w), the standardization factor can be calculated. One pitfall with this method is the solubility of the disodium tartrate dihydrate in methanol. It is recommended that the disodium tartrate dihydrate be finely divided and that a suitable extraction time be given for the solids to dissolve. 

Standardization of the KF titrant using purified water is widely accepted and probably the most commonly used in analytical laboratories. Using a suitable... 

Many reagents used as titrants, such as HC1, are not available as primary standards. Instead, we prepare titrant with approximately the desired concentration and use it to titrate a primary standard. By this procedure, called standardization, we determine the concentration of titrant. We then say that the titrant is a standard solution. The validity of the analytical result ultimately depends on knowing the composition of a primary standard. 

The volume of reagent (titrant) required for stoichiometric reaction of analyte is measured in volumetric analysis. The stoichiometric point of the reaction is called the equivalence point. What we measure by an abrupt change in a physical property (such as the color of an indicator or the potential of an electrode) is the end point. The difference between the end point and the equivalence point is a titration error. This error can be reduced by subtracting results of a blank titration, in which the same procedure is carried out in the absence of analyte, or by standardizing the titrant. using the same reaction and a similar volume as that used for analyte. 

The validity of an analytical result depends on knowing the amount of a primary standard. A solution with an approximately desired concentration can be standardized by titrating a primary standard. In a direct titration, titrant is added to analyte until the reaction is complete. In a back titration, a known excess of reagent is added to analyte, and the excess is titrated with a second standard reagent. Calculations of volumetric analysis relate the known moles of titrant to the unknown moles of analyte. 

Potassium permanganate can be standardized by titration of sodium oxalate (Na2C204) by Reaction 7-1 or pure electrolytic iron wire. Dissolve dry (105°C, 2 h) sodium oxalate (available in a 99.9-99.95% pure form) in 1 M H2S04 and treat it with 90-95% of the required KMn04 solution at room temperature. Then warm the solution to 55-60°C and complete the titration by slow addition of KMn04. Subtract a blank value to account for the quantity of titrant (usually one drop) needed to impart a pink color to the solution. 

Subtract the volume of titrant required for the blank from the titrant required for the sample. The difference represents the volume of DCIP that is equivalent to 0.50 mg of vitamin C. Calculate the standard deviation of your answer. What are the limits for 95°/o confidence Review Chapter 1, Section F for statistical analysis. 

An automated constant-current coulometric system employing electrogenerated iodine for assay of ascorbic acid or sodium ascorbate has been reported [38]. Using this apparatus, 25 samples of 30 mg could be determined in 2.5 h with an accuracy and precision of 0.3%. The automated system demonstrates accuracy and precision that are equivalent to or exceed the manual USP method, is significantly more rapid, and eliminates the need for preparation, standardization, and storage of titrant. 

---