Conductivity titration

Titrations conducted with microliter or picoliter sample volumes require a smaller absolute amount of analyte. For example, diffusional titrations have been successfully conducted on as little as 29 femtomoles (10 mol) of nitric acid. Nevertheless, the analyte must still be present in the sample at a major or minor level for the titration to be performed accurately and precisely. 

We now turn our attention to details of precipitation titrations as an illustration of principles that underlie all titrations. We first study how concentrations of analyte and titrant vary during a titration and then derive equations that can be used to predict titration curves. One reason to calculate titration curves is to understand the chemistry that occurs during titrations. A second reason is to learn how experimental control can be exerted to influence the quality of an analytical titration. For example, certain titrations conducted at the wrong pH could give no discernible end point. In precipitation titrations, the concentrations of analyte and titrant and the size of Ksp influence the sharpness of the end point. For acid-base titrations (Chapter 11) and oxidation-reduction titrations (Chapter 16). the theoretical titration curve enables us to choose an appropriate indicator. 

Mohr titration Argentometric titration conducted in the presence of chromate. The end point is signaled by the formation of red Ag2Cr04(.v). molality A measure of concentration equal to the number of moles of solute per kilogram of solvent. 

Materials. Sulfolane (99%purity) (Aldrich) was treated with calcium hydride and distilled under reduced pressure. The freshly prepared solvent had a specific conductivity of 1.0 X 10 7 O"1 cm"1 and a residual water content of 8 X 10"3M as determined by Karl Fisher titration. Conductivity water and reagent grade ether (Baker) were used. Glacial acetic acid (CIL), trifluoroacetic acid (Baker), and trifluoro-methanesulfonic acid (3M) were used as received. All these acids had a minimum purity of 99.5% as determined by titration with standard sodium hydroxide. Methanesulfonic acid (Eastman), distilled under reduced pressure, had a purity of 99.6%. Sulfolane solutions of these acids were prepared by weight, and the acid concentrations were checked by acidimetry after the samples were flooded with water. The solutions... 

The course of add-base titrations conducted in the presence of specifically adsorbing species is different from that observed at pristine conditions. The experimental limitations and difficulties encountered by potentiometric titration have been discussed in Section 3.I.B.1 and 3.LB.2. Cancellation of some errors simplifies the assessment of the effect of spedfic adsorption on surface charging (comparison of titrations in the presence and absence of specific adsoiption), but the compensation of errors is not complete, e.g. the solubility of materials (adsorbents) is strongly affected by specific adsorption. 

Values for the association of nickel with sulphate for temperatures from 25 to 95°C and ionic strengths to 2.6 M were measured using a variety of pH titration, conductivity and transport number measurement techniques. Although the reported association constants are in rough agreement with those from other studies, there are insufficient experimental details (or results) to use this work in deriving a recommended value for AT. ... 

Figure 5.15 shows a comparison of a low-frequency acid-base titration at two different ionic strengths with high-frequency titrations conducted at 3 and 10 MHz. In each case, 50 milliequivalents of HCl is titrated with 0.01 N NaOH. Obviously 10 MHz is the best frequency to use, but because of the curvature several additional titration points need to be taken to increase the precision of the endpoint determination. The M-shaped curve resulting at 3 MHz could lead to misinterpretation and an incorrect endpoint. 

It is possible to eliminate the second phase of an organic solvent during the titration. This change will usually permit the titration to be performed in less time and ease automation of the procedure. Properly, surfactant titrations conducted in an aqueous medium are not truly one-phase reactions, since the ion pair of the cationic and anionic surfactant is water-insoluble and constitutes a second dispersed phase of its own. The titration can be monitored by following the removal of the surfactant ion from the bulk aqueous phase or by observing the formation of the hydrophobic ion pair. 

Derivative methods work well only when sufficient data are recorded during the sharp rise in plT occurring near the equivalence point. This is usually not a problem when the titration is conducted with an automatic titrator, particularly when operated under computer control. Manual titrations, however, often contain only a few data points in the equivalence point region, due to the limited range of volumes over which the transition in plT occurs. Manual titrations are, however, information-rich during the more gently rising portions of the titration curve before and after the equivalence point. 

The majority of titrations involving basic analytes, whether conducted in aqueous or nonaqueous solvents, use HCl, HCIO4, or H2SO4 as the titrant. Solutions of these titrants are usually prepared by diluting a commercially available concentrated stock solution and are stable for extended periods of time. Since the concentrations of concentrated acids are known only approximately,the titrant s concentration is determined by standardizing against one of the primary standard weak bases listed in Table 9.7. 

A partial list of metallochromic indicators, and the metal ions and pH conditions for which they are useful, is given in Table 9.16. Even when a suitable indicator does not exist, it is often possible to conduct an EDTA titration by introducing a small amount of a secondary metal-EDTA complex, provided that the secondary metal ion forms a stronger complex with the indicator and a weaker complex with EDTA than the analyte. For example, calmagite can be used in the determination of... 

Each of these titrations was conducted on a 50.00-mF aliquot of the original 250.0-mF sample. The mass of each analyte, therefore, must be corrected by multiplying by a factor of 5. Thus, the grams of Ni, Fe, and Cr in the original sample are... 

Directions are provided for the quantitative analyses of Ck and P04 , and for conducting titrations of strong acids and strong bases. 

The free sulfur trioxide can be titrated with water the end point is deterrnined conductimetricaHy. The sulfuric acid content is deterrnined from the specific conductivity of the Hquid at the point in the titration where no free SO or excess water is present. If the presence of HF is suspected, a known amount of SO is added to the acid and the excess SO is deterrnined as above. The content of another common impurity, SO2, may be determined iodometricaHy in a dilute, aqueous solution. 

Table 4 lists the specifications set by Du Pont, the largest U.S. producer of DMF (4). Water in DMF is deterrnined either by Kad Fischer titration or by gas chromatography. The chromatographic method is more rehable at lower levels of water (<500 ppm) (4). DMF purity is deterrnined by gc. For specialized laboratory appHcations, conductivity measurements have been used as an indication of purity (27). DMF in water can be measured by refractive index, hydrolysis to DMA followed by titration of the Hberated amine, or, most conveniendy, by infrared analysis. A band at 1087 cm is used for the ir analysis. 

The free maleic acid content in maleic anhydride is determined by direct potentiometric titration (166). The procedure involves the use of a tertiary amine, A/-ethylpipetidine [766-09-6J, as a titrant. A tertiary amine is chosen as a titrant since it is nonreactive with anhydrides (166,167). The titration is conducted in an anhydrous solvent system. Only one of the carboxyhc acid groups is titrated by this procedure. The second hydrogen s dissociation constant is too weak to titrate (166). This test method is not only used to determine the latent acid content in refined maleic acid, but also as a measure of the sample exposure to moisture during shipping. 

The Reich test is used to estimate sulfur dioxide content of a gas by measuring the volume of gas required to decolorize a standard iodine solution (274). Equipment has been developed commercially for continuous monitoring of stack gas by measuring the near-ultraviolet absorption bands of sulfur dioxide (275—277). The deterrnination of sulfur dioxide in food is conducted by distilling the sulfur dioxide from the acidulated sample into a solution of hydrogen peroxide, foUowed by acidimetric titration of the sulfuric acid thus produced (278). Analytical methods for sulfur dioxide have been reviewed (279). 

Ash is a measure of residual sodium acetate. A simple method consists of dissolving the PVA in water, diluting to a known concentration of about 0.5 wt %, and measuring the electrical conductivity of the solution at 30°C. The amount of sodium acetate is estabUshed by comparing the result to a cahbration curve. A more lengthy method involves the extraction of the PVA with methanol using a Soxhlet extractor. The methanol is evaporated and water is added. The solution is titrated using hydrochloric acid in order to determine the amount of sodium acetate. 

Consistent with this, dissolution of KF increases the conductivity and KIFe can be isolated on removal of the solvent. Likewise NOF affords [NO]+[IF6] . Antimony compounds yield ISbFio, i-2. [IF4]+[SbF6], which can be titrated with KSbFfi. However, the milder fluorinating power of IF5 frequently enables partially fluorinated adducts to be isolated and in some of these the iodine is partly oxygenated. Complete structural identification of the products has not yet been established in all cases but typical stoichiometries are as follows ... 

It is necessary to draw attention to the variable pH of water which may be encountered in quantitative analysis. Water in equilibrium with the normal atmosphere which contains 0.03 per cent by volume of carbon dioxide has a pH of about 5.7 very carefully prepared conductivity water has a pH close to 7 water saturated with carbon dioxide under a pressure of one atmosphere has a pH of about 3.7 at 25 °C. The analyst may therefore be dealing, according to the conditions that prevail in the laboratory, with water having a pH between the two extremes pH 3.7 and pH 7. Hence for indicators which show their alkaline colours at pH values above 4.5, the effect of carbon dioxide introduced during a titration, either from the atmosphere or from the titrating solutions, must be seriously considered. This subject is discussed again later (Section 10.12). 

Cu, Ni, Co, Cr, Fe, or Al, even in traces, must be absent when conducting a direct titration of the other metals listed above if the metal ion to be titrated does not react with the cyanide ion or with triethanolamine, these substances can be used as masking reagents. It has been stated that the addition of 0.5-1 mL of 0.001 M o-phenanthroline prior to the EDTA titration eliminates the blocking effect of these metals with solochrome black and also with xylenol orange (see below). 

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