Indirect titration methods

J) Direct titration method, (ii) Indirect titration method, and... 

Indirect Titration (Method B). A weighed sample of salt was added to a flask containing methanol and a known excess of aqueous sodium hydroxide solution. The mixture was stirred and warmed on a hot plate for 1 h. After cooling, the amount of excess hydroxide present was determined by titration with standard aqueous hydrochloric acid. The titration was monitored using a pH electrode and meter, and the end point was determined from the resulting titration curve. 

Although originally developed for research purposes on soil and coal-derived HAs (Blom et al., 1957 Brooks and StemheU, 1957 Schnitzer and Gupta, 1965), indirect titration methods were also applied to aquatic samples in the 1970s. For example. Beck et al. (1974) analyzed 10 unfractionated DOM samples from a blackwater river in southeastern Georgia (USA), and Weber and Wilson (1975) analyzed three FAs from a darkly colored river and pond in New Hampshire (USA). It was later recognized (Sposito and Holtzclaw, 1979 Perdue et al., 1980) that incomplete removal of a humic substance from a Ca(CH3C02)2 reaction mixrnre causes carboxyl content to be overestimated. Davis (1982) found that incomplete removal of a humic substance from a Ba(OH)2 reaction mixture causes total acidity to be underestimated. The critical nature of the filtration step in indirect titrations was further discussed by Perdue (1985). 

Although less commonly employed than indirect titration methods, soil scientists also used direct titrations methods to study the acid-base chemistry and metal interactions of soil organic matter (PommerandBreger, 1960 Posner, 1966 Gamble, 1970, 1972 van Dijk, 1971 Khalaf et al, 1975 ... 

Bill Heineman s group developed an elegant indirect titration method in the 1970s [15], Indirect coulometric titrations and optically transparent thin-layer electrochemical cells were combined to provide a simple and quick means of making formal potential measurements on electron transfer proteins. Moreover, the amount of sample required for this measurement was quite small. This is now a routine method for measuring the formal potential of electron transfer proteins, which is used by a wide range of non-electrochemical scientists. Use of this method in our laboratory, greatly facilitated by help from the Heineman laboratory, led to our later efforts to develop direct electfochemical methods for protein and enzyme studies. 

Greater precision can be attained with a titration procedure but with a concomitant increase in experimental sophistication. Typically, precisions on the order of 0.1 % are achieved, while under optimum conditions this may be improved by one order of magnitude. A titration procedure has an additional advantage in that the number of species that can be determined is greatly increased by using indirect titration methods. 

The double titration method, which involves the use of ben2ylchloride, 1,2-dibromoethane, or aUyl bromide, determines carbon-bound lithium indirectly (101,102). One sample of the //-butyUithium is hydroly2ed directly, and the resulting alkalinity is determined. A second sample is treated with ben2ylchloride and is then hydroly2ed and titrated with acid. The second value (free base) is subtracted from the first (total base) to give a measure of the actual carbon-bound lithium present (active base). 

The direct iodometric titration method (sometimes termed iodimetry) refers to titrations with a standard solution of iodine. The indirect iodometric titration method (sometimes termed iodometry) deals with the titration of iodine liberated in chemical reactions. The normal reduction potential of the reversible system ... 

In the indirect amperometric method [560], saturated uranyl zinc acetate solution is added to the sample containing 0.1-10 mg sodium. The solution is heated for 30 minutes at 100 °C to complete precipitation. The solution is filtered and the precipitate washed several times with 2 ml of the reagent and then five times with 99% ethanol saturated with sodium uranyl zinc acetate. The precipitate is dissolved and diluted to a known volume. To an aliquot containing up to 1.7 mg zinc, 1M tartaric acid (2-3 ml) and 3 M ammonium acetate (8-10 ml) are added and the pH adjusted to 7.5-8.0 with 2 M aqueous ammonia. The solution is diluted to 25 ml and an equal volume of ethanol added. It is titrated amperometrically with 0.01 M K4Fe(CN)6 using a platinum electrode. Uranium does not interfere with the determination of sodium. 

An indirect oxidimetric titration method for alkylhthium compounds may be carried out, taking advantage of the fact that vanadium pentoxide causes fast and complete oxidation of alkyllithiums in solution. After the end of this reaction the reduced vanadium compounds are dissolved in sulfuric acid and titrated with Ce(IV) sulfate, using as indicator Ee(II) and 1,10-phenanthroline (64) sulfate. This method is inadequate for aryllithium compounds because the lithium phenoxide impurities reduce vanadium pentoxide. ... 

An indirect vanadometric method of assay for camphor was developed. The method involves the formation of 2, -dinitrophenylhydrazone of camphor. The nitro groups in the hydrazone are then reduced to amines by treatment with vanadium sulfate (VSOl ) and the excess of the reagent is back titrated with sodium dichromate (NapCrpO-y). The entire procedure is carried out in a modified separatory funnel into which standard vanadate, 6tJ and zinc amalgam... 

Numerous descriptions of methods of analysis of the acidic functional groups of humic substances have been published (e.g., Stevenson and Butler, 1969 Schnitzer and Khan, 1972 Stevenson, 1982). The published methods include direct titrations, discontinuous titrations, indirect titrations, indirect titrations coupled with either a distillation or ultrafiltration step, thermometric titration methods, nonaqueous titrations, irreversible reactions of acidic hydrogens with various organic and inorganic reagents, and so on. The two most commonly described methods are the barium hydroxide... 

TABLE 1.2 Determination of Optimal Titers for Indirect Immunoperoxidase Method—Checkerboard Titration ... 

In concluding the discussion of Tremillon s approach, it should be mentioned that most ionic melts were investigated by indirect methods, which excluded the addition of solid base into the melt. The upper limits of basicity were achieved at low O2- concentrations, which complicated investigations of other acids and bases by the weight-titration method. 

Indirect spectrophotometric methods are in most cases based on the formation of ion pairs that are extractable into organic solvents. These methods are often used in combination with volumetric and gravimetric methods, as many precipitated surfactant complexes can be dissolved in the appropriate solvents and analyzed colorimetrically. The spectrophotometric determination of the end-point in two-phase titration is often carried out. 

Despite these direct titration methods, the surfactant actives content is usually calculated indirectly by a subtraction method. For this, the water and sodium... 

Water hardness is the total calcium and magnesium ion concentration in a water sample and is expressed as the concentration of calcium carbonate. Temporary hardness is that part of the total hardness that disappears on boiling. Whilst not being accepted as a standard method, the use of ion-selective electrodes allows a rapid measurement of water hardness and can be used to determine changes in hardness. The direct potentiometric method is not recommended for the ion-selective electrode but an indirect potentiometric method involving ethylenediaminetetraacetic acid titration is recommended. The ion-selective electrode that is used is a liquid ion-exchange electrode that responds to the divalent ions magnesium and calcium. 

Test methods. There are a number of direct and indirect test methods used for the measurement of chlorine residual. The commonly used direct methods include, colorimetric and titration and the indirect method commonly used in commercial and some residential is oxidation-reduction potential. 

The number and variety of ion-specific electrodes is rapidly increasing with no end in sight. At the present writing, it is possible to use such electrodes to determine, either by direct or indirect measurement, ionic concentrations of the following species ammonia, bromide, cadmium, calcium, chloride, cupric, cyanide, fluoride, fluoroborate, iodide, lead, nitrate, perchlorate, potassium, sulfide, sodium, sulfur dioxide, and thiocyanate, all by direct measurement, and by titration methods aluminum, boron, chromium, cobalt, magnesium, mercury, nickel, phosphate, silver, sulfate, and zinc. 

---