Indirect titrations

Indirect titration using an excess of NaOH and back-titration to the first,... 

The acid in the receiving vessel can be either a dilute (perhaps 0.10 N) standardized solution of a strong acid, such as sulfuric acid, or a solution of boric acid. If it is the former, it is an example of a back titration. If it is the latter, it is an example of an indirect titration. 

Explain the difference between an indirect titration and a back titration. 

An indirect titration is one in which the ST is determined indirectly by titrating a second species that is proportional to the ST. A back titration is one in which the end point is intentionally overshot and the excess back titrated. 

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

A back titration - also known as an indirect titration - allows the concentration of an unknown solution to be determined by reacting it with a known excess of a reagent. The quantity of the excess reagent is determined by titration with a second reagent. 

Oxidation-Reduction Titrations. The extent of reduction resulting from reaction of niobium (V) chloride and bromide with pyridine was determined by indirect titration of crude reaction mixtures with standard ammonium tetrasulfato-cerate(IV) solution. Samples were stirred overnight in a stoppered flask with an excess of iron (III) ammonium sulfate. Any iron (II) formed by reaction with the niobium complex mixture was then titrated with the standard tetrasulfato-cerate(IV) solution using ferroin as indicator. Results of these determinations are given in Table III. 

Anions that precipitate with certain metal ions can be analyzed with EDTA by indirect titration. For example, sulfate can be analyzed by precipitation with excess Ba2+ at pH 1. 

Alternatively, an anion can be precipitated with excess metal ion. The precipitate is filtered and washed, and excess metal ion in the filtrate is titrated with EDTA. Anions such as CO, CrOg, S2, and SOg can be determined by indirect titration with EDTA.20... 

Sulfide ion was determined by indirect titration with EDTA. 

Sometimes it is not possible to perform a "direct" titration as illustrated in the last two problems In an indirect" titration, a known (but excess) volume of standard solution is used to insure a complete reaction with whatever is being analyzed for, then, at the end of the analysis, the unused quantity of the standard solution is determined by what is called aback-titrcition For example, such a procedure is useful for the determination of a gas that can be bubbled through the standard reaction solution Another example is illustrated by the next problem... 

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. 

The acid-base properties of DOM are of intrinsic interest because acidic functional groups contribute to the acid-base balance of natural waters, affect complexation and transport of dissolved metals, and interact with mineral surfaces. The concentrations of carboxyl and phenolic functional groups are among the most widely measured and reported properties of DOM. Methodologically, there are two basic approaches for measuring acidic group content—indirect titrations and direct titrations (Perdue et al., 1980 Perdue, 1985 Ritchie and Perdue, 2003). 

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). 

Even in the absence of filtration-related problems, indirect titrations do not provide insight into the strengths of acidic functional groups, because results are measured at only the two pH end points. 

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 ... 

A statistical analysis of the carboxyl and phenolic contents of FAs, HAs, and NOM is given in Figure 6. Separate statistical descriptions are provided for the results of indirect and direct titrations. Perhaps because of the aforementioned difficulties in indirect titrations of DOM, most of the reported carboxyl and phenolic contents were obtained using direct titrations. 

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... 

A second example of this type of indicator is illustrated in the Volhard titration. This is an indirect titration procedure for determining anions that precipitate with silver (Cl , Br , SCN"), and it is performed in acid (HNO3) solution. In this procedure, we add a measured excess of AgNOa to precipitate the anion and then determine the excess Ag" " by back-titration with standard potassium thiocyanate solution ... 

Indirect titration of sulphanilamides, determining unused reagent, has been performed with iodine monochloride384-386, iodine trichloride386 and bromine chloride387. 

Isocyanates can be determined by indirect titration of dibutylamine or by GC determination of this reagent, after reaction 39 (X = O R = R" = Bu) goes to completion. A variation of this procedure for isothiocyanate insecticides and other formulations consists of treating a solution in acetonitrile with excess primary amine, scavenging the excess amine with carbon disulphide, yielding a dithiocarbamate and determining the concentration by a 3-step titration with Cu(II) in acetonitrile... 

Anions that precipitate metal ions can be analyzed with EDTA by indirect titration. For example, sulfate can be analyzed by precipitation with excess Ba at pH 1. The resulting BaS04( ) is filtered, washed, and boiled with excess EDTA at pH 10 to bring Ba back into solution as Ba(EDTA). The excess EDTA is back titrated with Mg. ... 

Sulfide ion was determined by indirect titration with EDTA. To a solution containing 25.00 mL of 0.043 32 M Cu(C104)2 plus 15 mL of 1 M acetate buffer (pH 4.5) were added 25.00 mL of unknown sulfide solution with vigorous stirring. The CuS precipitate was filtered and washed with hot water. Ammonia was added to the filtrate (which con tains excess Cu " ) until the blue color of Cu(NH3)4 was observed. Titration of the filtrate with 0.039 27 M EDTA required 12.11 mL of EDTA to reach the end point with the indicator murexide. Find the molarity of sulfide in the unknown. 

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