Basic titrant

Although many quantitative applications of acid-base titrimetry have been replaced by other analytical methods, there are several important applications that continue to be listed as standard methods.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. 

Silica-aluminas contain acid sites stronger than those found in most halide-treated aluminas. We have attempted to tailor the acidity of amorphous silica-alumina by adding varying levels of "permanent" inorganic basic titrants. Such titrants were chosen in order to meet the following four criteria ... 

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],... 

Basic solvents A basic solvent can be used to enhance the acidic properties of weak acids. To take full advantage of a basic solvent requires a strongly basic titrant. Moss, Elliott, and Hall were able to observe separate end points for carboxylic acids and phenols using sodium aminoethoxide as titrant in ethylenediamine as solvent. 

ALKOXIDES OF QUATERNARY AMMONIUM BASES. A NEW TYPE OF STRONGLY BASIC TITRANT... 

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. 

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 quantitative analysis for NH3 in several household cleaning products is carried out by titrating with a standard solution of HGl. The titration s progress is followed thermometrically by monitoring the temperature of the titration mixture as a function of the volume of added titrant. Household cleaning products may contain other basic components, such as sodium citrate or sodium carbonate, that will also be titrated by HGl. By comparing titration curves for prepared samples of NH3 to titration curves for the samples, it is possible to determine that portion of the thermometric titration curve due to the neutralization of NH3. 

Under basic conditions, Mn04 can be used as a titrant for the analysis of Mn +, with both the analyte and the titrant ending up as Mn02. fn the analysis of a mineral sample for... 

Gravimetric and volumetric methods are practicable for the quantitative determination of the a-sulfo fatty acid esters. Using gravimetric methods the surfactant is precipitated with p-toluidine or barium chloride [105]. The volumetric determination method is two-phase titration. In this technique different titrants and indicators are used. For the analysis of a-sulfo fatty acid esters the quaternary ammonium surfactant hyamine 1622 (p,f-octylphenoxyethyldimethyl-ammonium chloride) is used as the titrant [106]. The indicator depends on the pH value of the titration solution. Titration with a phenol red indicator is carried out at a pH of 9, methylene blue is used in acid medium [106], and a mixed indicator of a cationic (dimidium bromide) and an anionic (disulfine blue VN150) dye can be used in an acid and basic medium [105]. 

In a typical acid—base titration, the analyte is a solution of a base and the titrant is a solution of an acid or vice versa. An indicator a water-soluble dye (Section J), helps us detect the stoichiometric point, the stage at which the volume of titrant added is exactly that required by the stoichiometric relation between titrant and analyte. For example, if we titrate hydrochloric acid containing a few drops of the indicator phenolphthalein, the solution is initially colorless. After the stoichiometric point, when excess base is present, the solution in the flask is basic and the indicator is pink. The indicator color change is sudden, so it is easy to detect the stoichiometric point (Fig. L.3). Toolbox L.2 shows how to interpret a titration the procedure is summarized in diagram (3), where A is the solute in the titrant and B is the solute in the analyte. 

The formation of a weakly basic tertiary amine in reaction 4.102 does not alter the titrant normality, but in the titration of an acid it may suppress the height of the titration curve on the basic side. In an extensive study of twelve quaternary ammonium titrants in non-aqueous media (mainly isopropyl alcohol), Harlow73 observed large differences in stability the presence of water had a profound stabilizing action but at the sacrifice of basic strength inert and basic solvents increased the rate of decomposition (see Fig. 4.18). 

Common chemical titrations include acid-base, oxidation-reduction, precipitation, and complexometric analysis. The basic concepts underlying all titration are illustrated by classic acid-base titrations. A known amount of acid is placed in a flask and an indicator added. The indicator is a compound whose color depends on the pH of its environment. A solution of base of precisely known concentration (referred to as the titrant) is then added to the acid until all of the acid has just been reacted, causing the pH of the solution to increase and the color of the indicator to change. The volume of the base required to get to this point in the titration is known as the end point of the titration. The concentration of the acid present in the original solution can be calculated from the volume of base needed to reach the end point and the known concentration of the base. 

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. 

A titration curve for an acid soil suspension to which 1 mL of a calcium hydroxide titrant is added and the change in pH followed for 2.3 minutes is shown in Figure 10.3. As can be seen, the pH initially increases and then falls back toward the original pH. The curve not only has a sawtooth pattern but is also curved in the reverse direction from a standard titration of an acid with a basic solution. 

Selective titration by using suitable solvent and titrant of acidic/basic components of physiologically active moiety of a salt,... 

Some acids or bases can donate or accept more than one proton, i.e. 1 mole of analyte is equivalent to more than 1 mole of titrant. If the pA"a values of any acidic or basic groups differ by more than ca 4, then the compound will have more than one inflection in its titration curve. Sodium carbonate is a salt of carbonic acid and it can accept two protons. The pKa values of carbonate and bicarbonate are sufficiently different (p/fa 10.32 and 6.38) for there to be two inflections in the titration curve. The two stages in the titration are ... 

The detailed synthesis procedure and textural properties (surface area, Sggy in m2 g-1 pore volume, V in ml g"1 and main pore diameter, d in nm), determined by nitrogen adsorption from 8.E.T. method have been published elsewhere (refs. 13-18) and are summarized in Table 1, where the surface acidity and basicity of supports are also collected. These values were determined by a spectro-photometric method described elsewhere (ref. 19), that allows titration of the amount (in tunol g 1) of irreversibly adsorbed benzoic acid (BA, pKa> 4.19), pyridine (PY, pka= 5.25) or 2,6-diterbutyl-4-methylpyridine (DTMPY, pKa 7.5) employed as titrant agents of basic and acid sites, respectively. Furthermore, the apparent rate constant values of different supports in the gas-phase skeletal isomerization of cyclohexene (CHSI), in Mmol atm"1 g"1 s-1, at 673 K, are also collected in Table 1, because these values are another way of measuring the stronger acid sites of supports (ref. 19). 

The uses of constant-current coulometry for the determination of drugs in biological fluids are few, basically due to sensitivity restriction. Monforte and Purdy [46] have reported an assay for two allylic barbituric acid derivatives, sodium seconal and sodium sandoptal, with electrogenerated bromine as the titrant and biamperometry for endpoint detection. Quantitative bromination required an excess of bromine hence back titration with standard arsenite was performed. The assay required the formation of a protein-free filtrate of serum with tungstic acid, extraction into chloroform, and sample cleanup by back extraction, followed by coulometric titration with electrogenerated bromine. The protein precipitation step resulted in losses of compound due to coprecipitation. The recoveries of sodium seconal and sodium sandoptal carried through the serum assay were approximately 81 and 88%, respectively. Samples in the concentration range 7.5-50 pg/mL serum were analyzed by this procedure. 

Perform analysis as described (see Basic Protocol 7, steps 2 to 7), but use 100 g solids and bring to 3000 g total weight, and reduce concentration of titrant to 0.0247 N. 

Strategy The main species in solution at the stoichiometric point is sodium formate, which has a basic anion. The number of moles of HC02 ions in the solution at the stoichiometric point is equal to the initial number of moles of HCOOH in the analyte, which we can calculate from the initial volume of analyte solution and its molarity. The molarity of HC02" at the stoichiometric point is this number of moles of ions divided by the total volume of the solution. To find the total volume, we calculate the volume of titrant solution needed to reach the stoichiometric point and add it to the initial volume of analyte. At this stage, we have the molarity of the basic anion. The pH of the salt solution is then calculated as described in Toolbox 10.2. The Kh of HC02 is related to the K, of its conjugate acid HCOOH by Ka X Kh = Kw K3 is listed in Table 10.1. Assume that the autoprotolysis of water has no significant effect on the pH, then check that assumption. 

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