Primary standards for aqueous acidic

Table 11.27 Primary Standards for Aqueous Acid-Base Titrations Table 11.28 Titrimetric (Volumetric) Factors...

Potassium hydrogen phthalate has many uses in analytical chemistry. It is a primary standard for standardization of bases in aqueous solutions. Its equivalent weight is 204.2. It also is a primary standard for acids in anhydrous acetic acid. Other applications are as a buffer in pH determinations and as a reference standard for chemical oxygen demand (COD). The theoretical COD of a Img/L potassium hydrogen phthalate is 1.176mg O2. 

Constant-boiling aqueous HCI can be used as a primary standard for acid-base titrations. When 20 wt% HQ (FM 36.461) is distilled, the composition of the distillate varies in a regular manner with the barometric pressure ... 

In general, to standardize a particular protein, a purified form of that protein is used as a primary calibrator (see Chapter 20). The purified preparation must, however, express the same immunoreactivity as the native protein. Unfortunately, once removed from its natural milieu, apo B-lOO is insoluble in aqueous buffers. This phenomenon is attributed to the very hydrophobic nature of apo B-100. An LDL preparation with density of 1,030 to 1.050 g/mL, often referred to as narrow-cut LDL, is generaUy used as the primary standard for apo B-100. The protein concentration of the purified preparation is determined by amino acid analysis. In contrast, freshly purified apo A-I is soluble in aqueous buffers and is suitable as a primary standard. 

Halates are readily prepared by the base hydrolysis of the halogen. Bromates and iodates can also be synthesized by the oxidation of the halide. Only iodic acid can be isolated outside of aqueous solution. Chlorates are excellent oxidizing agents, lodate occurs naturally, is a source of elemental iodine, and is a primary standard for iodimetry. The perhalic acids of chlorine and iodine are much easier to prepare than that of bromine. Perchloric acid, like sulfuric and phosphoric acid in previous groups, has a tetrahedral structure around the central atom and involves dir-pir... 

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. 

The most common strong base for titrating acidic analytes in aqueous solutions is NaOH. Sodium hydroxide is available both as a solid and as an approximately 50% w/v solution. Solutions of NaOH may be standardized against any of the primary weak acid standards listed in Table 9.7. The standardization of NaOH, however, is complicated by potential contamination from the following reaction between CO2 and OH . 

The primary standard betaine dye (44) is only sparingly soluble in water and less polar solvents it is insoluble in nonpolar solvents such as aliphatic hydrocarbons. In order to overcome the solubility problems in nonpolar solvents, the more lipophilic penta-t-butyl-substituted betaine dye (45) has additionally been used as a secondary reference probe [174]. The excellent Hnear correlation between the Ej values of the two dyes allows the calculation of t(30) values for solvents in which the solvatochromic indicator dye (44) is not soluble. Introduction of electron-withdrawing substituents e.g. Cl [323], F, CF3, C6F13 [324]) in the betaine molecule reduces the basicity of its phenolate moiety, which allows the direct determination of x(30) values for somewhat more acidic solvents. Moreover, the Hpophilic and fluorophilic penta(trifluoromethyl)-substituted betaine dye (46) is more soluble in nonpolar solvents e.g. hexafluoro-benzene) than the standard dye (44) [324]. Conversely, the solubility in aqueous media can be improved through replacement of some of the peripheral hydrophobic phenyl groups in (44) by more hydrophilic pyridyl groups, to yield the more water-soluble betaine dye (47) [325]. The Ej values of these new secondary standard betaine dyes correlate linearly with the x(30) values of (44), which allows the calculation of x(30) values for solvents in which only betaine dyes (45)-(47) are sufficiently stable and soluble for the UV/Vis spectroscopic measurements [324, 325]. 

The solubility of carbon dioxide in aqueous or nonaqueous media depends on three primary factors temperature, partial pressure of carbon dioxide, and acid-base reactions in the solution. Accurate data for solubility and equilibria are well-known for aqueous solutions (1-3), but not for nonaqueous solutions. Neither the standard compilations of equilibrium constants (1,2) nor recent reviews on nonaqueous electrolytes ( ) cover what appears to be a large and poorly indexed literature. 

Acids and bases in Table 10-3 can be purchased in forms pure enough to be primary standards. NaOH and KOH are not primary standards because the reagent-grade materials contain carbonate (from reaction with atmospheric CO2) and adsorbed water. Solutions of NaOH and KOH must be standardized against a primary standard. Potassium hydrogen phthalate is convenient for this purpose. Solutions of NaOH for titrations are prepared by diluting a stock solution of 50 wt% aqueous NaOH. Sodium carbonate is relatively insoluble in this stock solution and settles to the bottom. 

One of the assumptions of one-way (and other) ANOVA calculations is that the uncontrolled variation is truly random. However, in measurements made over a period of time, variation in an uncontrolled factor such as pressure, temperature, deterioration of apparatus, etc., may produce a trend in the results. As a result the errors due to uncontrolled variation are no longer random since the errors in successive measurements are correlated. This can lead to a systematic error in the results. Fortunately this problem is simply overcome by using the technique of randomization. Suppose we wish to compare the effect of a single factor, the concentration of perchloric acid in aqueous solution, at three different levels or treatments (0.1 M, 0.5 M, and 1.0 M) on the fluorescence intensity of quinine (which is widely used as a primary standard in fluorescence spectrometry). Let us suppose that four replicate intensity measurements are made for each treatment, i.e. in each perchloric acid solution. Instead of making the four measurements in 0.1 M acid, followed by the four in 0.5 M acid, then the four in 1 M acid, we make the 12 measurements in a random order, decided by using a table of random numbers. Each treatment is assigned a number for each replication as follows ... 

There are separate ISO standards for primary and secondary alkyl sulfates (i.e., for products made from primary or secondary alcohols). The ISO standard covering primary alkyl sulfates specifies neutral oil by extraction from ethanol/water solution, total combined alcohols by ether extraction after acid hydrolysis, pH of a 10% aqueous solution, water by titration if below 10% or by azeotropic distillation if above 5%, and chloride by titration (60). The standard covering secondary alkyl sulfates is similar, but combined alcohols are not determined. Assay is by determination of total solids after extraction of the neutral oil, corrected for the presence of other impurities (61). 

The single largest use of ammonia is its direct apphcation as fertdizer, and in the manufacture of ammonium fertilizers that have increased world food production dramatically. Such ammonia-based fertilizers are now the primary source of nitrogen in farm soils. Ammonia also is used in the manufacture of nitric acid, synthetic fibers, plastics, explosives and miscellaneous ammonium salts. Liquid ammonia is used as a solvent for many inorganic reactions in non-aqueous phase. Other apphcations include synthesis of amines and imines as a fluid for supercritical fluid extraction and chromatography and as a reference standard in i N-NMR. 

In aqueous solution, primary interest centers on the production of CO, formic acid, methanol and alcohols, and methane and hydrocarbons. The standard redox potentials (versus the saturated calomel electrode, SCE) for the common C02 reduction products of formic acid, CO, formaldehyde, methanol, and methane in aqueous solution at pH 7.0 are given as [42] ... 

Coupling of the free amino derivative with Fmoc protected commercially available amino acids (alanine and phenylalanine) was accomplished via standard reaction conditions activated by DCC in anhydrous tetrahydrofiiran (THF) solution. Deprotection of synthesized peptides by treatment with aqueous methanol solution and catalytic amount of trimethylamine produced a new class of 5-thio-carbo peptides in 86% yield. This particular family of new stable peptidomimetics is conveniently protected and could be used for further additional functionalization at the primary -OH at C-6 of the thioglucose moiety. Further deprotection of the 1,2-0-isopropylidene block created another... 

The details of hydrogen transfer from carbon are unclear. Analogy with other reactions of this type, particularly those of TEMPO, would suggest a cyclic transition state, characterised by primary hydrogen effects of 2, under basic conditions and a general base-catalysed process under acidic conditions, characterised by isotope effects around 5. However, the cyclic mechanism was also proposed for the standard oxidation conditions of aqueous acetic acid, originally on the basis of a rate decrease in aqueous acetic acid as sodium acetate was added, despite such an addition possibly deprotonating the... 

Now let us return to the approaches connected with the estimation of the primary medium effect for protons, log y0 n+, that are used for obtaining quantitative information on the acidity of pure protolytic or aprotic solvents relative to the standard solution of a strong acid in water. From the thermodynamics, these are known to be a measure of the Gibbs free energy of proton transfer from the standard solution in water to the one in a non-aqueous solvent (M). This parameter is connected with the energy of proton resolvation in the following way ... 

Following the discovery of the hydrated electron in radiation chemistry, the reexamination of some fields of aqueous chemistry gave rise to a new concept of primary reduction processes. This paper surveys aspects of these investigations in which it appears that e aq, as opposed to its conjugate acid (H atom), is invariably the precursor to H2 when water is reduced. Evidence is reviewed for the production of e aq (a) photochemically, (b) by chemical reduction of water, (c) electrolytically, (d) by photo-induced electron emission from metals, (e) from stable solvated electrons, and (f) from H atoms. The basis of standard electrode potentials and various aspects of hydrated electron chemistry are discussed briefly. 

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