Solution basicity measurement

The acidity constant for an acid-base indicator was determined by preparing three solutions, each of which has a total indicator concentration of 5.00 X 10- M. The first solution was made strongly acidic with HCl and has an absorbance of 0.250. The second solution was made strongly basic and has an absorbance of 1.40. The pH of the third solution was measured at 2.91, with an absorbance of 0.662. What is the value of K, for the indicator ... 

Another example of enhanced sensitivity to substituent effects in the gas phase can be seen in a comparison of the gas-phase basicity for a series of substituted acetophenones and methyl benzoates. It was foimd that scnsitivtiy of the free energy to substituent changes was about four times that in solution, as measured by the comparison of A( for each substituent. The gas-phase data for both series were correlated by the Yukawa-Tsuno equation. For both series, the p value was about 12. However, the parameter r" ", which reflects the contribution of extra resonance effects, was greater in the acetophenone series than in the methyl benzoate series. This can be attributed to the substantial resonance stabilization provided by the methoxy group in the esters, which diminishes the extent of conjugation with the substituents. 

In the discussion of the relative acidity of carboxylic acids in Chapter 1, the thermodynamic acidity, expressed as the acid dissociation constant, was taken as the measure of acidity. It is straightforward to determine dissociation constants of such adds in aqueous solution by measurement of the titration curve with a pH-sensitive electrode (pH meter). Determination of the acidity of carbon acids is more difficult. Because most are very weak acids, very strong bases are required to cause deprotonation. Water and alcohols are far more acidic than most hydrocarbons and are unsuitable solvents for generation of hydrocarbon anions. Any strong base will deprotonate the solvent rather than the hydrocarbon. For synthetic purposes, aprotic solvents such as ether, tetrahydrofuran (THF), and dimethoxyethane (DME) are used, but for equilibrium measurements solvents that promote dissociation of ion pairs and ion clusters are preferred. Weakly acidic solvents such as DMSO and cyclohexylamine are used in the preparation of strongly basic carbanions. The high polarity and cation-solvating ability of DMSO facilitate dissociation... 

This technique provides quantitative information about tautomeric equilibria in the gas phase. The results are often complementary to those obtained by mass spectrometry (Section VII,E). In principle, gas-phase proton affinities, as determined by ICR, should provide quantitative data on tautomeric equilibria. The problem is the need to correct the measured values for the model compounds, generally methyl derivatives, by the so-called N-, 0-, or S-methylation effect. Since the difference in stability between tautomers is generally not too large (otherwise determination of the most stable tautomer is trivial) and since the methylation effects are difficult to calculate, the result is that proton affinity measurements allow only semi-quantitative estimates of individual tautomer stabilities. This is a problem similar to but more severe than that encountered in the method using solution basicities (76AHCS1, p. 20). 

The linearity of the detector can be obtained by diluting the analyte stock solution and measuring the associated responses, while the linearity of the analytical method can be determined by making a series of concentrations of the analyte from independent sample preparations (weighing and spiking) [15]. It is also essential that the basic calibration linear curve be obtained by using independent samples, and not by using samples that have been prepared by dilution and injected into HPLC/GC, or spotted on one TLC plate. 

To understand VD at its most basic element, begin by considering a vessel with no volume markings on it that contains an unknown volume of solvent. Into this volume of solvent is dissolved a known mass of solute. Then, a sample of the solution is removed and the concentration of the solute is measured. By knowing the mass of solute added and subsequently measuring the concentration of the solution, the volume of the solvent can be computed ... 

O <2ZD In a titration, a basic solution is added to an acidic solution, and measurements of pH are taken. Compare a strong acid-strong base titration and a strong acid-weak base titration in terms of... 

Basicity in the gas phase is measured by the proton affinity (PA) of the electron donor and in solution by the pAj,. A solution basicity scale for aldehydes and ketones based on hydrogen bond acceptor ability has also been established [186]. Nucleophilicity could be measured in a similar manner, in the gas phase by the affinity for a particular Lewis acid (e.g., BF3) and in solution by the equilibrium constant for the complexation reaction. In Table 8.1 are collected the available data for a number of oxygen systems. It is clear from the data in Table 8.1 that the basicities of ethers and carbonyl compounds, as measured by PA and p , are similar. However, the nucleophilicity, as measured by the BF3 affinity, of ethers is greater than that of carbonyl compounds, the latter values being depressed by steric interactions. 

The oxygen and sulfur bases are weaker than the nitrogen bases, and accurate solution basicities are not available. Arnett s heat of protonation studies indicate that the order of decreasing basicity is ROR > ROH > HaO,98 a result that is in agreement with gas-phase measurements.99 Hydrogen sulfide in the gas phase has basicity comparable to that of water (Table 3.18), and substitution of H by alkyl produces stronger gas-phase bases just as does similar substitution on oxygen. 

Measurements that can be expressed as a mole/mole ratio, the most basic measurements in chemistry, are typified by processes which react, interact, blend, or replace a described amount of substance A with a described amount of substance B. Included are solution concentration measurements when all solutes are known in a known solvent. Note especially that these measurements are independent of the magnitude of the unit mole. Note also that if these measurements are made by mass or volume determinations, the uncertainties in the corresponding atomic or molecular mass values must be taken into account. 

The 1,3,4-thiadiazole ring system, with three heteroatoms, does not exhibit tautomerism in its fully conjugated form. However, when certain substituents are present, tautomerism is possible. l,3,4-Thiadiazolin-2-ones (39 X = 0) and -2-thiones (39 X = S) exist in the oxo and thione forms, respectively, as shown by spectroscopic and LCAO-MO calculations. 2-Amino-l,3,4-thiadiazoles exist in the amino form in solution and in the solid state the Kt value is 10s as shown by basicity measurements. UV spectroscopy and LCAO-MO calculations show that the amino tautomer is also the main species when there is an alkoxy group in the 5-position (40 R1 = alkoxy, R2 = NH2), or if the exocyclic nitrogen atom is part of a hydrazone group (40 R2 = NHN=CR2). 

The acidity or basicity of an aqueous (water) solution is measured by the concentration of H30+. This value also implies the concentration of OH because these two concentrations are related by the water ion-product constant ... 

It follows that a standard crystal with a natural frequency of 9 MHz and a surface area of about one square centimeter will manifest a change in frequency of about 200 Hz for each microgram of adsorbed solute. Now frequency changes can be measured to within 0.1 Hz with normal equipment consequently a change in mass adsorbed of 0.2 ng (lO g) should be detectable. It follows that this type of device should be very sensitive but it appears that, so far, it has not been made available commercially, at least, not as a GC detector. One attractive feature of this detecting system is that it basically measures mass and therefore could be considered to be an absolute detector. 

A 0.4 g sample of dry adsorbent was added to 20 mL of deionized water and the suspension stirred overnight to reach equilibrium. The sample was filtered and the pH of solution was measured using an Accumet Basic pH meter (Fisher Scientific, Springfield, NJ, USA). Thermal analysis... 

The book is designed to introduce a basic knowledge of solutions and measurement of their concentration. We will study principally the nature of solutions, their components with their physical and chemical properties. This book will also show you how crucial to life are solutions and how important is measuring concentration. 

The anionic thiocyanate complex of ruthenium(IIl), Ru(SCN)6, reacts with Capri Blue (basic oxazine dye, formula 4.31) to form a sparingly soluble ion-associate which precipitates at the phase boundary and on the wall of the separating funnel during shaking of the aqueous phase with DIPE. After separation, the ion-associate is dissolved in methanol and the absorbance of the solution is measured [41]. 

Unsubstituted and N-substituted bipyridyls 76 (R = H, Me, Ph, MeCO, 4-MeC6H4S02) were investigated for amino-imino tautomerism using basicity measurements and UV spectral data. The bipyridyls 76 (R = H, Me, MeCO) were shown to exist predominantly in the amino form both in aqueous solution and in non-polar media. The tautomeric equilibrium of 76 (R = 4-McC6H4S02), however, depends on the solvent polarity whereas the imino tautomer predominates in water and ethanol, significant amounts of the amino tautomer were observed in dioxane and cyclohexane (78JCS(P2)1215). 

Metals react with acids and bases. Acids are chemical compounds that donate hydrogen ions (H+) to other atoms. Bases are compounds that accept these ions and are often oxides or hydroxides (OH ). An alkali is a base that can be dissolved in water. Acids and bases react with each other to produce a salt. This reaction typically occurs in a water, or aqueous, solution. The concentration of hydrogen ions in solution is measured on the pH scale. Solutions with pH levels below 7 are acidic, with acidity increasing as the pH decreases. As the pH increases above 7, the solution becomes more basic. A pH of 7 indicates that a solution is neutral. 

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