Acid-base chemistry indicators

Another example of a case where acid-base chemistry competes with cyclization is found in efforts to construct an analog of the Corey lactone [45], The enantiomerically pure unsaturated ester 91 was assembled and subjected to the conditions indicated in Eq. (26). In this instance, dimethyl methylmalonate was used as the proton donor to avoid 1,4-addition of the conjugate base to 91. Cyclization afforded a combined 77% isolated yield of the y-hydroxy ester 92 and the lactone 93 the former could be converted to the lactone in the... 

Adenosine is similar to cytosine in its acid-base chemistry N-l, adjacent to the -NH2 group, is the principal site of protonation. A tautomer of the cation protonated at N-3 is formed in smaller amounts. Guanosine is electronically more complex, being protonated mainly at N-7 and to a lesser extent at N-329. This can be understood in terms of electronic interaction with the adjacent oxygen as indicated in the resonance structure to the right in the following diagram ... 

As indicated by the flow diagram in Figure 3.5, using molarity is critical for carrying out stoichiometry calculations on substances in solution. Molarity makes it possible to calculate the volume of one solution needed to react with a given volume of another solution. This sort of calculation is particularly important in acid-base chemistry, as shown in Worked Example 3.14. 

Hardness and softness as chemical concepts were presaged in the literature as early as 1952, in a paper by Mulliken [138], but did not become widely used till they were popularized by Pearson in 1963 [139]. In the simplest terms, the hardness of a species, atom, ion or molecule, is a qualitative indication of how polarizable it is, i.e. how much its electron cloud is distorted in an electric field. The adjectives hard and soft were said to have been suggested by D.H. Busch [140], but they appear in Mulliken s paper [138], p. 819, where they characterize the response to spatial separation of the energy of acid-base complexes. The analogy with the conventional use of these words to denote resistance to deformation by mechanical force is clear, and independent extension, by more than one chemist, to the concept of electronic resistance, is no surprise. The hard/soft concept proved useful, particularly in rationalizing acid-base chemistry [141]. Thus a proton, which cannot be distorted in an electric field since it has no electron cloud (we ignore the possibility of nuclear distortion) is a very hard acid, and tends to react with hard bases. Examples of soft bases are those in which sulfur electron pairs provide the basicity, since sulfur is a big fluffy atom, and such bases tend to react with soft acids. Perhaps because it was originally qualitative, the hard-soft acid-base (HSAB) idea met with skepticism from at least one quarter Dewar (of semiempirical fame) dismissed it as a mystical distinction between different kinds of acids and bases [142]. For a brief review of Pearson s contributions to the concept, which has been extended beyond strict conventional acid-base reactions, see [143],... 

The reduction of the pyrimidine to dihydropyrimidine is the reverse of the oxidation reaction carried out by DHODs. The structure of the FMN/pyrimidine-binding site is very similar to the structure of L. lactis DHODs. Three Asn residues form hydrogen bonds with the nitrogens and carbonyls of the pyrimidine analogous to DHODs. DPD has an active site cysteine proposed to act in acid/base chemistry similar to Class 1 DHODs. When mutated to alanine, only 1% of the wild-type activity was retained, indicating the importance of this residue in catalysis. Secondary tritium isotope effects using 5- H-uracil were determined in both H2O and D2O an inverse isotope effect was observed in H2O and the value became more inverse in D20. " This was taken as evidence of a stepwise mechanism in which hydride transfer to C6 is followed by protonation at C5. 

As indicated above a model of DOC biodegradation including acid-based chemistry and solid carbonate precipitation/dissolution is a more realistic approach to verify the hydrogeochemical situation along the transect. 

As outlined above for the generic precipitation and dissolution also for precipita-tion/dissolution reactions including acid-based chemistry the kinetic solid carbonate precipitation/dissolution rate constant is a critical and sensitive factor influencing the Fe and Mn distribution along a given flow path. Depending from actual pH conditions at stationary conditions saturation of the solid carbonates with saturation indices Q can be expected. 

Preparation of 2 M Sodium Hydroxide (from Red Devil Lye or Diano ) Preparation of Phenolphthalein Indicator (using Ex-Lax ) pH CMbration of C abbage Juice Indicator pH Determination of Some Household Products Acid/Base Chemistry (with consumer products)... 

Because the steady-state response of the fiber-optic asssonia probe is based on simple acid-base chemistry, the development of a function that describes the probe s steady-state response is straight forward (15, 16). The following equation gives the response function for the case when a fluorescent indicator dye is used ... 

We conclude our study of acid-base chemistry by examining acid-base titration in more detail. We learn to calculate the pH during any stage of titration involving strong and/or weak acids and bases. In addition, we see how acid-base indicators are used to determine the end point of a titration. (16.4 and 16.5)... 

The phenomena of reversible reactions and dynamic equilibria are widespread and relevant to many areas of chemistry. Consider, for instance, the whole field of acid-base chemistry (Chapters 8 and 18), where these ideas are crucial to our understanding of what an acid is, as well as to the use of indicators and buffers. In a similar way our conceptual grasp of electrochemistry is very much dependent on the interplay of reversible reactions (Chapter 19). We have commented earlier on how certain important industrial processes are dependent on some key reversible reactions. The ability to predict tbe effects of changes in physical conditions provided by Le Chatelier s principle is very useful indeed in establishing the best conditions to use for these processes. Such considerations help us to adapt conditions so as to maximize the yield of product. However, these are not the only considerations to be kept in mind. The rate at which a given yield is produced is also important economically, so the time taken to achieve a particular equilibrium is also of significance. Quite often these different considerations work in opposite directions and a compromise set of conditions is employed wbicb gives an acceptable yield in an economically viable time. 

During previous studies, the role of acid/base chemistry of the pigment was not thoroughly explored. The ammonia adsorption study indicates that the surface does have some acidic characteristic towards NH3. Ammonia has a relatively low pKb value in comparison with typically amine additives for polymer systems. As such, any acidic surface on the pigment would have some interaction with the ammonia. A HALS additive may not have the same level of interaction with a pigment surface due to higher pKb values. 

Fig. 1. A multilayer coating dry chemistry test for blood urea nitrogen (BUN) where HI and I represent the acid base forms of a pH indicator, respectively...

The first strategy is to rmdertake a superficial scan of mainstream textbooks that everyday situations have been connected to cormnon school chemistry textbooks. For example, student-exercises may contain informatiorr, about contaminants in a river such as lead salts, about acid-base indicators in plants or about food additives for the preservation of wine. However, implicit confusion may (and frequently will) occur when the textbook and the teacher aim at reaching the right answer, for example the correct calculation of the concentration of an additive in gram per litre or parts per million (ppm). Students may still pose questions such as How many glasses of wine can I drink before 1 will get sick What is the effect of alcohol on my body Why is the addition of sulphite to wine important Is the same fact tme for red wine Or even further Shouldn t the government prohibit the addition of sulphite In this way students can become personally involved in subjects that can be related to their learning of chemical substances, and even to atoms and molecules. But, the student-activities in mainstream school chemistry textbooks often are not focused on this type of involvement they do not put emphasis in the curriculum on personal, socio-scientific and ethical questions that are relevant to students lives and society. 

Phenolphthalein is another acid-base indicator. It is often used by magicians (and chemistry teachers) to perform a trick that turns water into wine. In acidic and neutral conditions, phenolphthalein is colorless and looks like water. A pH of approximately 8.3, however, turns phenolphthalein a deep reddish-violet color. In basic conditions, phenolphthalein looks like red wine. 

The sol-gel process involves the preparation of inorganic matrices via three steps. Components of the sol-gel cocktail are the sol-gel precursor (e.g. tetramethoxysilane), water, a catalyst (acids or bases), the indicator chemistry and a solvent such as ethanol. Mixing these components causes hydrolysis of the ester, silanol-ester condensation, and silanol-silanol condensation of the precursors ... 

The fascinating book Sensational Chemistry, in the Open University s Our Chemical Environment series, OU Press, Milton Keynes, 1995, is an excellent introduction to the topic. The Web has several hundred relevant sites, most of which are simple to follow just go to a reputable search engine, and type acid-base indicator . 

An acid-base titration is a laboratory procedure commonly used to determine the concentration of an unknown solution. A base solution of known concentration is added to an acid solution of unknown concentration (or vice versa) until an acid-base indicator visually signals that the end point of the titration has been reached. The equivalence point is the point at which a stoichiometric amount of the base has been added to the acid. Both chemists and chemistry students hope that the equivalence point and the end point are close together. 

Acid-base, redox, precipitation and chelometric titrations are usually dealt with in textbooks on analytical chemistry. The titration curves in these titrations can be obtained potentiometrically by use of appropriate indicator electrodes, i.e. a pH-glass electrode or pH-ISFET for acid-base titrations, a platinum electrode for redox titrations, a silver electrode or ISEs for precipitation titrations, and ISEs for... 

We now turn our attention to details of precipitation titrations as an illustration of principles that underlie all titrations. We first study how concentrations of analyte and titrant vary during a titration and then derive equations that can be used to predict titration curves. One reason to calculate titration curves is to understand the chemistry that occurs during titrations. A second reason is to learn how experimental control can be exerted to influence the quality of an analytical titration. For example, certain titrations conducted at the wrong pH could give no discernible end point. In precipitation titrations, the concentrations of analyte and titrant and the size of Ksp influence the sharpness of the end point. For acid-base titrations (Chapter 11) and oxidation-reduction titrations (Chapter 16). the theoretical titration curve enables us to choose an appropriate indicator. 

From an acid-base titration curve, we can deduce the quantities and pK.d values of acidic and basic substances in a mixture. In medicinal chemistry, the pATa and lipophilicity of a candidate drug predict how easily it will cross cell membranes. We saw in Chapter 10 that from pKa and pH, we can compute the charge of a polyprotic acid. Usually, the more highly charged a drug, the harder it is to cross a cell membrane. In this chapter, we learn how to predict the shapes of titration curves and how to find end points with electrodes or indicators. 

Bassam Z. Shakhashiri, "Color- ful Acid-Base Indicators, " Chemical Demonstrations, A Handbook for Teachers of Chemistry, Vol. 3 (The University of Wisconsin Press, Madison, 1989), pp. 33-40. 

The pL-independent IEs have therefore been interpreted580 as indicating that DOD shifts the acid-base equilibrium from (Fem—OO-) to the protonated intermediate (Fenl—OOH) and increases in that way the rate of synthesis of products formed via oxene chemistry (inverse SDIE) and decreases the rate of products formed through the peroxide chemistry... 

Blood chemistry. Pretest, and at least prior to termination, electrolytes and electrolyte balance, acid-base balance, glucose, urea nitrogen, serum lipids, serum proteins (albumin-globulin ratio), enzymes indicative of organ damage such as transaminases and phosphatases should be measured. Toxicant and metabolite levels should be assessed as needed. 

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