Water bases, ionization

Neutralization. In water, lime ionizes readily to Ca ", Mg ", and OH , forming a strong base or alkaU. Both Ca(OH)2 and Mg(OH)2 are strong diacid bases neutralizing such strong monobasic acids as HCl and HNO, yielding neutral salts and heat. 

C. Aguilar, I. Feirer, R Bonnll, R. M. Marce and D. Barcelo, Monitoring of pesticides in river water based on samples previously stored in polymeric cartridges followed by on-line solid-phase extraction-liquid cliromatography-diode array detection and confirmation by atmospheric pressure chemical ionization mass spectrometry . Anal. Chim. Acta 386 237-248 (1999). 

The ability of water to ionize, while shght, is of central importance for life. Since water can act both as an acid and as a base, its ionization may be represented as an intermolecular proton transfer that forms a hydronium ion (HjO ) and a hydroxide ion (OH ) ... 

In the presence of a base B, water acts as an acid by donating a proton, and the equiiibrium constant for the transfer of a proton from water to a base is caiied the base ionization constant (Zjj) ... 

Most reported triazine LC applications are reversed-phase utilizing C-8 and C-18 analytical columns, but there are also a few normal-phase (NH2,CN) and ion-exchange (SCX) applications. The columns used range from 5 to 25-cm length and from 2 to 4.6-mm i.d., depending on the specific application. In general, the mobile phases employed for reversed-phase applications consist of various methanol and/or acetonitrile combinations in water. The ionization efficiency of methanol and acetonitrile for atmospheric pressure chemical ionization (APcI) applications were compared, and based on methanol s lower proton affinity, the authors speculated that more compounds could be ionized in the positive ion mode when using methanol than acetonitrile in the mobile phase. 

Mole ratios can be used to determine the amount of one substance needed to react with a given amount of another substance. In this experiment, you will react a substance called an acid with another substance called a base. Acids can be defined as substances that dissociate and produce hydrogen (H+) ions when dissolved in water. Bases are substances that ionize to produce hydroxide (OH ) ions when they dissolve in water. When acids and bases react with each other, the H+ ions and OH ions join to form water (H20). The resulting solution no longer has an excess of either H+ ions or OH- ions. The solution has become neutral. This process is called neutralization. By using the mole ratio of hydrogen ions and hydroxide ions in the balanced chemical equation, you can predict the point at which a solution becomes neutral. 

In the above example, the anion (A) functions as a base when it combines with a hydrogen ion. (By definition, any substance that combines with hydrogen ions is a base. Like strong acids, strong bases ionize completely in a dilute aqueous solution.) Thus NaOH dissolves in water to form hydroxide ions, which in turn function as a base when they combine with hydrogen ions to form water, as shown by the general equations... 

Table 3 gives an overview of the environmental impact categories that are taken into account in the environmental impact assessment according to the baseline method that is recommended in the Dutch LCA Handbook [1], The characterization for toxicity is based on factors derived by the Usetox model [12], The impact categories depletion of water and ionizing radiation are not taken into account because reliable characterization factors are not available. 

Bases are those substances that ionize in water to release OH- ions. NaOH is a strong base, ionizing completely in water to yield Na+ and OH- ions. However, even those hydroxide bases that do not dissolve... 

Bases accept (or take) hydrogen ions. In water, the strongest possible base is the hydroxide ion, OH-. A strong base ionizes essentially 100% to produce the OH- ion, so a strong base is a soluble ionic hydroxide. Like all soluble ionic compounds, ionic hydroxides are strong electrolytes and dissociate completely... 

The modeling data based on nonsteady-state equilibrium predict that volatilization of 4-nitrophenol will be insignificant (Yoshida et al. 1983). The Henry s law constant (H) values for these two compounds (see Table 3-2) and the volatility characteristics associated with various H values (Thomas 1982) can be used to predict that volatilization from water will not be important. The dissociation constant (pKa) values of the two compounds (see Table 3-2) indicate that significant fractions of these nitrophenols will be dissociated at pHs above 6. Since ionic species do not volatilize significantly from water, the ionization may further limit volatilization. 

The neutral pH of 7 is based on the fact that water self-ionizes to form hydronium (hydrogen) ions and hydroxide ions in equal amounts. The concentration of these ions is equal to 1.0 X 10 7, which corresponds to a pH of 7. 

A substance that produces OH (aq) ions in aqueous solution. Strong soluable bases are soluble in water and are completely dissociated. Weak bases ionize only slightly. 

Figure 2 The role of the zinc and its bound water in catalysis. The water can ionize either (a) without or (b) with the help of an adjacent base, B, supplied by an amino acidresidue. The zinc-bound water can also (c) be displaced by substrate, S or (d) the coordination sphere be expanded upon interaction with substrate...

Although a nitro group has a large effect on thermodynamic acidity, rates of proton transfer are not increased by similar amounts. For nitroethane (pA- 8.60 [99]), water catalysed ionization occurs with a rate coefficient [37] of 3.7 x 10"8 sec-1 compared with the value of 1.32 x 10 2 sec-1 for acetylacetone [89] (pif 8.9 [17]) and the value (ca. 102 sec"1) which would be observed for a normal acid [59] with pK 8.5. Since nitroparaffins ionize slowly the reaction may be followed using classical methods and rates of proton transfer to a wide variety of bases have been measured nitroparaffins [100] and ketones were among the first carbon acids to be studied. Corrections may be required for the presence of the aci-isomer. Figure 4 shows results for the rates of base catalysed ionization of ethyl nitroacetate [101] obtained by spectro-photometric measurement of the increase in concentration of the anion (74) or by measuring the rate of bromination, (74) and (75), viz. 

A strong base is a substance that reacts completely with water, so that the base ionization constant defined in Eq. (14) or (15) is effectively infinite. This situation can only be realized if the conjugate acid of the strong base is very weak. A weak base will be characterized by a base ionization constant that is considerably less than unity, so that the position of equilibrium in the reaction represented in Eq. (12) favors the existence of unreacted free base. 

When a weak acid, HA, is dissolved in water, it ionizes as shown previously to foiTJi HjO and the corresponding conjugate base. A". A JL. expression can be written for the ionization. 

If we start with the conjugate base, A , and dissolve it in water, it ionizes as a typical base it accepts a proton from H2O to form OH and the corresponding conjugate acid, HA. A Kb expression can be written for this ionization. 

In contrast to strong bases, a weak base ionizes only partially in dilute aqueous solution to form the conjugate acid of the base and hydroxide ion. The weak base methylamine (CH3NH2) reacts with water to produce an equilibrium mixture of CH3NH2 molecules, CH3NH3+ ions, and OH ions. 

Base ionization constants You won t be surprised to leam that like weak acids, weak bases also form equihbrium mixtures of molecules and ions in aqueous soluhon. Therefore, the equilibrium constant provides a measure of the extent of the base s ionization. The equilibrium constant for the ionization of methylamine in water is defined by this equilibrium constant expression. 

Write the equation for the ionization reaction and the base ionization constant expression for ethylamine (C2H5NH2) in water. 

Unless conditions require the use of the exact solution, approximate equations are preferable because they are easier to apply and provide greater physical insight. If a calculation (ignoring water autoionization) of the ionization of a weak acid gives a concentration of H30 smaller than 10 M or if a calculation of base ionization gives a concentration of OH smaller than M, then we have to use the more exact treatment. For buffer solutions, a pH near 7 does not necessarily mean that water ionization is important, unless the acid or base concentration becomes very small. 

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