Hydroxyl ion concentration

Aqueous ammonia can also behave as a weak base giving hydroxide ions in solution. However, addition of aqueous ammonia to a solution of a cation which normally forms an insoluble hydroxide may not always precipitate the latter, because (a) the ammonia may form a complex ammine with the cation and (b) because the concentration of hydroxide ions available in aqueous ammonia may be insufficient to exceed the solubility product of the cation hydroxide. Effects (a) and (b) may operate simultaneously. The hydroxyl ion concentration of aqueous ammonia can be further reduced by the addition of ammonium chloride hence this mixture can be used to precipitate the hydroxides of, for example, aluminium and chrom-ium(III) but not nickel(II) or cobalt(II). 

Activators enhance the adsorption of collectors, eg, Ca " in the fatty acid flotation of siUcates at high pH or Cu " in the flotation of sphalerite, ZnS, by sulfohydryl collectors. Depressants, on the other hand, have the opposite effect they hinder the flotation of certain minerals, thus improving selectivity. For example, high pH as well as high sulfide ion concentrations can hinder the flotation of sulfide minerals such as galena (PbS) in the presence of xanthates (ROCSS ). Hence, for a given fixed collector concentration there is a fixed critical pH that defines the transition between flotation and no flotation. This is the basis of the Barsky relationship which can be expressed as [X ]j[OH ] = constant, where [A ] is the xanthate ion concentration in the pulp and [Oi/ ] is the hydroxyl ion concentration indicated by the pH. Similar relationships can be written for sulfide ion, cyanide, or thiocyanate, which act as typical depressants in sulfide flotation systems. 

Diazophenols, ie, o-hydroxyaryldiazonium salts, couple to 1-naphthol in weaMy basic solution primarily in the para position, but as the hydroxyl ion concentration is increased, formation of the ortho isomer is favored and is frequentiy the sole product. Pyridine and pyridine derivatives, urea, and acetate, etc, used as buffers can also catalyze azo coupling reactions (28). l-amino-2-naphthol-4-sulfonic acid [116-63-2] (1,2,4-acid) and 1-naphthol yield the important Eriochrome Black A [3564-14-5] (18a, R = H) (Cl Mordant Black 3 Cl 14640) which is reportedly (20) a mixture of ortho and para isomers. 

The hydrogen ion and hydroxyl ion concentrations are given in moles per liter at 25""C. 

Combination of the hydroxyl ion with the mesomeric cation involves the removal of a double bond. For the quaternary pyridinium compounds this causes the total loss of the aromaticity. For quaternary quinolinium and isoquinolinium compounds, the aromatic character of one of the two rings is lost, and for the quaternary acridinium compounds that of one out of three. Hence., the order of stabilities of these compounds (determined by Hantzsch ) is explained. - Comparison of quaternary 3,4-dihydroisoquinolinium compounds and their isoquinolinium analogs with respect to the equilibrium (5) (4) shows that a much higher hydroxyl ion concentration is necessary for the isoquinolinium ions to form the carbinolamine. This is because the transition from the quaternary 3,4-dihydroisoquinolinium ions into the undissociated carbinolamine involves significantly smaller loss of mesomeric energy than that for the quaternary isoquinolinium hydroxides. ... 

At a the net anodic reaction rate is zero (there is no metal dissolution) and a cathodic current equal to I" must be available from the external source to maintain the metal at this potential. It may also be apparent from Fig. 10.4 that, if the potential is maintained below E, the metal dissolution rate remains zero = 0), but a cathodic current greater than /"must be supplied more current is supplied without achieving a benefit in terms of metal loss. There will, however, be a higher interfacial hydroxyl ion concentration. 

In the experiments descrihed above no tendency was found for the (ClOi)- ion to form a molecular ion by combination with Fe+++. The absorption spectrum characteristic of the Fe+++ ion in aqueous solution was therefore determined by studying solutions of FeC104 as a function of the hydroxyl-ion concentration. 

The pH-dependence of the rate is complicated. The rate increases very rapidly between pH values of 8 and 10 and the authors correlate this with the literature value of p bh+ of benzylamine of 9.34. The rate levels off at pH 11-12, but at pH 13-14 a reaction the rate of which is linearly dependent on hydroxyl ion concentration occurs. 

The above reactions show hydroxyl ion formation in the dissolution. Uranates will precipitate when the hydroxyl ion concentration becomes sufficiently high, according to the reaction ... 

G. Horvai and E. Pungor, Amperometric determination of hydrogen and hydroxyl ion concentrations in unbuffered solutions in the pH range 5-9. Anal. Chim. Acta 243, 55-59 (1991). 

The polymerization is an anionic mechanism initiated by hydroxide ions or any bases present [42], The reaction scheme can be seen in Figure 5. The polymerization rate is regulated by hydroxyl ion concentration and hence is carried out at pH values below 3.5. Above this pH, the reaction rate is too rapid to allow discrete particle formation [55, 56]. 

Figure 1. Base hydrolysis of 30 ppb solution of carbofuran with various hydroxyl ion concentrations at 15 °C. 1) 3.73 x 10" M,...

The hydroxyl ion concentration is initially reduced due to the retardation of the C3S hydration to form Ca (0H)2 and the sudden increase in hydroxyl ion concentration is smoothed out probably due to the gradual breakdown of the inhibiting admixture monolayer to give a faster diffusion of hydration products. 

The alkali metal ions Na+ and K+ behave in a similar manner to the hydroxyl ion concentration, again due to the delay in C3S hydration, since the majority of the soluble alkaline metal ions originates from the... 

When present in micelles, ester quats hydrolyze faster than free unimers in the bulk phase. This is due to an increased hydroxyl ion concentration around the micelle, i.e., the local pH in the vicinity of the micelle surface is higher than in the bulk. The phenomenon is referred to as micellar catalysis and is further discussed in the Betaine esters section. 

Hackh s Dictionary, p 422 (Ref 13) gives a table of relation between pH, pOH, Hydrogen-ion concentration (normality) and hydroxyl ion concentration (normality)... 

Under conditions of constant water and hydroxyl ion concentrations, Eq. (19) becomes ... 

Pour 3 ml of a 1 W alkali solution into each of two test tubes. Add about 1 g of solid ammonium chloride to one of them and dissolve it by shaking the tube. Pour 2 ml of a magnesium chloride solution into both tubes. Why does no precipitate form in one case Explain the observed phenomenon proceeding from the change in the hydroxyl ion concentration. 

Explain why the equality of the hydrogen ion and hydroxyl ion concentrations is violated when certain salts are dissolved in water. Compare the values of the dissociation constants of water, acetic acid, carbonic acid, the bicarbonate ion, and aluminium hydroxide. How can the hydrolysis process be explained from the viewpoint of the law of mass action In what cases is hydrolysis reversible and in what cases does it proceed virtually to the end ... 

A final explanation of the slow deswelling is that above pH 6.5, the gel must deprotonate by transferring its protons to hydroxyl ions and buffer species. Except at very high pH, hydroxyl ion concentrations are quite low. Neither citrate nor phosphate buffer can be very helpful. The highest pK for citrate is approximately 6.4, well below the pKa of the gel amines (7.8). Thus proton transfer from amine to citrate is not favored. Phosphate buffer has pK values 2.15,7.10, and 12.12. The first two pKs are lower than 7.8, so the corresponding phosphate groups are weaker bases than the gel amines. At and below pH 11.0, the phosphate group with pK = 12.12 will retain its acidic proton, and therefore will also not be able to extract a proton from the gel. 

Clearly the position of the homogeneous reaction (la) will depend on the concentration of the free metal ions which can be modified by an auxiliary complexing (or masking) agent (see Section 10.3). It will move increasingly to the right as the (overall) stability constant of the complex, ML , increases and to the left as the solution becomes more acidic. Increase of pH should lead to more complete reaction but since this implies a concomitant increase in hydroxyl ion concentration there will now be increasing competition between the tendencies of IT and OH- to coordinate to the cation basic species and even metal hydroxides may form and precipitate. 

The composition of cuprammonium varies widely with ammonia concentration7 and, probably, with hydroxyl ion concentration as well. 

To 1 cc. of a magnesium sulphate solution add 5 cc. of water and then a few drops of sodium hydroxide solution. Look up the solubility and the degree of ionization of magnesium hydroxide (page 101). Calculate the hydroxyl-ion concentration in a saturated solution of Mg (OH)2. 

Make a list of the percentage of ionization of the hydroxides of the alkali metals, of ammonium, and of the alkaline earth metals in 0.1 A1 solution if the substance is soluble to that extent. Give figures for the hydroxyl-ion concentration in 0.1 A NH4OH solution and in saturated solutions of Ca(OH)2 and Mg(OH)2. Describe and discuss the results of Experiment 4 in the light of these figures. 

---