Dihydrogen phosphate anions

C17-0018. Identify the conjugate acid and the conjugate base of the dihydrogen phosphate anion, H2 PO4, and the hydrogen phosphate anion, HP04. ... 

This situation is relatively common. If we look, for example, at the values of Ka in Table 6.5, we see that phosphoric acid is a strong acid insofar as the loss of the first proton occurs with Ka = 7.5 x 10 3, but the loss of the second proton, to form HPOq-, is difficult, as characterized by Ka = 6.2 x 10-8. In other words, the dihydrogen phosphate anion H2PO4 is a very weak acid. And the hydrogen phosphate di-anion HPO4- has a low a value of Ka = 2.2 x 10-11, causing us to say that the POq- anion does not normally exist. Even the loss of the second proton of sulphuric acid is characterized by a modest value of Ka = 10-2. 

The results of electrochemical investigations are summarized in Table 19. It is noteworthy that chloride, by virtue of its higher charge density, causes relatively larger cathodic perturbations than bromide. Interestingly, as observed with monosubstituted cobaltocenium derivatives, the dihydrogen phosphate anion produces the largest cathodic shifts. 

Beer, P. D., Szemes, F., Remarkable chloride over dihydrogen phosphate anion selectivity exhibited by novel macrocyclic bis [ruthenium(II) bipyridyl] and ruthenium(II) bipyridyl-metallocene receptors. J. Chem. Soc., Chem. Commun. 1995, 2245-2247. 

The comparison of lower rim tetrasubstituted calix[4]arene 13a with the corresponding pentasubstituted calix[5]arene 13b (both compounds immobilised in the cone conformation) revealed that the higher number of ferrocene units in 13b does not positively influence the complexation ability of the receptor. The stability constants of 13a and 13b with chloride anion were found to be 55 and 15 M"1, respectively. As indicated by cyclic voltammetry, both receptors are selective towards the dihydrogen phosphate anion (cathodic shift -160 mV). 

Table 2. Thermodynamic parameters of complexation of calixpyrrole receptors with halides and dihydrogen phosphate anions (tetra-n-butylammonium as counterion) in acetonitrile, dichloromethane, A/,A/-dimethylformamide, dimethyl sulphoxide and propylene carbonate... 

Of all the compounds described above only the tetrakis-O-galactosyl derivative 32a was used in complexation studies68 with D-glucosamine hydrochloride and tetrabutylammonium dihydrogen phosphate. It forms 1 1 and 1 2 (host-guest) complexes with D-glucosamine and 1 1 complexes with the dihydrogen phosphate anion (Ka = 31 4M , DMSO-d6). 

Notice the diversity in structure of lhe.se proton dunurs. They include the classical hydrochloric acid (reaction a). Ihc weakly acidic dihydrogen phosphate anion (reaction b). the ammonium cation as is found in ammonium chloride (reaction c), the carboxylic acetic acid (reaction d). Ihc cnolic form of phenobarbital (reaction e), Ihe carboxylic acid moiety of indomelhacin (reaction j), Ihc imidc of saccharin (reaction g). and the prolonaied amine of ephedrine (reaction h). Because all are proton donors, they mu.st be treated as acids when calculating the pH uf a solution or percent ionization of the drug. At the same lime, as nuted below, there are important differences in the pharmaceutical properties of ephedrine hydrochloride (an acid salt of an amine) and lho.se of indomelhacin. phenobarbital. or saccharin. 

Figure 16. Crystal structure of receptor 124 with two bound dihydrogen phosphate anions (tetra-butylammonium cations and solvent are omitted for clarity). [Reprinted with permission from D. W. Rudkevich, W. Verboom, Z. Brzozka, M. J. Palys, P. R. V. Stauthamer, G. J. von Hummel. S. M. Franken. S. Harkema, J. F. Engbersen. and D. N. Relnhoudt. J. Am. Chem. Soc., 116. 4341 (1994). Copyright American Chemical Society.]...

We recently incorporated the ruthenium(II) bipyridyl moiety into acyclic, macrocyclic, and lower rim caUx[4Jarene structural frameworks to produce a new class of anion receptor capable of optical and electrochemical sensing (226, 253. 254). Stability constant determinations in DMSO using H NMR titration techniques demonstrated that these acyclic receptors (131 and 132) form strong complexes with chloride and dihydrogen phosphate anions (stronger than with analogous monopositive cobaltocenium based receptors). The ruthenium ion is dipositive and hence the electrostatic interactions are particularly favorable. The 4,4 -substituted ruthenium bipyridyls were observed to bind anions more... 

We also prepared an acyclic mixed-ruthenium(II) bpy-ferrocene receptor 135 (257). The emission of the ruthenium center in the free receptor is quenched by the ferrocene units. However, on addition of dihydrogen phosphate anion the emission increases 20-fold, being switched on by the presence of the anion... 

Upper rim calix[4]arenes functionalized with two and four ruthenium(II) bi-pyridyl amide groups (136 and 137) have very recently been prepared (258). These receptors also sense the presence of dihydrogen phosphate anions selectively. 

Naphthalene and binaphthalene appended with thiourea groups (24 and 25, respectively) have been synthesized by Kondo and co-workers in order to investigate potential cooperative binding between two thiourea groups in 25 [38]. This group found that 1 1 complexes were formed between 25 and fluoride, acetate and dihydrogen phosphate anion, which was confirmed by Job plots in acetonitrile and ESI-MS. 

Beer. P.D. Chen. G.Z. Goulden. A.J. Graydon, A.R. Stokes. S.E. Wear, T. Selective electrochemical recognition of the dihydrogen phosphate anion in the presence of hydrogen sulfate and chloride ions by new neutral ferrocene anion receptors. Chem. Commun. 1993, 24. 1834. 

Inorganic mono or dihydrogen phosphate anions are sometimes represented as Pi and acid pyrophosphate anions as PPi. 

Cytc ILs with dicyanamide, sacchaiinate, and dihydrogen phosphate anions FTIR Retention of secondary structure to extreme temperatures. The nature of the anion appears to be the important factor [112]... 

The buffering effect of the dihydrogen phosphate anion makes Cyastat AA-SP more suitable for alkaline systems, such as nonacid cured coating systems. At concentration loadings of 1% in acid cured coating systems, cure interference may occur. 

The anion after the first dissociation, H2P04, is the dihydrogen phosphate anion. The anion after the second dissociation, HP04, is the hydrogen phosphate anion. The anion after the third dissociation, P04, is the phosphate or orthophosphate anion. For each of the dissociation reactions shown above, there is a separate acid dissociation constant, called. ai, Ko2, given at 25 °C. Associated with these three dissociation constants... 

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