Diprotic monoprotic

Triprotic Acids and Bases, and Beyond The treatment of a diprotic acid or base is easily extended to acids and bases having three or more acid-base sites. For a triprotic weak acid such as H3PO4, for example, we can treat H3PO4 as if it was a mono-protic weak acid, H2P04 and HP04 as if they were intermediate forms of diprotic weak acids, and P04 as if it was a monoprotic weak base. 

Thus, if we titrate a monoprotic weak acid with a strong base, the EW and FW are identical. If the weak acid is diprotic, however, and we titrate to its second equivalence point, the FW will be twice as large as the EW. 

If the weak acid is monoprotic, then the FW must be 58.78 g/mol, eliminating ascorbic acid as a possibility. If the weak acid is diprotic, then the FW may be either 58.78 g/mol or 117.6 g/mol, depending on whether the titration was to the first or second equivalence point. Succinic acid, with a formula weight of 118.1 g/mol is a possibility, but malonic acid is not. If the analyte is a triprotic weak acid, then its FW must be 58.78 g/mol, 117.6 g/mol, or 176.3 g/mol. None of these values is close to the formula weight for citric acid, eliminating it as a possibility. Only succinic acid provides a possible match. 

For the primary stage (phosphoric) V) acid as a monoprotic acid), methyl orange, bromocresol green, or Congo red may be used as indicators. The secondary stage of phosphoric) V) acid is very weak (see acid Ka = 1 x 10 7 in Fig. 10.4) and the only suitable simple indicator is thymolphthalein (see Section 10.14) with phenolphthalein the error may be several per cent. A mixed indicator composed of phenolphthalein (3 parts) and 1-naphtholphthalein (1 part) is very satisfactory for the determination of the end point of phosphoric(V) acid as a diprotic acid (see Section 10.9). The experimental neutralisation curve of 50 mL of 0.1M phosphoric(V) acid with 0.1M potassium hydroxide, determined by potentiometric titration, is shown in Fig. 10.6. 

The theory of titrations between weak acids and strong bases is dealt with in Section 10.13, and is usually applicable to both monoprotic and polyprotic acids (Section 10.16). But for determinations carried out in aqueous solutions it is not normally possible to differentiate easily between the end points for the individual carboxylic acid groups in diprotic acids, such as succinic acid, as the dissociation constants are too close together. In these cases the end points for titrations with sodium hydroxide correspond to neutralisation of all the acidic groups. As some organic acids can be obtained in very high states of purity, sufficiently sharp end points can be obtained to justify their use as standards, e.g. benzoic acid and succinic acid (Section 10.28). The titration procedure described in this section can be used to determine the relative molecular mass (R.M.M.) of a pure carboxylic acid (if the number of acidic groups is known) or the purity of an acid of known R.M.M. 

Neutralisation reactions. The equivalent of an acid is that mass of it which contains 1.008 (more accurately 1.0078) g of replaceable hydrogen. The equivalent of a monoprotic acid, such as hydrochloric, hydrobromic, hydriodic, nitric, perchloric, or acetic acid, is identical with the mole. A normal solution of a monoprotic acid will therefore contain 1 mole per L of solution. The equivalent of a diprotic acid (e.g. sulphuric or oxalic acid), or of a triprotic acid (e.g. phosphoric( V) acid) is likewise one-half or one-third respectively, of the mole. 

STRATEGY Verify that Eq. 14 can be used by checking that S 5i> K JKal and S 5i> fC,. If so, we use Eq. 14 to determine the pH of the salts of the diprotic conjugate base (H,A ") of a triprotic acid (H SA) and the monoprotic conjugate base (HA ) of a diprotic acid (H2A). However, when the solure is a salt of an anion that has lost two protons, such as HP042-, we must adjust the expression to use the appropriate neighboring pkas. 

The basic relationships between solubility and pH can be derived for any given equilibrium model. In this section simple monoprotic and diprotic molecules are considered [26,472-484,497]. 

Equations have been published [16] which relate pKa and p0Ka values to partition coefficient (P) values for monoprotic acids and bases, and diprotic acids, bases and ampholytes. For example, P1 for a monoprotic acid is calculated from... 

M. H., Determination of acidity constants of monoprotic and diprotic acids by capillary electrophoresis,... 

Does the solid acid appear to be monoprotic or diprotic and why ... 

Surface complexation models for the oxide-electrolyte interface are reviewed two models for surface hydrolysis reactions are considered (diprotic surface groups and monoprotic surface groups) and four models for the electric double layer (Helmholtz,... 

Gouy-Chapman, Stern, and triple layer). Methods which have been used for determining thermodynamic constants from experimental data for surface hydrolysis reactions are examined critically. One method of linear extrapolation of the logarithm of the activity quotient to zero surface charge is shown to bias the values which are obtained for the intrinsic acidity constants of the diprotic surface groups. The advantages of a simple model based on monoprotic surface groups and a Stern model of the electric double layer are discussed. The model is physically plausible, and mathematically consistent with adsorption and surface potential data. 

In order to maintain the complexity of the model at a level consistent with the resolution of the experimental data, the reactivity of these surface groups has been described by relatively simple models (i) as diprotic weak acids, and (ii) as monoprotic... 

The monoprotic surface group model can be described in terms of the more familiar diprotic surface group model. In the diprotic model, the surface is thought of as an ensemble of Ng diprotic surface groups, which, under the condition of zero protonic charge, are occupied by Ns protons. 

Ka2. °r even < K 2. Such models are mathematically similar to the monoprotic model, since the diprotic model becomes similar to a monoprotic model if the acidity constants in the diprotic acid model are such that the neutral XOH group is insignificant in the material balance equations. 

The monoprotic model is appealing since it is very simple, realistic, and based on one less adjustable parameter than the diprotic model. The value of the parameter Ka can be found directly from the H+ concentration in the bulk of solution at Oq =0, since Ka = a + at this condition, according to Equation 6. Since the surface complexation models are already recognized as being underdetermined, any physically realistic model with fewer adjustable parameters is welcomed. 

The chemical model for charge in the surface plane (ctq) was given by Equations 1-4 for the diprotic model or Equations 6-8 for the monoprotic model. In general, either of these sets of equations can be represented by... 

Two models of surface hydrolysis reactions and four models of the electrical double layer have been discussed. In this section two examples will be discussed the diprotic surface group model with constant capacitance electric double layer model and the monoprotic surface group model with a Stern double layer model. More details on the derivation of equations used in this section are found elsewhere (3JL). ... 

If the constraint that is small is removed in interpretation of the data, one may consider the physical nature of the interface and other forms of experimental data in deciding what combinations of parameters are appropriate for describing the interface. In particular, the relationship of the diprotic surface group model to the monoprotic surface group model can be examined. 

Written in this form, you see that the equivalents (or milliequivalents) of acid are equal to the equivalents of base. The equivalent weight of the acid is the grams of acid divided by the equivalents of base. The equivalent weight of monoprotic acid is equal to the molecular weight. The equivalent weight of diprotic acid is equal to half the molecular weight. 

FIGURE 2-16 Conjugate acid-base pairs consist of a proton donor and a proton acceptor. Some compounds, such as acetic acid and ammonium ion, are monoprotic they can give up only one proton. Others are diprotic (H2C03 (carbonic acid) and glycine) or triprotic... 

Our approximation is confirmed by this last result. The concentration of L" is about eight orders of magnitude smaller than that of HL. The dissociation of HL is indeed negligible relative to the dissociation of H2L+. For most diprotic acids, Aj is sufficiently larger than K2 for this approximation to be valid. Even if K2 were just 10 times less than A h [H+] calculated by ignoring the second ionization would be in error by only 4%. The error in pH would be only 0.01 pH unit. In summary, a solution of a diprotic acid behaves like a solution of a monoprotic acid, with Ku = Kal. 

A buffer made from a diprotic (or polyprotic) acid is treated in the same way as a buffer made from a monoprotic acid. For the acid H2A. we can write two Henderson-Hasselbalch equations, both of which are always true. If we happen to know [H2A] and [HA ], then we will use the pA, equation. If we know [HA ] and [ A2 ], we will use the pK2 equation. 

The derivation of fractional composition equations for a diprotic system follows the same pattern used for the monoprotic system. 

The principal species of a monoprotic or polyprotic system is found by comparing the pH with the various pKa values. For PH < pKh the fully protonated species, H A, is the predominant form. For pA) < pH < pK2, the form H , A is favored and, at each successive pK value, the next deprotonated species becomes principal. Finally, at pH values higher than the highest pK, the fully basic form (A"-) is dominant. The fractional composition of a solution is expressed by a, given in Equations 10-17 and 10-18 for a monoprotic system and Equations 10-19 through 10-21 for a diprotic system. 

The figure compares the titration of a monoprotic weak acid with a monoprotic weak base and the titration of a diprotic acid with strong base. 

Uses of ionization constants to compute concentrations of the ions present in solution and the pH of the solution are illustrated in the following problems. First we consider the dissociation of a monoprotic acid, using acetic acid as an example. Later we examine the dissociation of a diprotic acid, H2S, in connection with precipitation of metal sulfides. 

Acids with one, two, and / V I three dissociable protons are called monoprotic, diprotic, and triprotic, respectively. 

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