This paper recalls the well-known property of different functions represented by the curves with sigmoidal shape (S-shape) [1] , involved with in­flection (inf) point. An inflection point is the point on 2D plane where the curvature of the curve changes direction. The S-shape is characteristic, among others, for potentiometric titration curves [2] . Different methods of equivalence (eq) point determination are based on location of the inflec­tion point on the curves pH = pH(V) or E = E(V), where E—potential, V—volume of titrant added. The inflection points are registered also in different physicochemical studies.

Generalizing, we refer to a monotonic function y = y(x). The inflection point (x_inf, y_inf) corresponds to maximal slope, where

Applying the relation

at the inflection point on the curve y = y(x) we have

and then at dy/dx ¹ 0 we get

It means that the maximal slope is equivalent with the relation (4) valid for the inverse function x = x(y). This property is important for pH titration curves; namely, the functions V = V(pH) assume relatively simple form [3] .

In this paper, we refer to a simple acid-base ti­tration (y = pH, x = V), and to the relationship s = s(pH) for surface tension (y = s, x = pH).

The main task of titration made for analytical purposes is the estimation of the equivalence vol­ume (V_eq). Let us consider the simplest case of titration of V₀ mL of C₀ mol/L HCl as titrand (D) with V mL of C mol/L NaOH as titrant (T). At V = V_eq, the fraction titrated

i.e., CV_eq = C₀V₀. In this D+T system, the titration curve V = V(pH) has the form

where

To facilitate the calculations, it is advisable to re­write (6) into the form

From (5) and (6) we get

From (8)

Setting d²V/dpH² = 0 and writing, from (10) we get, by turns,

From (11) we obtain for z = z_inf

and then for V = V_inf [3] [4]

Analogous result can be obtained for titration of V₀ mL of C₀ mol/L NaCl with V mL AgNO₃ [5] . Denoting [Ag⁺][Cl^–] = K_so we get (13), where [5]

At pK_so = 9.75 for AgCl, V₀ = 100 mL, C₀ = 10^–4 and C = 10^–3, we get V_eq – V_inf = 0.16 mL.

Many physicochemical processes are graphi­cally represented by the curves with the sigmoidal shape. In this section, we refer to the function s = s(pH) obtained on the basis of Szyszkowski’s em­pirical formula [6]

expressing the relationship between surface tension s and concentration [HL] of uncharged form HL of an aliphatic fatty acid as a surfactant in aqueous media; s₀—surface tension of pure water, a, b—constants.

Denoting and

we get, by turns:

Putting d²s/dpH² = 0, from Equation (16) we get [H⁺]∙(1 + b∙C)^1/2 = K₁, and then

From Equation (17) it results that the abscissa (pH_inf) corresponding to inflection point does not overlap with pK₁ value for HL.