Potentiometric Acid-base Titrations Research Articles

Potentiometric Determination of Acids and Bases using Silicone Rubber Based Graphite Electrode

Abstract A silicone rubber based graphite electrode has been used as an indicator electrode for potentiometric acid-base titrations. The electrode potential changes an average of 30 mV per pH-unit. If the electrodes are pretreated with a solution of an oxidant before use, the pH-sensitivity is increased. The electrode can be used in both aqueous and non-aqueous solutions. A measuring cell, composed of a silicone rubber based graphite electrode of small surface area and a chloride-selective reference electrode, can be used for acid-base titrations in the micro range.

Potentiometric acid-base titration in molten salts. The acid character of Li +, Na +, K +, Ca 2+, Sr 2+, Ba 2+ and Pb 2+ as inferred from the reaction of their carbonates with K 2Cr 2O 7 in molten KNO 3

Summary The acid, K 2 Cr 2 O 7 , has been titrated potentiometrically with the carbonates of Li, Na, K, Ca, Sr, Ba and Pb in fused KNO 3 at 350°. Both “forward” and “backward” neutralization curves (except for the case of Pb) consist of a single step corresponding usually to formation of CrO 4 2− . “Cyclic” neutralization curves with NaOH and KOH suggest that Na + is acid with respect to K + . The acidity of Ca 2+ , Sr 2+ and Ba 2+ decreases in the order given. CaCrO 4 and SrCrO 4 add an extra molecule of Na 2 CO 3 to give CaCrO 4 ·Na 2 CO 3 (SrCrO 4 ·Na 2 CO 3 ); BaCrO 4 does not. Pb 2+ is the most acid cation studied. PbCrO 4 adds another molecule of PbCO 3 and gives a second inflexion in the titration curves, corresponding to the formation of PbCrO 4 ·PbCO 3 . PbCO 3 decomposes thermally in fused KNO 3 to give white lead, 2PbCO 3 ·PbO. The reaction is zero-order with respect of PbCO 3 and has an activation energy of 38.2 kcal mole −1 .

Potentiometric acid-base titrations in molten salts. The acid character of Li+, Na+, K+, Ca2+, Sr2+, Ba2+ and Pb2+ as inferred from the reaction of their carbonates with K2Cr2O7 in molten KNO3

The acid, K 2 Cr 2 O 7 , has been titrated potentiometrically with the carbonates of Li, Na, K, Ca, Sr, Ba and Pb in fused KNO 3 at 350°. Both “forward” and “backward” neutralization curves (except for the case of Pb) consist of a single step corresponding usually to formation of CrO 4 2− . “Cyclic” neutralization curves with NaOH and KOH suggest that Na + is acid with respect to K + . The acidity of Ca 2+ , Sr 2+ and Ba 2+ decreases in the order given. CaCrO 4 and SrCrO 4 add an extra molecule of Na 2 CO 3 to give CaCrO 4 ·Na 2 CO 3 (SrCrO 4 ·Na 2 CO 3 ); BaCrO 4 does not. Pb 2+ is the most acid cation studied. PbCrO 4 adds another molecule of PbCO 3 and gives a second inflexion in the titration curves, corresponding to the formation of PbCrO 4 ·PbCO 3 . PbCO 3 decomposes thermally in fused KNO 3 to give white lead, 2PbCO 3 ·PbO. The reaction is zero-order with respect of PbCO 3 and has an activation energy of 38.2 kcal mole −1 .

Potentiometric acid-base titrations in molten salts—V. A comparative study on the acid character of some of group 1 and group 2 cations

The acid NaPO3 has been potentiometrically titrated with the carbonates of Li, Na, K, Ca, Sr and Ba in molten KNO3 at 350°C. Both “forward” and “backward“ titration curves show marked potential inflexions at simple acid: oxide-ion donor ratios. The reaction products depend on the cation of the titrant, and in some cases on the route of the reaction. The P:O and P:e ratios of the compounds formed at the disappearance of PO3− (in forward experiments) or CO32− (in backward ones) are proposed to characterize the acid character of the cations. Acidity decreases in the succession Ca > Li > Sr > Ba > Na, K.

Potentiometric acid-base titrations in molten salts—IV. Basic character of some oxide-ion donors in molten KNO3: their reaction with NaPO3 and K2Cr2O7

The basic character of electrogenerated O2−, OH−, O22−, CO32−, HCO3−, HCOO−, (COO)22− and CH3COO− in fused KNO3 at 350°C was studied by forward and backward potentiometric neutralization experiments with the two acids NaPO3 and K2Cr2O7. The anions fall into two distinct groups. To the first, the “oxide” group, belong O2−, OH− and O22−, which give rise to O2-electrode potentials varying between −850 and −92O mV, and allow the formation of the basic compounds PO43−·O2− and 2CrO42−·O2−. Cyclic neutralization curves suggest the presence of two phosphate glasses, namely, an acid glass nPO3−·P2O74−, and a basic glass nPO43−·O2−. The other anions, forming the “carbonate’, give rise to O2-electrode potentials of ca −550 to −600 mV, and do not form the basic phosphate or chromate.The thermal decomposition of bicarbonate, oxalate, formate, and acetate in fused KNO3 was studied in detail. Bicarbonate changes into carbonate. Oxalate produces carbonate and not oxide as previously claimed, together with an equivalent quantity of nitrite. The same occurs also with formate and acetate, but in the latter case decomposition follows a different route.

Graphic method for the determination of titration end-points

An extension of Gran's method for the détermination of equivalence points in potentiometric acid-base titrations is presented. Equations are derived assuming that the stability constants of the reactions are known. The equations make it possible to locate the equivalence point with fair accuracy when acids with stability constants ranging from K(H)(HA) ~ 10(7) to K(H)(HA) ~ 10(10) are titrated. The limit of K(H)(HA) above which Gran's equations do not yield satisfactory results is about 10(7). Conditional constants and a-coefficients are used to extend the method to apply to more complex systems, where more than one species reacts with the titrant.

Potentiometric acid-base titrations in molten salts: The acid character of group (VI) oxides as inferred from their reaction with molten KNO 3

The reaction between CrO 3, MoO 3, and WO 3, and fused KNO 3 at 350° has been studied by measuring the variation in the potential of an oxygen electrode relative to an Ag/Ag(I) reference electrode. When the reactions come to an end, the reaction products are determined by acid-base titrations in situ, using Na 2O 2 as titrant. The acid strength of the three oxides decreases in the order CrO 3 > MoO 3 > WO 3. This is evident from the variation of the potential of the oxygen electrode with time, and from the fact that, whereas dichromate is formed in case of CrO 3, only the triomolybdate is obtained from the reaction of MoO 3 with fused KNO 3. WO 3 is stable under the same experimental conditions. Neutralization of all the reaction products with Na 2O 2 leads to the formation of the most basic anion, XO 4 2−. Acid-base equilibrium constants for the reactions: 2 CrO 3 + O 2− → Cr 2O 7 2− CrO 3 + O 2− → CrO 4 2− MoO 3 + O 2− → MoO 4 2− Mo 3O 10 2− + 2O 2− → 3 MoO 4 2− WO 3 + O 2− → WO 4 2− have been calculated. The kinetics of the reaction: 2 CrO 3 + 2 NO 3- → Cr 2O 7 2- + N 2O 5 are established. The reaction is bimolecular with respect to CrO 3 and has an activation energy of 25.3 kcal/mole.

Potentiometric acid-base titrations in molten salts: The acid character of group V oxides as inferred from their reaction with molten KNO 3

The acid characters of P 2O 5, As 2O 5 and V 2O 5 were compared on the basis of their reaction with fused KNO 3 at 350°. The attack on the nitrate was found to decrease in the series P 2O 5 > As 2O 5 > V 2O 5. The products of these reactions were determined by conducting in situ potentiometric acid—base titrations using an oxygen electrode as indicator electrode and Na 2O 2 as titrant. In principle, the three pentoxide reacted with the nitrate melt in a similar fashion to yield the corresponding meta-salts. Metavanadate is, however, a strong Lux acid which directly attacks the base electrolyte to yield pyro-vanadate. Similarly, the controlled neutralization of meta-phosphate and meta-arsenate with Na 2O 2 gives rise to the corresponding pyro-salts. AII pyro-compounds react with Na 2O 2 to yield the ortho-salts. The neutralization of KH 2AsO 4 in molten potassium nitrate occurs in two successive steps due to the formation of the di- and tri-sodium salts. Acidity (basicity) numbers and equilibrium constants of the different oxy-anions have been determined. On the basis of these figures, the different oxy-anions are arranged in the descending order of their strength as: ▪

Pyrolytic graphite cup as both vessel and electrode for micro redox and acid-base potentiometric titrations

A potentiometric titration vessel of pyrolytic graphite is described for use in microtitrations, which are usually performed with colored indicators. The vessel also serves simultaneously as the indicator electrode, the reference electrode being an in-burette platinum wire. Thus, the need for internal electrodes is eliminated. The vessel has been used satisfactorily in both manual and automatic titrations.

Potentiometric acid-base titrations in molten salts: The application of the principle of bimetallic electrodes for the determination of equivalence points

Silver, nickel, copper, iron and tungsten were successfully applied as indicator electrodes in the titration of the acid K 2Cr 2O 7 in molten KNO 3 at 350°. With all electrodes sharp potential drops were recorded at the theoretically calculated equivalence points. Because of differences in E° potentials (as oxygen electrodes), the measurement of potential differences between any two indicator electrodes allowed the determination, of equivalence points. In this type of titration no reference electrode is needed. Bimetallic combinations were equally suitable for the determination of equivalence points of acids neutralizing in more than one step, e.g., NaPO 3 and NaH 2PO 4.

Potentiometric acid—base titrations in molten salts—III. The use of a metal/ metal-oxide electrode as an indicator electrode and the determination of acid—base equilibria in fused KNO3

The feasibility of the Cu/CuO electrode for O2−-ion determination and for acid—base titrations in molten KNO3 at 350°C has been established. The potential of the electrode varies linearly with log [O2−] and the slope of the calibration curve is 2·303(RT/2F) at the working temperature. This allows the determination of the Cu/CuO/O2− standard potential. The CuO electrode is used for titrating the acid K2Cr2O7 with Na2O2 in molten KNO3. Sharp potential drops are recorded at the equivalence points. The equilibrium constant of the reaction Cr2O72− + O2− = 2CrO42− is calculated as ca 1·8 × 1012.The standard O2/O2− potential in fused KNO3 at 350°C has been calculated indirectly. With this value and the potentiometric titration curves obtained with the oxygen electrode, the first known acid—base equilibrium constants in fused salts are computed, as follows: ▪A correlation is drawn between the equilibrium constants calculated in the present study and the acidity numbers of the same oxy-anions previously proposed.

Theory of titration curves: Locations of points at which ph=pk a on potentiometric acid-base titration curves; end-point errors in titrations to predetermined ph values

For the titration of a pure solution of a weak monobasic acid with a strong base, it is shown that the fraction of the equivalent volume of base added at the point where pCH = p K α is exactly equal to 1 2 if and only if p K α = p K ω 2 , in which case it is independent of dilution. It is less than 1 2 if p K α < p K ω 2 and is greater than 1 2 if p K α > p K ω 2 , and in either of these cases it depends on the concentrations of both acid and base. Explicit descriptions are given of the conditions under which pcH = p K α either at the start of the titration or at the equivalence point. For the titration of either a weak or a strong acid with a strong base, exact equations are given for the titration error resulting from the termination of the titration at any preselected pcH value.

Potentiometric acid-base titrations in molten salts the neutralization of sodium metavanadate and sodium metaphosphate in chloride and nitrate melts

Potentiometric acid-base titrations in molten salts the neutralization of sodium metavanadate and sodium metaphosphate in chloride and nitrate melts

Potentiometric acid-base titrations in molten salts: The system Cr 2O 72−-CrO 42−-O 2−

The acid K 2Cr 2O 7 was titrated potentiometrically against the three bases K 2CO 3, Na 2O 3, and NaHCO 3 in molten KNO 3 at 350°. An oxygen electrode was used as an indicator electrode, while a Ag/Ag(I) electrode acted as a suitable reference electrode. At the equivalence point, a sharp drop in the potential of the indicator electrode was measured, whose magnitude depended on the base used. The points of maximum inflexion in the titration curves were identical with the theoretical quantities of the bases necessary for neutralisation of the acid. Contamination of the melt with traces of water did not affect the measured potentials.

Potentiometric acid-base titration in acetone- water solutions of nitrocellulose

The development of a method for carrying out potentiometric acid-base titrations in acetone-water solutions of nitrocellulose is described. The special problems inherent in the titration of solutions with acetone concentrations greater than 90% are discussed and an instrumental method of ascertaining the attainment of equilibrium after each addition of titrant is presented. It is concluded that the optimum conditions are a maximum concentration of 0.5 % nitrocellulose in a 99.5 % acetone-water solution, and that the concentration of titrant must be sufficiently high for the alteration of this ratio during the titration to be very small. The care and treatment of the glass and calomel electrodes used in this titration are described and a procedure for preparing them for future use is given. Finally, mention is made of an automatic apparatus designed to carry out titrations involving slow reactions of this kind.

The aluminium indicator electrode in acid-base potentiometric titrations

Aluminium has been employed as an indicator electrode in potentiometric acid-base titrations. The titration curves obtained were approximately 3.6 times broader than those obtained with the antimony electrode.