Two-phase potentiometric

The partitioning behaviour of an ionisable compound can also be followed directly using a two-phase potentiometric titration using commercial instrumentation (Sirius analytical Instruments) as shown by Avdeef et al. (1998). 

Figure 5. Two-phase potentiometric acid-base titration data for PSl6o in n-octanol Na" " (x), K" " (o), Cs" " (A) in the presence of 10 2 M NaCl, KCl, CsCl (aqueous phase).

Figure 6. Two-phase potentiometric acid-base and titration data for PSlSo, Na form in the presence of 10 2 (+), io l (o) and 1 M (A), NaCl aqueous solutions (upper curves) lower curves according to Eq. 8.

It can be used for two-phase potentiometric titration from a three- or four-components system. Typ 1 is used for the three-components system HpE Cj and Typ = 8 for the four-components system HpBpCjL,. In the potentiometric titration either acid solution or alkaline solution is added to the two-phase system, and the value of - log [H+] in the aqueous phase is measured. 

The potentiometric micro detection of all aminophenol isomers can be done by titration in two-phase chloroform-water medium (100), or by reaction with iodates or periodates, and the back-titration of excess unreacted compound using a silver amalgam and SCE electrode combination (101). Microamounts of 2-aminophenol can be detected by potentiometric titration with cupric ions using a copper-ion-selective electrode the 3- and... 

The most popular theoretical description of the potentiometric behavior of ion-selective membranes makes use of the three-segmented membrane model introduced by Sollner53), Teorell 30,54), and Meyer and Sievers 31-5S). In this model the two phase boundaries and the interior of the membrane are treated separately. Here, the... 

Active matter (anionic surfactant) in AOS consists of alkene- and hydroxy-alkanemonosulfonates, as well as small amounts of disulfonates. Active matter (AM) content is usually expressed as milliequivalents per 100 grams, or as weight percent. Three methods are available for the determination of AM in AOS calculation by difference, the two-phase titration such as methylene blue-active substances (MBAS) and by potentiometric titration with cationic. The calculation method has a number of inherent error factors. The two-phase titration methods may not be completely quantitative and can yield values differing by several percent from those obtained from the total sulfur content. These methods employ trichloromethane, the effects from which the analyst must be protected. The best method for routine use is probably the potentiometric titration method but this requires the availability of more expensive equipment. 

Johansson, P.-A. Gustavii, K., Potentiometric titration of ionizable compounds in two phase systems. 2. Determination of partition coefficients of organic acids and bases, Acta Pharm. Suecica 13, 407 -20 (1976). 

Potentiometric measurements are done under the condition of zero current. Therefore, the domain of this group of sensors lies at the zero-current axis (see Fig. 5.1). From the viewpoint of charge transfer, there are two types of electrochemical interfaces ideally polarized (purely capacitive) and nonpolarized. As the name implies, the ideally polarized interface is only hypothetical. Although possible in principle, there are no chemical sensors based on a polarized interface at present and we consider only the nonpolarized interface at which at least one charged species partitions between the two phases. The Thought Experiments constructed in Chapter 5, around Fig. 5.1, involved a redox couple, for the sake of simplicity. Thus, an electron was the charged species that communicated between the two phases. In this section and in the area of potentiometric sensors, we consider any charged species electrons, ions, or both. 

Carboxybetaines Low pH two-phase titration, gravimetric analysis, or potentiometric titration... 

Clarke, F.FI. Cahoon, N.M. Partition coefficients by curve fitting the use of two different octanol volumes in a dual-phase potentiometric titration. J. Pharm. Sci. 1996, 85, 178-183. 

As shown in Section 2.3.6, AG j j j, and hence can be obtained from thermochemi-cal, solubility, or potentiometric measurements (with an extrathermodynamic assumption). Thus by sweeping the potential drop across the ITIES, the activities of species in the two phases can be varied, with a resulting flow of current as ions cross the interface. 

Metrohm markets microprocessor-controlled Instruments for routine coulo-metric (KF 652 Processor) and amperometrlc-voltammetrlc (EP/KF 678 Processor) analyses. These Instruments are frequently used for research purposes. Thus, the Mettler Memotltrator and the Metrohm Tltroprocessor 636/Rod Stirrer 622 were used by Johansson et al. [62] to demonstrate that potentiometric two-phase titrations can be carried out in an automatic fashion. The typical background noise from the electrodes can be reduced by Introducing hydrophobic anions or cations in the aqueous phase. These ions also affect the acid-base equilibrium by extracting the sample ions as ion-pairs into the organic phase, which allows the conditional acidity constant (apparent K ) to be manipulated to make selective titrations possible. 

All titrimetric methods of analysis require some means of detecting the equivalence point. This could be an abrupt change of color (colorimetric titrations), a sudden change in the potential difference between two electrodes (potentiometric titrations), a change in current flow through two electrodes (amperometric titrations), and so on. Similarly, the radioactivity of either the titrant or the substance titrated can be employed for detecting the equivalence point. This type of analysis is called radio-metric titration. It should be noted that the sole purpose of the radioactivity is to signal attainment of the equivalence point and that it takes no part in the titration process. The technique can be employed in all classes of titrations, provided that a phase separation can be effected. 

Surfactant-sensitive electrodes are commercially available, and they are also easily made in the laboratory. They can be used to detect the end-point in titrations of anionic and cationic surfactants with surfactants of opposite charge. They are used in exactly the same way as a glass electrode in acid-base titrations, or a silver-silver chloride electrode in titrations of chloride with silver nitrate. The main advantages of potentiometric as opposed to two-phase titration are ... 

These materials are readily determined by two-phase titration (ISO 2271) or potentiometric titration with benzethonium chloride. On acid hydrolysis they yield the corresponding alcohol, a sulphate ion and a hydrogen ion, and this affords three additional approaches—determination of the increase in acidity, of the amount of fatty alcohol or ethoxylated alcohol liberated, and of the sulphate ion. The experimental procedure may be varied within limits for example ISO 2870 [5] and ASTM D 1570-89 [6] differ with respect to the identity of the acid used, duration of boiling, choice of indicator and other details. The following procedures are similar to both of those standard methods. The choice of indicator is immaterial for these particular compounds, but is of crucial importance in some other cases. 

Titration with benzethonium chloride in acid solution measures the sulphonated ester plus the sulphonated carboxylic acid, only the sulphon-ate group of the latter being titrated. In alkaline solution the titration measures the sulphonated ester plus twice the sulphonated carboxylate, both the sulphonate and the carboxylate group being titrated. Alpha-sulphonated esters give poor potentiometric titration curves, and two-phase titration is strongly preferred. 

The ester cannot be determined by saponification because this slowly removes the sulphonate group as well as hydrolysing the ester group. It can be determined by acid hydrolysis followed by measurement of the a-sulphonated carboxylate salt produced, either by potentiometric titration of the weak acid or by two-phase titration with benzethonium chloride. Both procedures measure the a-sulphonated ester plus the unsulphonated ester. If the latter is present at a significant level, it can be determined (section 5.11.2) and corrected for. 

All may be determined by two-phase or potentiometric titration with benzethonium chloride. Titration of sulphosuccinamates in acid solution measures the sulphonate group alone (but see section 5.13.3), and titration in alkaline medium measures the carboxylate group as well (cf. section 5.11.3). Sulphosuccinamates, possessing a carboxylate group, can also be determined by potentiometric acid-base titration. 

Possible surface-active impurities are soap in the taurine derivatives and fatty amine in sulphosuccinamates. They are easily determined by extraction with petroleum ether or by two-phase titration in acid and alkaline solution. Soap may also be determined by potentiometric acid-base titration. 

Fatty acids and acyl sarcosines may be extracted with petroleum or diethyl ether from an acidified aqueous solution if no other surfactants are present, or with petroleum ether from acidified 50% ethanol if they are. The extract is evaporated and the residue weighed. Alkylether carboxylic acids cannot be quantitatively extracted with petroleum ether. They can be extracted from aqueous solution with chloroform, but they are best determined by two-phase titration with benzethonium chloride in akaline solution (bromophenol blue method) or direct potentiometric acid-base titration. 

C. Titration with NaTPB. The solution must be at least 0.1 M in hydrogen ion. Potentiometric titration is preferred, but two-phase titration should be satisfactory. It may be more effective to add a measured excess of NaTPB, filter or centrifuge (or run off the chloroform layer) and determine the excess by precipitation with potassium, filtration and weighing. 

SW sulphobetaines are more like anionics than cationics, and can be titrated with benzethonium chloride in alkaline solution, by either two-phase or potentiometric procedures. They cannot be titrated with NaTPB. 

This section includes methods for determining the total, primary, secondary and tertiary amine contents of fatty amines which are nominally primary, secondary or tertiary or mixtures. For determination, another approach is to titrate with sodium dodecyl sulphate. Either two-phase or potentiometric titration may be used. The solution must be distinctly acid, pH 3 or lower. Two-phase or potentiometric titration with NaTPB is also applicable. Procedures for all of these are given in section 7.1. The difficulty with these approaches is in deciding what value to use for the molecular weight. 

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