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An ion-selective membrane is the key component of all potentiometric ion sensors. It establishes the preference with which the sensor responds to the analyte in the presence of various interfering ions from the sample.

If ions can penetrate the boundary betweentwo phases, then an electrochemical equilibrium will be reached, in which different potentials in the two phases are formed. If only one type of an ion can be exchanged between the two phases, then the potential difference formed between the phases is governed only by theactivities of this target ion in these phases.

Eletrodos Ion-Seletivos When the membrane separates two solutions of different ionic activities a1 and a2 and provided the membrane is only permeable to this single type of ion, the potential difference E across the membrane is described by the Nernst equation: Eletrodos Ion-Seletivos The potential difference can be measured between two identical reference electrodes placed in the two phases.

In practice the potential difference i.

An examplary potencjometria for the measurement of electromotive force is precented in figure 1. It is important to note that this is a measurement at zero current i. Equilibrium means that the transfer of ions from the membrane into solution is equal to the transfer from the solution to the membrane. The measured signal is the sum of different potentials generated at all solid-solid, solid-liquid and liquid-liquid interfaces.


[Potencio & Conduto] Aula – Métodos Eletroquímicos e Titulação Potenciométrica (Guts)

Typical calibration curve of a potentiometric sensor determined in this way is shown in figure 2. The linear range of the calibration curve is usually applied to determine the activity fundakentos the target ionin any unknown solution.

However, it should be pointed out that only at constant ionic strength a linearrelationship between the signal measured and the concentration of the analyte is maintained because of the clear cut relationship between ion activity and concentration, occurring in such condition.

However, a membrane truly selective for a single typeof an ion and completely non-selective for other ions does not exist.

Meaning of “potenciometria” in the Portuguese dictionary

For this reason the potential of sucha membrane is governed mainly by the activity of the primary target ion and also by the activity of othersecondary interfering ions. The influence of the presence of interfering species in a sample solution on themeasured potential difference is taken into consideration in the Nikolski-Eisenman formalism: The selectivity is one of the most important characteristics of an electrode, as it oftendetermines whether a reliable measurement in the sample is possible or not.

The selectivity coefficient Kxy has been introducedin the Nikolski-Eisenman equation. Most often it is expressed as the logarithm of Kxy. Negative values indicate a preference for the target ion relative to the interfering ion. Positive values of log Kxy indicate the preference of an electrode for theinterfering ion. The experimental selectivity coefficients depend on the activity and a method of their determination.

Different methods of the selectivity determination can be found in the literature. There is also an alternative method of the selectivity determination called matchedpotential method MPM.

Each of them has got advantages and drawbacks, va there are not general rules pointing which method gives the true result. The theoretical valueaccording to the Nernst equation is: A useful slope can be regarded as [mV per decade] [mV per decade] for double charged ion respectively.


However, in certain applications the value of the electrode slope is not criticaland worse value does not exclude its usefulness.

Potentiometric titration applied to free fatty acid determination of edible oils and fats

At high and very low target ion activities there are deviations from linearity. Typically, the electrode calibration curve exhibits linear response range between M and M. According the IUPAC recommendation the detection limit is defined by the cross-sectionof fundamejtos two extrapolated linear parts od the ion-selective calibration curve.

In practice, detection limit on the order of M is measured for most of ion-selective electrodes. The observed detection limit is often governedby the presence of other interfering ions or impurities. If for example metal buffers are used to eliminate the effectswhich lead to the contamination of very dilute solutions it is possible to enhance the detection limit down to M.

This definition can be extended to consider the drift of the system.

However, it should be pointed out that a single time constant does not describe the form of the electrode response function. Moreover, in many investigationsthe response time of the overall measuring system is determined, which influences on the response time of the ISE.

This membrane can be prepared as: Typically such a membrane contains an analyte-selective component whichis responsible for the recognition process. According to the nature of the binding sites, the membranes can be classified as: