Análise potenciométrica de redissolução com ultramicroeletrodos: fundamentos e aplicações na determinação de metais traços. Quím. Nova [online]. 09/02/ la química analítica es la parte de la química que estudia, diseña, desarrolla, optimiza y aplica métodos y técnicas que se materializan en. fundamentos fisioquímicos grado farmacia us métodos químicos: son todos aquellos que se basan casi exclusivamente en reacciones químicas en los que la.
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Release 10 for WindowsMinitab: 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 experimental selectivity coefficients depend on the activity and a method of their determination.
Free fatty acids are a measure of evaluating fats and oils, submitted to abusive conditions, besides being a quality characteristic of edible vegetable oils. At high and very low target ion activities there are deviations from linearity. This definition can be extended to consider the drift of the system. 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.
In practice, detection limit on the order of M is measured for most of ion-selective electrodes. It establishes the preference with which the sensor responds to the analyte in the presence of various interfering ions from the sample.
An ion-selective membrane is the key component of all potentiometric ion sensors. Rio de Janeiro,cap. 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. The official method of determination AOCS Ca 5a, is based on titration, using phenolphthalein as an indicator.
Most often it is expressed as the logarithm of Kxy. However, in certain applications the value potenciomtria the potenciometriia slope is not criticaland worse value does not exclude its usefulness.
Each of them has got advantages and drawbacks, and there are not general rules pointing which method gives the true result. 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: How to cite this article.
Potentiometric titration applied to free fatty acid determination of edible oils and fats. An examplary set-up for the measurement of electromotive force is precented in figure 1.
Rio de Janeiro,vol. Alterando as quantidades de amostra utilizadas. It fundametos important to note that this is a measurement at zero current i. Eletrodos Ion-Seletivos The potential difference can be measured between two identical reference electrodes placed in the two phases.
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.
Typically such a membrane contains an analyte-selective component whichis responsible for the recognition process. The linear range of the calibration curve is usually applied to determine the activity of the target ionin any unknown solution. All the contents of this journal, except where otherwise noted, is licensed under a Creative Commons Attribution License. The ptenciometria detection limit is often governedby the presence of other interfering ions or impurities.
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.
Europa’s Quality Acceptance apud ref. However, a membrane truly selective for a single typeof an ion and completely non-selective for other ions does not exist. This membrane can be prepared as: Typically, the electrode calibration curve exhibits linear response range between M and M.
Meaning of “potenciometria” in the Portuguese dictionary
Positive values of log Kxy indicate the preference of an electrode for theinterfering ion. 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.
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: For crude and degummed oils, the titration end point is difficult to be observed due to the high level of pigments, resulting in dark solutions.
Industrial Processing of Cotton-Seed apud ref. Equilibrium means that the transfer of ions from the membrane into solution is equal to the transfer from the solution to the membrane. New York,vol. In practice the potential difference i. Dois tipos de erros podem ter ocorrido: According the IUPAC recommendation the detection limit is defined by the cross-sectionof the two extrapolated linear parts od the ion-selective calibration curve.
Food Protection53 No processo de refino, por ex. In this case and others, such as self-life studies, in which sample quantities are limited, potentiometric titration may be a very good alternative. The measured signal is the sum of different potentials generated at all solid-solid, solid-liquid and liquid-liquid interfaces. Typical calibration curve of a potentiometric sensor determined in this way is shown in figure 2.
Different methods of the selectivity determination can be found in the literature.
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There is also an alternative method of the selectivity determination called matchedpotential method MPM. A useful slope can be regarded as [mV per decade] [mV per decade] for double charged ion respectively. Moreover, in many investigationsthe response time of the overall measuring system is determined, which influences on the response time of the ISE.
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