Equivalence point Definition of end point - MywallpapersMobi

Equivalence point Definition of end point

Equivalence point

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This article relies largely or entirely on a single source . Relevant discussion may be found on the talk page . Please help improve this article by introducing citations to additional sources. (July 2013)

The equivalence point, or stoichiometric point, of a chemical reaction is the point at which chemically equivalent quantities of bases and acids have been mixed. In other words, the moles of acid are equivalent to the moles of base, according to the equation (this does not necessarily imply a 1:1 molar ratio of acid:base, merely that the ratio is the same as in the equation). It can be found by means of an indicator, for example phenolphthalein or methyl orange .

The endpoint (related to, but not the same as the equivalence point) refers to the point at which the indicator changes colour in a colourimetric titration .

Contents

  • 1 Methods to determine the equivalence point
  • 2 See also
  • 3 References
  • 4 External links

Methods to determine the equivalence point[ edit ]

Different methods to determine the equivalence point include:

pH indicator
A pH indicator is a substance that changes color in response to a chemical change. An acid-base indicator (e.g., phenolphthalein ) changes color depending on the pH . Redox indicators are also frequently used. A drop of indicator solution is added to the titration at the start; when the color changes the endpoint has been reached, this is an approximation of the equivalence point.
Conductance
The conductivity of a solution depends on the ions that are present in it. During many titrations, the conductivity changes significantly. (For instance, during an acid-base titration, the H3O+ and OH ions react to form neutral H2O. This changes the conductivity of the solution.) The total conductance of the solution depends also on the other ions present in the solution (such as counter ions). Not all ions contribute equally to the conductivity; this also depends on the mobility of each ion and on the total concentration of ions ( ionic strength ). Thus, predicting the change in conductivity is harder than measuring it.
Color change
In some reactions, the solution changes colour without any added indicator. This is often seen in redox titrations, for instance, when the different oxidation states of the product and reactant produce different colours.
Precipitation
If the reaction forms a solid, then a precipitate will form during the titration. A classic example is the reaction between Ag+ and Cl to form the very insoluble salt AgCl. Surprisingly, this usually makes it difficult to determine the endpoint precisely. As a result, precipitation titrations often have to be done as back titrations .
Isothermal titration calorimeter
An isothermal titration calorimeter uses the heat produced or consumed by the reaction to determine the equivalence point. This is important in biochemical titrations, such as the determination of how substrates bind to enzymes .
Thermometric titrimetry
Thermometric titrimetry is an extraordinarily versatile technique. This is differentiated from calorimetric titrimetry by the fact that the heat of the reaction (as indicated by temperature rise or fall) is not used to determine the amount of analyte in the sample solution. Instead, the equivalence point is determined by the rate of temperature change. Because thermometric titrimetry is a relative technique, it is not necessary to conduct the titration under isothermal conditions, and titrations can be conducted in plastic or even glass vessels, although these vessels are generally enclosed to prevent stray draughts from causing “noise” and disturbing the endpoint. Because thermometric titrations can be conducted under ambient conditions, they are especially well-suited to routine process and quality control in industry. Depending on whether the reaction between the titrant and analyte is exothermic or endothermic , the temperature will either rise or fall during the titration. When all analyte has been consumed by reaction with the titrant, a change in the rate of temperature increase or decrease reveals the equivalence point and an inflection in the temperature curve can be observed. The equivalence point can be located precisely by employing the second derivative of the temperature curve. The software used in modern automated thermometric titration systems employ sophisticated digital smoothing algorithms so that “noise” resulting from the highly sensitive temperature probes does not interfere with the generation of a smooth, symmetrical second derivative “peak” which defines the endpoint. The technique is capable of very high precision, and coefficients of variance (CV’s) of less than 0.1 are common. Modern thermometric titration temperature probes consist of a thermistor which forms one arm of a Wheatstone bridge . Coupled to high resolution electronics, the best thermometric titration systems can resolve temperatures to 10−5K. Sharp equivalence points have been obtained in titrations where the temperature change during the titration has been as little as 0.001K. The technique can be applied to essentially any chemical reaction in a fluid where there is an enthalpy change, although reaction kinetics can play a role in determining the sharpness of the endpoint. Thermometric titrimetry has been successfully applied to acid-base, redox, EDTA, and precipitation titrations. Examples of successful precipitation titrations are sulfate by titration with barium ions, phosphate by titration with magnesium in ammoniacal solution, chloride by titration with silver nitrate , nickel by titration with dimethylglyoxime and fluoride by titration with aluminium (as K2NaAlF6) Because the temperature probe does not need to be electrically connected to the solution (as in potentiometric titrations), non-aqueous titrations can be carried out as easily as aqueous titrations. Solutions which are highly colored or turbid can be analyzed by thermometric without further sample treatment. The probe is essentially maintenance-free. Using modern, high precision stepper motor driven burettes, automated thermometric titrations are usually complete in a few minutes, making the technique an ideal choice where high laboratory productivity is required.
Spectroscopy
Spectroscopy can be used to measure the absorption of light by the solution during the titration, if the spectrum of the reactant, titrant or product is known. The relative amounts of the product and reactant can be used to determine the equivalence point. Alternatively, the presence of free titrant (indicating that the reaction is complete) can be detected at very low levels. An example of robust endpoint detector for etching of semiconductors is EPD-6 a system probing reaction at up to six different wavelengths [1]
Amperometry
Amperometry can be used as a detection technique ( amperometric titration ). The current due to the oxidation or reduction of either the reactants or products at a working electrode will depend on the concentration of that species in solution. The equivalence point can then be detected as a change in the current. This method is most useful when the excess titrant can be reduced, as in the titration of halides with Ag+. (This is handy also in that it ignores precipitates.)

See also[ edit ]

  • Titration

References[ edit ]

  1. ^ “Page Title” . www.zebraoptical.com.

External links[ edit ]

  • Equivalence points of virtual and real acid-base titrations – Software Program
  • Example of robust industrial endpoint detector
  • Graphical method to solve acid-base problems, including titrations
  • Graphic and numerical solver for acid-base problems -Software Program for phone and tablets

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      Definition of ‘end point’

      Word Frequency







      end point in British

      or endpoint (ˈɛndˌpɔɪnt)

      noun

      1. chemistry

      the point at which a titration is complete, usually marked by a change in colour of an indicator
      2. 

      the point at which anything is complete

      Collins English Dictionary. Copyright © HarperCollins Publishers

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      She had reached that end point in any medical soap , the terminal illness storyline . Times, Sunday Times (2010)The trouble is, it is far from clear what the end point is. Times, Sunday Times (2011)That remit is now what is telling us the programme has reached a natural end point and it’s time to move on. The Sun (2009)

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      End point

      measurement
      THIS IS A DIRECTORY PAGE. Britannica does not currently have an article on this topic.

      Learn about this topic in these articles:

      titration

      • Electrometric titration of glycine.

        In titration

        …some signal is called the end point. This signal can be the colour change of an indicator or a change in some electrical property that is measured during the titration. The difference between the end point and the equivalence point is the titration error, which is kept as small as…

        Read More

      • pH paper

        In chemical analysis: Classical quantitative analysis

        …indicator colour change is the end point of the titration. The end point is used as an approximation of the equivalence point and is employed, with the known concentration of the titrant, to calculate the amount or concentration of the analyte.

        Read More

      • pH paper

        In chemical analysis: Ultraviolet-visible spectrophotometry

        …is possible to determine the end point. The end-point volume is used with the concentration of the reagent and the initial volume of the sample solution to calculate the concentration of the analyte.

        Read More







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