Gran plot

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The equivalence volume is used to compute whichever of [H^+] _{0^{ or [OH^-] _{0^{ is unknown.

The pH meter is usually calibrated with buffer solutions at known pH values before starting the titration. The ionic strength can be kept constant by judicious choice of acid and base. For instance, HCl titrated with NaOH of approximately the same concentration will replace H+ with an ion (Na+) of the same charge at the same concentration, to keep the ionic strength fairly constant. Otherwise, a relatively high concentration of background electrolyte can be used, or the activity quotient can be computed.

Titrating strong base with strong acid

Mirror-image plots are obtained if titrating the base with the acid, and the signs of the slopes are reversed.

frac{v_0 [OH^-] _0-v_i [H^+] _0}{v_0+v_i} egin{cases} approx [OH^-] _i ext{ or } K_w 10^{pH_i} & ext{ when } v_{0^{ [OH^-] _0 > v_i [H^+] _0 ext{ (alkaline region)} \\= 0 & ext{ when } v_{0^{ [OH^-] _0 = v_i [H^+] _0 ext{ (equivalence point)} \\approx - [H^+] _i ext{ or } -10^{-pH_i} & ext{ when } v_{0^{ [OH^-] _0 < v_i [H^+] _0 ext{ (acidic region)} end{cases}

[
Click on the image to view in full size.
Figure 3. Sample Gran plots using data from a titration of Cl- by Ag+ monitored potentiometrically. The potentials were converted to [Ag+] values for plotting. Note that the filled circles indicate the data points included in the least-squares computations to give the fitted dashed lines.]

Hence,

Figure 1 gives sample Gran plots of a strong base-strong acid titration.

Concentrations and Dissociation Constants of Weak Acids

The method can be used to estimate the dissociation constants of weak acids, as well as their concentrations (Gran, 1952). With an acid represented by HA, where :K_a = frac{ [H^+] _i [A^-] _i}{ [HA] _i} ,we have at any "i"'th point in the titration of a volume v_0 of acid at a concentration [HA] _0 by base of concentration [OH^-] _0. In the linear regions away from equivalence, : [HA] _i approx frac{v_0 [HA] _0-v_i [OH^-] _0}{v_0+v_i} and
: [A^-] _i approx frac{v_i [OH^-] _0}{v_0+v_i}
are valid approximations, whence:K_a approx frac{10^{-pH_i} v_i [OH^-] _0}{v_0 [HA] _0-v_i [OH^-] _0} , or:K_a (v_0 frac{ [HA] _0}{ [OH^-] _0}-v_i) approx 10^{-pH_i} v_i or, because v_e = v_0 frac{ [HA] _0}{ [OH^-] _0} ,:K_a (v_e -v_i) approx 10^{-pH_i} v_i. A plot of 10^{-pH_i} v_i versus v_{i^{ will have a slope -K_{a^{ over the linear acidic region and an extrapolated x-intercept v_{e^{ , from which either [HA] _{0^{ or [OH^-] _{0^{ can be computed. If the ionic strength is not constant, one can correct this expression for the changing activity quotient. See Harris (1998) or this cite web |url=http://chemistry.depaul.edu/wwolbach/205_207/9.pdf |title=on-line document |accessdate=2008-02-17] The alkaline region is treated in the same manner as for a titration of strong acid. Figure 2 gives an example; in this example, the two x-intercepts differ by about 0.2 mL but this is a small discrepancy, given the large equivalence volume (0.5% error).

Similar equations can be written for the titration of a weak base by strong acid (Gran, 1952; Harris, 1998).

Carbonate content

Martell and Motekaitis (1992) use the most linear regions and exploit the difference in equivalence volumes between acid-side and base-side plots during an acid-base titration to estimate the adventitious CO2 content in the base solution. This is illustrated in the sample Gran plots of Figure 1. In that situation, the extra acid used to neutralize the carbonate, by double protonation, in volume v_{0^{ of titrate is (v_e-v_e^{prime}) [H^+] _{0^{ = 2v_0 [CO_2] _0. In the opposite case of a titration of acid by base, the carbonate content is similarly computed from (v_e^{prime}-v_e) [OH^-] _{0^{ = 2v_e^{prime} [CO_2] _0, where v_e^{prime} is the base-side equivalence volume (from Martell and Motekaitis).

When the total CO2 content is significant, as in natural waters and alkaline effluents, two or three inflections can be seen in the pH-volume curves owing to buffering by higher concentrations of bicarbonate and carbonate. As discussed by Stumm and Morgan (1981), the analysis of such waters can use up to six Gran plots from a single titration to estimate the multiple end points and measure the total alkalinity and the carbonate and/or bicarbonate contents.

Electrode offset

If the pH electrode is not well calibrated, an offset correction might be appropriate and can be computed from the acid-side Gran slopes. Let the offset be represented by b (= pH_0), then:-log_{10}( [H^+] _{i^{ ) = b + pH_iIncorporating this into the equations above results in the following modified slopes:
* For a titration of acid by base, one obtains an acid-side slope of - [OH^-] _{0^{ 10^b from which b_{ ^{ can be computed, and a base-side slope of [OH^-] _{0^{ 10^{-b}/K_w, from which K_{w^{ can be computed.
* For a titration of base by acid, as illustrated in the sample plots, the acid-side slope, [H^+] _{0^{ 10^b, is similarly used to compute b_{ ^{ , and the base-side slope of - [H^+] _{0^{ 10^{-b}/K_w is used to compute K_{w^{ . In the sample data illustrated in Figure 1, the offset was not insignificant, at -0.054 pH units.

The x-intercepts are unaffected.

Potentiometric data

To use potentiometric measurements E_{i^{ in monitoring the H^{+_{ concentration in place of pH_{i^{ readings, one can trivially set -log_{10} [H^+] _i = b_0 - b_1E_{i^{ and apply the same equations as above, where b_{0^{ is the offset correction nFE_{0^{ /RT corresponding to the b_{ ^{ used earlier, and b_{1^{ is a slope correction nF^{ _{ /RT (1/59.2 pH units/mV at 25°C), such that -b_1E_{i^{ replaces pH_{i^{ .

When monitoring another species S^{1_{ by potentiometry, one can apply the same formalism with -log_{10} [S^1] _i = b_0 - b_1E_{i^{ . Thus, a titration of a solution of another species S^{2_{ by species S^{1_{ is analogous to a pH-monitored titration of base by acid, whence either ({v_0+v_i})10^{b_1E_{i or ({v_0+v_i})10^{-b_1E_{i plotted versus v_{i^{ will have an x-intercept v_0 [S^0] _0 / [S^1] _{0^{ . In the opposite titration of S^{1_{ by S^{2_{ , the equivalence volumes will be v_0 [S^1] _0 / [S^0] _{0^{ . The significance of the slopes will depend on the interactions between the two species, whether associating in solution or precipitating together (Gran, 1952). Usually, the only result of interest is the equivalence point. However, the before-equivalence slope could in principle be used to assess the solubility product K_{sp^{ in the same way as K_{w^{ can be determined from acid-base titrations, although other ion-pair association interactions may be occurring as well. Gran "et al." (1981) give a more detailed treatment that takes into account other complex species in a titration of Cl- by Ag+ (Ag2Cl+ and AgCl2-, notably) and in other precipitation titrations, in order to compute equivalence volumes of dilute silutions, when precipitation is incomplete. ]

To illustrate, consider a titration of Cl- by Ag+ monitored potentiometrically: frac{v_0 [Cl^-] _0-v_i [Ag^+] _0}{v_0+v_i} egin{cases} approx [Cl^-] _i ext{ or } K_{sp} 10^{-b_1E_i+b_0} & ext{ when } v_{0^{ [Cl^-] _0 > v_i [Ag^+] _0 ext{ (before equivalence)} \\= 0 & ext{ when } v_{0^{ [Cl^-] _0 = v_i [Ag^+] _0 ext{ (equivalence point)} \\approx - [Ag^+] _i ext{ or } -10^{b_1E_i-b_0} & ext{ when } v_{0^{ [Cl^-] _0 < v_i [Ag^+] _0 ext{ (after equivalence)} end{cases}

Hence,

Figure 3 gives sample plots of potentiometric titration data.

Non-ideal behaviour

In any titration lacking buffering components, both before-equivalence and beyond-equivalence plots should ideally cross the x axis at the same point. Non-ideal behaviour can result from measurement errors ("e.g." a poorly calibrated electrode, an insufficient equilibration time before recording the electrode reading, drifts in ionic strength), sampling errors ("e.g." low data densities in the linear regions) or an incomplete chemical model ("e.g." the presence of titratable impurities such as carbonate in the base, or incomplete precipitation in potentiometric titrations of dilute solutions, for which Gran "et al." (1981) propose alternate approaches). Buffle "et al." (1972) discuss a number of error sources.

Because the 10^{pH_i} or 10^{-pH_i} terms in the Gran functions only asymptotically tend toward, and never reach, the x axis, curvature approaching the equivalence point is to be expected in all cases. However, there is disagreement among practitioners as to which data to plot, whether using only data on one side of equivalence or on both sides, and whether to select data nearest equivalence or in the most linear portions: [ Instructions given to students are particularly indicative of the different recommended practices. For instance, C. Chambers of George Fox University recommends using the acid-side data "just before" the equivalence point in acid-base titrations (cite web |url=http://academic.georgefox.edu/~cchamber/analytical/NeutTit.pdf |title= On-line document |accessdate=2008-02-17). Following Harris (1998), M. El-Koueidi and M. Murphy of the University of North Carolina (Charlotte) (cite web |url=http://www.chem.uncc.edu/courses/3111/Chem3111Expt2.pdf |title= On-line document |accessdate=2008-03-22) and W. Wolbach of Depaul University (cite web |url=http://chemistry.depaul.edu/wwolbach/205_207/9.pdf |title= On-line document |accessdate=2008-02-17) recommend using last 10-20% volume data before the equivalence point, and the latter recognizes that base-side Gran plots from titrations of acid by base ("i.e" after the equivalence point) can be used to assess the CO2 content in the base. Similarly, W. E. Brewer and J. L. Ferry of the University of South Carolina recommend using those data within 10% before equivalence (cite web |url=http://www.chem.sc.edu/analytical/chem321L/labs/Expt5.pdf |title= On-line document |accessdate=2008-02-17). K. Kuwata of Macalester College recommends that students choose whichever data region gives the straightest line before equivalence (cite web |url=http://www.macalester.edu/~kuwata/classes/2005-06/Chem%20222/Gran%20Plot%20Lab%202006.pdf |title= On-line document |accessdate=2008-02-17). D. L. Zellmer of the California State University at Fresno asks students to plot data from both sides of equivalence, using data furthest from equivalence, and to assess the errors in order to determine whether or not the two estimates of the equivalence volumes are significantly different (pH data: cite web |url=http://zimmer.csufresno.edu/~davidz/Chem102/pHGrans/pHGrans.html |title= On-line document |accessdate=2008-02-17; potentiometric titration of chloride ion with silver ion: cite web |url=http://zimmer.csufresno.edu/~davidz/Chem102/GransPlot/GransPlot.html |title= On-line document |accessdate=2008-02-17). ] [Butler (1991) discusses the issue of data selection, and also examines interferences from titratable impurities such as borate and phosphate. ] using the data nearest the equivalence point will enable the two x-intercepts to be more coincident with each other and to better coincide with estimates from derivative plots, while using acid-side data in an acid-base titration presumably minimizes interference from titratable (buffering) impurities, such as bicarbonate/carbonate in the base (see Carbonate content), and the effect of a drifting ionic strength. In the sample plots displayed in the Figures, the most linear regions (the data represented by filled circles) were selected for the least-squares computations of slopes and intercepts. Data selection is always subjective.

Electrode calibration

Another use of the Gran plot is for electrode calibration. [F.J.C. Rossotti and H. Rossotti, "J. Chem. Ed"., (1965), 42, 375] The advantage of this method of electrode calibration is that it can be performed in the presence of a background medium of constant ionic strength, the same medium which may later be be used for the determination of equilibrium constants. The electrode is assumed to obey a modified form of the Nernst equation.:E=E^0+s ln [H^+] ,"E" is a measured electrode potential, E^0 and "s" are calibration constants and [H^+] is the hydrogen ion concentration. The parameter "s" is ideally equal to "RT/F" (59.1 mV at 298 K) but in practice may not be exactly equal to the ideal value. Note that the electrode response is assumed to be linear in hydrogen ion "concentration" (not activity). This assumption is valid when the ionic strength is effectively constant. Rearrangement of this equation gives: [H^+] =10^{frac{E^0-E}{sThe analytical concentration of the hydrogen ion, T_H in a titration of a strong acid, concentration a_H, with a strong base, concentration b_H is given by:(T_H)_i=frac{a_Hv_0-b_H v_i}{v_0+v_i}By convention the concentration of hydroxide ions is given as minus the corresponding concentration of hydrogen ions. Note that for this application the concentrations of acid and base must "both" be "known". The concentrations should be directly related to primary standards.

As described above, in acid solutions with a pH of 5 or less, the concentration of hydoxide ions is negligible compared to the concentration of hydrogen ions, so [H^+] =T_H. Therefore:(v_0+v_i)10^{-frac{E^0-E}{s=a_Hv_0-b_H v_iand a plot of the function on the left-hand side, sometimes called the Gran function, against titre, v_i should be a straight line cutting the x-axis at the (acid) equivalence point. The parameters E^0 and "s" can be obtained by the method of least squares. By a similar argument, with alkaline solutions of pH 9 or above the left-hand side of the equation:(v_0+v_i)10^{-frac{E-E^0}{s}pK_w}=a_Hv_0-b_H v_iwill yield a straight line with respect to titre, cutting tha axis at the (alkaline) equivalence point. The difference between the two equivalence points can be used to calculate a carbonate content value.

A computer program, GLEE, implementing this approach to electrode calibration is available. [http://www.hyperquad.co.uk/glee.htm GLass Electrode Evaluation]

References

* Buffle, J., Parthasarathy, N. and Monnier, D. (1972): Errors in the Gran addition method. Part I. Theoretical Calculation of Statistical Errors; "Anal. Chim. Acta" 59, 427-438; Buffle, J. (1972): "Anal. Chim. Acta" 59, 439.

* Butler, J. N. (1991): Carbon Dioxide Equilibria and Their Applications; CRC Press: Boca Raton, FL.

* Butler, J. N. (1998): Ionic Equilibrium: Solubility and pH Calculations; Wiley-Interscience. Chap. 3.

* Gran, G. (1950): Determination of the equivalence point in potentiometric titrations, "Acta Chemica Scandinavica", 4, 559-577.

* Gran, G. (1952): Determination of the equivalence point in potentiometric titrations-- Part II, "Analyst", 77, 661-671.

* Gran, G., Johansson, A. and Johansson, S. (1981): Automatic Titration by Stepwise Addition of Equal Volumes of Titrant Part VII. Potentiometric Precipitation Titrations, "Analyst", 106, 1109-1118.

* Harris, D. C.: Quantitative Chemical Analysis, 5th Ed.; W.H. Freeman & Co., New. York, NY, 1998.

* Martell, A. E. and Motekaitis, R. J.: The determination and use of stability constants, Wiley-VCH, 1992.

* Skoog, D. A., West, D. M., Holler, F. J. and Crouch, S. R. (2003): Fundamentals of Analytical Chemistry: An Introduction, 8th Ed., Brooks and Cole, Chap. 37.

* Stumm, W. and Morgan, J. J. (1981): Aquatic chemistry, 2nd Ed.; John Wiley & Sons, New York.

Notes


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