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Classification Of Reactions In Titrimetrlc Analysis

Titrimetric methods generally employed in analysis may be broadly classified into the following types:

1. Acid-base or Neutralization reactions, or acidimetry and alkalimetry.

These include the titration of free bases, or those formed from salts of weak acids by hydrolysis, with a standard acid (acidirnetry), and the titration of free acids, or those formed by the hydrolysis of salts of weak bases, with a standard base (alkalirnetry). The reactions involve the combination of hydrogen and hydroxide ions to form water.

2. Complexometric Titration

Complexometric titration (sometimes chelatometry) is a form of volumetric analysis in which the formation of a colored complex is used to indicate the end point of a titration.

These depend upon the combination of ions, other than hydrogen or hydroxide ions, to form a soluble, slightly dissociated ion or compound, as in the titration of a solution of a cyanide with silver nitrate  (2CN^– + Ag⁺ -[Ag(CN),]-) or of chloride ion with mercury(I1) nitrate solution (2C1^– + Hg²⁺=Hg

Ethylenediaminetetra-acetic C1₂). acid, largely as the disodium salt of EDTA, is a very important reagent for complex formation titrations and has become one of the most important reagents used in titrimetric analysis. Equivalence point detection by the use of metal-ion indicators has greatly enhanced its value in titrimetry.

3. Precipitation reactions.

 These depend upon the combination of ions to form a simple precipitate as in the titration of silver ion with a solution of a chloride. No change in oxidation state occurs.

4.  Oxidation-reduction reactions.

Under this heading are included al1 reactions involving change of oxidation number or transfer of electrons among the reacting substances. The standard solutions are either oxidising or reducing agents. The principal oxidising agents are potassium permanganate, potassium dichromate, cerium(1V) sulphate, iodine, potassium iodate, and potassium bromate.

 Frequently used reducing agents are iron(I1) and tin(I1) compounds, sodium thiosulphate, arsenic(II1) oxide, mercury(1) nitrate, vanadium(I1) chloride or sulphate, chromium(I1) chloride or sulphate, and titanium(II1) chloride or sulphate.

Iodometric Titration

Iodometric titration involves the reaction of iodine with a known amount of reducing agent, usually sodium thiosulfate (Na₂S₂O₃) or PAO. Starch solution is used as an indicator to detect the end point of the titration. Thus, the exact amount of iodine that would react with a measured volume of sodium thiosulfate of known strength is determined. From this, the concentration of the analyte in the sample, which is proportional to the amount of iodine reacted with thiosulfate or PAO, is then calculate

Titrimetric Procedures Applied in Environmental Analysis

INDICATORS

A useful indicator has a strong color that changes quickly near its pKa. These traits are desirable so only a small amount of an indicator is needed. If a large amount of indicator is used, the indicator will effect the final pH, lowering the accuracy of the experiment.

 The indicator should also have a pKa value near the pH of the titration’s endpoint. For example a analyte that is a weak base would require an indicator with a pKa less than 7. Choosing an indicator with a pKa near the endpoint’s pH will also reduce error because the color change occurs sharply during the endpoint where the pH spikes, giving a more precise endpoint.

Figure 1: A Basic Titration Curve, The horizontal lines show the range of pH in which phenolphthalein (blue) and methyl orange (red) changes color. The middle line represents the pKa, while the two outer lines represent the end or start of the color changes. The peak and light blue highlights show the range in which the color changes will occur based on the amount of titrant added 

Notice that this reaction is between a weak acid and a strong base so phenolphthalein with a pKa of 9.1 would be a better choice than methyl orange with a pKa of 3.8. If in this reaction we were to use methyl orange as the indicator color changes would occur all throughout the region highlighted in pink.

The data obtained would be hard to determine due to the large range of color change, and inaccurate as the color change does not even lie with the endpoint region. Phenolphthalein on the other hand changes color rapidly near the endpoint allowing for more accurate data to be gathered.

Error in Titration Calculations

Different methods are used to determine the equivalence point of a titration. No matter which method is used, some error is introduced, so the concentration value is close to the true value, but not exact. For example, if a colored pH indicator is used, it might be difficult to detect the color change. Usually, the error here is to go past the equivalence point, giving a concentration value that is too high.

Another potential source of error when an acid-base indicator is used is if water used to prepare the solutions contains ions that would change the pH of the solution. For example, if hard tap water is used, the starting solution would be more alkaline than if distilled deionized water had been the solvent.

If a graph or titration curve is used to find the endpoint, the equivalence point is a curve rather than a sharp point. The endpoint is a sort of “best guess” based on the experimental data.

The error can be minimized by using a calibrated pH meter to find the endpoint of  an acid-base titration rather than a color change or extrapolation from a graph.

PRECIPITATION TITRATION

This  is a titrimetric method which involves the formation of precipitates during the experiment of titration. The titrant reacts with the analyte and forms an insoluble substance. The titration is continued till the last drop of the analyte is consumed.

If you don’t know the concentration of the second substance in solution, but know its volume, then you can figure out how much of the substance is present by slowly adding the substance of known concentration (the titrant) until the second substance (the analyte) is gone.

This is made possible by knowing the stoichiometry  of  how molecules combine in specific reactions. The amount of titrant consumed translates into the amount of product formed and hence the amount of analyte that was present in the pre-reaction solution. Dividing this amount by the volume gives the molar concentration of the second substance.In reactions that result in a precipitate rather than products that remain dissolved in the solution, it can be difficult to visually determine the reaction end point, which is why numerous precipitation titration indicators  exist to more precisely signal this point.

These involve indicators that form a second precipitate when the amount of titrant added exceeds the amount of analyte present and reacts with the indicator substance.

Indicators for Precipitation Titrations

Three broad classes of indicators exist for precipitation titrations. To look for chloride ions; one looks for the cation silver.

Precipitation Titration Example

To determine the concentration of chloride ion in a certain solution we can titrate this solution with silver nitrate solution (whose concentration is known). The chemical reaction occurs as follows:

Ag⁺(aq) + Cl^–(aq)→ AgCl(s).

AgCl in the form of a white precipitate can be seen settled at the bottom of the flask during titration. The quantity of silver ion used to equivalence point is equal to the quantity of chloride ion which was originally present.

To calculate the number of moles of chloride ion or silver ion we can use

n = cV (molarity definition)                                                                 

To calculate the volume of the added solution or molar concentration of ion the corresponding values of either of the ions should be known.