Intro to Titrations

Volumetric Analysis Volumetric analysis is when the volume of a reactant required to complete a chemical reaction is measured. As the stoichiometry of the reaction is known, the amount of the second reactant can be calculated from the known volume added of the first reactant.

Definitions Titration: the process in which small increments of a known solution of a reagent is added to a flask containing a known mass or volume of a second reactant (analyte). Titrations are usually, but not always, carried out with a buret so that the small increments can be accurately and precisely measured. The substance in the buret is the titrant.

Principles Examples of titration: acid/base neutralization titrations, redox reactions, precipitation reactions, and complex formation reactions. The titration ends when the analyte in the flask is completely consumed by the reagent being added from the buret. This point, the stoichiometric end of the reaction, is called the equivalence point.

Principles But how do you know when the reaction is completed? What we “see” as the end of the reaction is the endpoint, which is when a sudden change in a physical property is observed. Physical properties which are commonly followed to give an endpoint: color change (such as when a color indicator is used), voltage or current, absorbance of light by a species in the reaction flask.

Principles Indicators, usually color, are compounds (which may be a reactant in the reaction or not) which show an abrupt change when the reaction is complete, and so mark the endpoint. The indicator or other physical property followed is either indicating the disappearance of the analyte or the appearance of an excess of the titrant.

Principles Unfortunately, the endpoint and the equivalence point are not exactly the same. The difference between the two is the titration error. For many titrations, this error is caused by the fact that the endpoint is measured by the appearance of excess titrant in the flask. So the endpoint is slightly beyond the equivalence point.

Principles Blank titrations can be run to determine the volume of titrant required to give the physical change with no analyte. This volume can be subtracted from the endpoint of an actual titration to give a more accurate picture of the equivalence point. But it is important to remember that there will be a slight error in the endpoint.

Principles In a titration, one of the reactants must be of known concentration or of known purity. If the known is a solid, then it should be a primary standard, with a purity of at least 99.9%. The primary standard must also be dried and cooled in a dessicator to remove any water which may be adsorbed. Reagent solutions of known concentrations (hopefully to 4 or 5 sig figs) may be made or purchased and are called standard solutions.

Principles If you make a standard solution, you titrate a primary standard with a solution of the reagent until the endpoint is reached, and then calculate the concentration of the solution using the stoichiometry of the balanced equation. This is called standardization. For acid-base reactions, primary standard KHP (potassium hydrogen phthalate, an acid) is commonly used to standardize a base solution (NaOH). The standardized base solution may then be used to standardize HCl solutions or other acidic solutions.

Principles There are two ways a titration may be conducted: direct titration and back titration. A direct titration is when the titrant is added to the analyte until the endpoint is reached. This is the classic titration.

Principles In a back titration, a known volume of a standard reagent is added to the analyte. Some of the standard reagent reacts with the analyte, but an excess remains. There must be an excess of the standard reagent. At this point, no analyte remains. The excess standard reagent is then titrated with a different standard reagent until the endpoint of this second reaction is reached.

Principles Back titrations are often conducted when the endpoint for the analyte reaction with a reagent is not obvious. If the second reaction has a very obvious, sharp endpoint, it would be the preferred titration to give an accurate and precise measurement of the endpoint. For either type of titration, once the endpoint is measured, the math begins!

Titration Calculations In a titration calculation you must: 1.from the volume of titrant used or the known mass of the primary standard, calculate the moles of titrant or primary standard consumed 2.from the stoichiometry of the balanced equation, convert the moles of the known titrant or primary standard to moles of analyte or unknown solution 3.if the molarity of the unknown solution is desired, divide the number of moles by the volume of unknown solution.

Solubility Product of Ionic Compounds You should remember that many ionic compounds are only moderately soluble or are classified as insoluble in water. That means if you add an insoluble ionic compound like silver bromide to water, only a small fraction of the silver bromide will dissolve into silver ions and bromide ions. This means that this is an equilibrium process and can be written as an equilibrium reaction with an equilibrium constant:

Solubility Product of Ionic Compounds The equilibrium constant for the dissolution of an ionic compound in water is called the solubility product. The solubility product tells you how much of the compound will dissolve to make a saturated solution of the compound. The K sp gives you the molar solubility or the mass solubility of the compound.

Common Ion Effect on Solubility The above was for the dissolution of the ionic compound in pure water, but many times the ionic compound is added to an aqueous solution with other substances. These other substances may affect the equilbrium and thus the solubility. A common case is when the ionic compound is added to a solution containing a common ion, that is, an ion which is also contained in the ionic compound.

Common Ion Effect on Solubility The common ion in the solution affects the equilibrium and thus how much of the compound dissolves. So if AgCl is added to an aqueous solution of HCl, the common ion is chloride. Using Le Chatelier’s Principle, it should be obvious that adding AgCl to a solution already containing dissociated chloride ions will suppress the dissolution of AgCl. This will decrease its solubility.

Common Ion Effect on Solubility Mathematically, this may also be proven :

Titrations of Ion Mixtures Usually only one analyte ion is titrated in a solution (of course this is excepting an indicator). But mixtures of 2 analytes ions (or more) may also be titrated. This is only possible when the two ions have very different solubility products for the resulting ionic compounds. They need to differ by about 10 3 or more to get a sufficient separation. The further apart they are, the clearer the two separate endpoints will be.

Silver Ion Titrations There are two common methods of titration using the silver ion as a reagent: –the Volhard Method –the Fajans Method. These are called argentometric methods.

Volhard Method: A Back Titration This method is used to determine the amount or concentration of any anion which forms an insoluble silver compound. A known volume of silver nitrate is added to the flask containing the anion to force the complete precipitation of the silver compound. Excess silver ion is present.

Volhard Method: A Back Titration The ppt may be filtered out (if the silver compound is more soluble than the silver thiocyanate produced in the next step, it must be filtered at this point). The filtrate with excess silver ion is then back titrated with KSCN - (potassium thiocyanate ion) with ferric ion present to make solid silver thiocyanate.

Volhard Method: A Back Titration After the silver ion has been removed by the thiocyanate ion, the ferric ion in the solution then reacts with the thiocyanate ion to produce a orange-red solution of ferric thiocyanate complex ion:

Volhard Method: A Back Titration The appearance of the orange/red color marks the endpoint of the silver reaction with thiocyanate. The amount of silver used in the thiocyanate reaction is then calculated, which gives the amount of silver that reacted with the anion.

Fajans Method The Fajans titration is actually very interesting as it uses an adsorption indicator. It is used to analyze a variety of ions, not just anions which form ppts with silver Commonly, it is used to analyze the amount of an anion by titrating with silver ion. The insoluble silver salt crystal starts to form.

Fajans Method Interestingly, as there is a large excess of the anion, some anions are adsorbed on the surface of the crystal. They are not part of the crystal lattice, just adsorbed. However, this gives the crystal an overall negative charge. At the equivalence point, all of the anion has crystallized with the silver ion.

Fajans Method Now, as more silver ion is added, the silver ion adsorbs onto the crystal, giving the crystal a positive charge. The adsorption indicator, usually a negatively charged species, then is attracted to the positively charged crystal and interacts with it. This interaction between the indicator and the crystal causes a color change, the end point. Note that the end point is past the equivalence point.