TITRATION>>PRINCIPLES & APPLICATIONS OF INSTRUMENTS & TECHNIQUES<<

By: Saloni Shroff

Semester 1 – Roll no. 04

FN (Nutrigenomics)

INTRODUCTION . . .

Titration is a technique to determine the concentration of an unknown solution.

Titration is the slow addition of one solution of a known concentration (called a titrant or titrator) to a known volume of another solution of unknown concentration (called a titrand or analyte) until the reaction reaches neutralization, which is often indicated by a color change.

Also known as Titrimetry or Volumetric Titration.

Elements of Titration . . . The standard solution: the

solution of known concentration. An accurately measured amount of standard solution is added during titration to the solution of unknown concentration until the equivalence or endpoint is reached.

The analyte: the solution of unknown concentration is known as the analyte. During titration the titrant is added to the analyte in order to achieve the equivalence point and determine the concentration of the analyte.

The equivalence point: the point when the reactants are done reacting.The equivalence point is the ideal point for the completion of titration. At the equivalence point the correct amount of standard solution must be added to fully react with the unknown concentration.

The end point: it indicates once the equivalence point has been reached. It is indicated by some form of indicator which varies depending on what type of titration being done. For example, if a color indicator is used, the solution will change color when the titration is at its end point.

Equivalence point & End point are not necessarily equal.

An endpoint is indicated by some form of indicator at the end of a titration.

An equivalence point is when the moles of a standard solution (titrant) equal the moles of a solution of unknown concentration (analyte).

The calibrated burette: it is the main piece of equipment required for a titration method. Calibration is important because it is essential for the burette to be as accurate as possible in order to dispense very precise amounts of liquid into the sample.

A burette is a long cylindrical piece of glass with an open top for pouring in the titrant. At the bottom there is a carefully formed tip for dispensing.

Burettes usually have a plastic stopper that can easily be turned to deliver mere fractions of a drop of titrant, if needed. Burettes come in many sizes and are marked in millilitres and fractions of millilitres.

The Indicator: the use of an indicator is key in performing a successful titration reaction. The purpose of the indicator is to show when enough standard solution has been added to fully react with the unknown concentration. 

Indicators must only be added to the solution of unknown concentration when no visible reaction will occur. Depending on the solution being titrated, the choice of indicator can become key for the success of the titration.

Titration with an Indicator . . .

THE TITRATION SET UP

MATERIALS . . .~Erlenmeyer flask or Beaker

~Excess amount of standard solution (titrant)

~A precisely measured amount of analyte; this will be used to make the solution of unknown concentration

~Indicator

~Calibrated Burette

~Burette Stand

http://chemwiki.ucdavis.edu/@api/deki/files/462/new_burette.jpg

PROCEDURE . . .Measure out a precise amount of analyte & make up the solution of unknown concentration

Quantitatively transfer the analyte into a beaker or Erlenmeyer flask

Add four to five drops of the appropriate color indicator into the beaker

Fill the burette with an excess amount of titrant, the standard solution of known concentration and should be in aqueous form

Place the beaker containing the aqueous solution of unknown concentration under the burette

Record the initial volume of the burette. Make sure to measure at the bottom of the meniscus

Measure and record your final volume of the burette. Calculate the volume of standard solution used by subtracting the initial volume measurement from the final volume measurement of the burette

Now perform the necessary calculations in order to obtain the concentration of the unknown solution

TYPES Of Titrations . . . There are many types of titrations with

different procedures and goals.

Acid – Base titration Redox titration Gas phase titration Complexometric titration Back titration Karl Fischer titration (Potentiometric)

Acid – Base titration:

Acid-base titrations depend on the neutralization between an acid and a base when mixed in solution.

In addition to the sample, an appropriate indicator is added to the titration chamber, reflecting the pH range of the equivalence point.

The acid-base indicator indicates the endpoint of the titration by changing color.

The final solution after titration should be neutralized and contain equal moles of hydroxide and hydrogen ions. So the moles of acid should equal the moles of base:

Some common Indicators used in acid – base titration:

Redox titration: Redox titrations are based on a reduction-oxidation

reaction between an oxidizing agent and a reducing agent.

A potentiometer or a redox indicator is usually used to determine the endpoint of the titration.

Some redox titrations do not require an indicator, due to the intense color of the constituents.

For instance, in permanganometry a slight persisting pink color signals the endpoint of the titration because of the color of the excess oxidizing agent potassium permanganate

Gas phase titration: Gas phase titrations are titrations done in the gas

phase, specifically as methods for determining reactive species by reaction with an excess of some other gas, acting as the titrant.

In one common gas phase titration, gaseous ozone is titrated with nitrogen oxide according to the reaction.

O3 + NO → O2 + NO2

After the reaction is complete, the remaining titrant and product are quantified (e.g., by FT-IR) - this is used to determine the amount of analyte in the original sample.

Complexometric titration: Complexometric titrations

rely on the formation of a complex between the analyte and the titrant.

In general, they require specialized indicators that form weak complexes with the analyte.

Common examples are Eriochrome Black T for the titration of calcium and magnesium ions, and the chelating agent EDTA used to titrate metal ions in solution.

Back titration: Back titration is a titration done in reverse- instead

of titrating the original sample, a known excess of standard reagent is added to the solution, and the excess is titrated.

A back titration is useful if the endpoint of the reverse titration is easier to identify than the endpoint of the normal titration, as with precipitation reactions.

Back titrations are also useful if the reaction between the analyte and the titrant is very slow, or when the analyte is in a non-soluble solid.

Karl Fischer titration: A potentiometric method to

analyze trace amounts of water in a substance.

A sample is dissolved in methanol, and titrated with Karl Fischer reagent. The reagent contains iodine, which reacts proportionally with water.

Thus, the water content can be determined by monitoring the potential of excess iodine.

Titration CURVES . . . The graphs of titration curves effectively

show the relationship between the pH of the solution of unknown concentration as the standard solution is added to it in order to reach neutralization.

APPLICATIONS OF

TITRIMETRY

In biodiesel: Waste vegetable oil (WVO) must be neutralized before a batch may be processed. A portion of WVO is titrated with a base to determine acidity, so the rest of the batch may be properly neutralized. This removes free fatty acids from the WVO that would normally react to make soap instead of biodiesel.

Kjeldahl method: A measure of nitrogen content in a sample. Organic nitrogen is digested into ammonia with sulfuric acid and potassium sulfate. Finally, ammonia is back titrated with boric acid and then sodium carbonate.

Winkler test for dissolved oxygen: Used to determine oxygen concentration in water. Oxygen in water samples is reduced using manganese(II) sulfate, which reacts with potassium iodide to produce iodine. The iodine is released in proportion to the oxygen in the sample, thus the oxygen concentration is determined with a redox titration of iodine with thiosulfate using a starch indicator.

Vitamin C: Also known as ascorbic acid, vitamin C is a powerful reducing agent. Its concentration can easily be identified when titrated with the blue dye Dichlorophenolindophenol (DCPIP) which turns colorless when reduced by the vitamin.

Ester value (or ester index): A calculated index. Ester value = Saponification value – Acid value.

Acid value: The mass in milligrams of potassium hydroxide (KOH) required to neutralize carboxylic acid in one gram of sample. An example is the determination of free fatty acid content. These titrations are achieved at low temperatures.

Saponification value: The mass in milligrams of KOH required to saponify carboxylic acid in one gram of sample. Saponification is used to determine average chain length of fatty acids in fat. These titrations are achieved at high temperatures.

Benedict's reagent: Excess glucose in urine may indicate diabetes in the patient. Benedict's method is the conventional method to quantify glucose in urine using a prepared reagent. In this titration, glucose reduces cupric ions to cuprous ions which react with potassium thiocyanate to produce a white precipitate, indicating the endpoint.

. . REFERENCES http://www.metrohmusa.com/Products/Titration/Food-Titration.html

http://ca.mt.com/ca/en/home/applications/Application_Browse_Laboratory_Analytics/Application_fam_browse_main.tabs.applications.html

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