Volumetric Analysis Volumetric analysis is a form of quantitative analysis involving the measuring of volumes of reacting solutions, it involves the use of titrations. When buying food we often have two questions: What is in it? Quantitative analysis answers the second question How much of it is in there? When chemists talk about the amount of two substances being equal, they are talking about an equal number of particles not an equal mass or volume.the unit of measurement used is called a mole, which is the mass of approx 6.02 x 10 23 atoms. The number of moles can be calculated from the following equation. n (mol) = m (g) M (g mol-1)
Concentration Usually when solutions are analysed by titration the results are presented as solution concentration.molar concentration or molarity is the amount of a substance in moles in one litre of solution, where Molar concentration = amount of solute in moles / volume of solution in litres the units of measurement is the mol.l -1 C Mass concentration represents the mass of solute in one litre of solution. n V Mass concentration= mass of solute in grams/ volume of solution in litres the units for this would be in grams.l -1 Other concentration units include: m C V Percent weight per volume: the mass of solute in grams, in 100mL of solution. 1%W/V= 1g.100mL -1 Parts per million (ppm): 1ppm equals 1 milligram per litre 1ppm= 1mg.L -1 Parts per billion (ppb): 1ppb= 1 g.l -1 These can be calculated from the mass concentration. mol L -1 M 10 mol L -1 M mol L -1 10 1000 1000 ppb 1000 1000 ppb
Stoichiometry Equations are used to describe chemical reactions. Reactants Products They give lots of information. Ex H 2 + O 2 H 2 O This doesn t balance so coefficients are used to balance it out (Law of conservation of matter) 2H 2(g) + O 2(g) 2H 2 O (l) The physical state is also shown This tells us that 2 moles of hydrogen molecules reacts with 1 mole of oxygen molecules to produce 2 moles of water molecules.
Volumetric Glassware As you know there are often three ways of using the apparatus, whatever it is: The correct way, the wrong way and my way. (Stanley et al, 2005) The following gives a guide on the correct use of some volumetric glassware that you will be using during this topic. Remember when reading all volumes off apparatus: Keep your eye level with the meniscus The volume reading is made with the bottom of the meniscus Do not use hot solutions when making readings. Read from here
Volumetric Flask A volumetric flask is used in the preparation of standard solutions. A standard solution is simply one of a known concentration. Cleaning - Rinse with distilled water and shake out the excess. - Water will be used as the solvent so there is no need to dry. Adding the solute Method 1 Method 2 Labelling - Ensure the flask is labelled with Your name Solution Name Concentration Date prepared After use - Rinse with distilled water and drain
Pipette A pipette is used to deliver a known volume of liquid. It takes into account any liquid sticking to the walls. Cleaning - Must be kept extremely clean - Rinse with distilled water - The water inside could dilute the sample. To get it out draw about 5ml of the sample in and rotate it (see diagram). Drain. Filling - Place the tip well into the solution, you don t want to suck up any air. - Using filler bulb or syringe overfill the pipette calibration mark - Use the release button to allow the level to fall the graduation mark. Delivery - Liquid must run out under its own weight. - Hold the pipette vertical and the receiving flask at an angle. - The tip of the pipette must touch the neck of the flask - After the pipette is drained wait 5-10 seconds to make sure it is completely drained. - There will be one drop left in the tip, it has been accounted for so you don t have to get this out. - Using distilled water wash any drops stuck to the flask down into the flask. This will change the concentration but not the amount of substance. After use - Rinse with distilled water.
Burette A burette is designed to deliver known but variable volumes of liquid (accounts for liquid sticking to the walls) Cleaning - Rinse the burette with 5-10 ml of the solution you are going to use. Do this by holding the burette horizontal and rotating it (same as with the pipette. - Do not pour the liquid out, let it run through the tap so that the whole burette is clean. Filling - Set up the burette in its stand ensuring that it is carefully clamped and vertical. - Place a plastic funnel in the top of the burette and check the tap is in the off position. - Pour the required solution into the burette until the level is about the 0.0 ml mark. It does not have to be exact but it cannot go past the 0.0 mark. - Remove the plastic funnel Delivery - Turn the tap so that the burette tip is filled with liquid below the tap. This will now mean you can accurately read off the volume. - Use the Left hand turn Right hand shake rule when using a burette. Using the left hand to turn the tap and the right hand to gently shake or swirl the flask. (see diagram below) After Use - Rinse with distilled water and allow to drain.
Performing a titration 1. Rinse a conical flask with distilled water and fill it with a known amount of the substance being analysed using a pipette. 2. Fill the burette with your standard solution and record the initial burette reading in your book. 3. Perform a scout titration by opening the tap of the burette so that it flows into the conical flask, swirl the flask while this is happening. 4. Close the tap when the endpoint is seen (permanent colour change) 5. Record the final burette reading. 6. Calculate the volume of titrant delivered known as the titre. 7. Perform the actual titration by setting up as with the scout titration and perform the following steps. 8. Turn the tap and let the titrant flow rapidly out of the tip until you are within 2 ml of the expected endpoint, as found above in the scout titration. 9. Rinse the walls of the flask with the wash bottle again. 10. Continue adding the titrant one drop at a time until you have reached the endpoint. 11. Turn off the tap and record the final burette reading 12. Calculate the titre. 13. Repeat steps 7-12 Concordant titres are those that agree within 0.1 ml generally the aim is to achieve two or more concordant titres.
CHROMATOGRAPHY Adsorption Adsorption is the attachment of molecules to the surface of a solid. The attachment occurs when secondary bonds form between the molecules and atoms at the surface of the solid. Depending on the strength of the secondary bonds the molecules will be adsorbed accordingly. Eg for a polar surface, if polar molecules come into contact with the surface the stronger the polarity of the molecules the more strongly they will be adsorbed. Note that absorption is different from adsorption as in absorption the particles are incorporated into the body of another material. Adsorption Chromatography Adsorption chromatography is a widely used technique for separating and identifying the components of mixtures. The separation occurs because of the variation in the strengths of the adsorption of molecules to different compounds to a solid surface. It can also be used to do quantitative determinations of concentrations. Chromatography is usually used to separate mixtures of organic compounds. It is usually done with other analytical techniques. There are several forms of adsorption chromatography, they are all based on the principle of different rates of movement of each component of a mixture through a solid called the stationary phase. The components are transported through the stationary phase by a liquid or gas called the mobile phase. The rate of movement depends on the strengths of the secondary bonds formed between the stationary and mobile phase. Stationary phase: the material on to which the components of the mixture are initially adsorbed. Mobile phase: the liquid or gas that moves across the stationary phase carrying the mixture to be separated with it.
For each type of adsorption chromatography the most common materials used for the solid stationary phase are finely divided silica, SiO 2, and alumina, Al 2 O 3. These both have polar O-H groups at their surface when they are hydrated. Silica and alumina are used in normal phase adsorption chromatography where the polarity of the stationary phase is greater than the mobile phase. Reverse Phase chromatography is more commonly used. Reverse phase uses a mobile phase with more polarity than the stationary phase. For this the hydrated ends of the silica are replaced with non polar hydrocarbon chains.
The separation process in adsorption chromatography A sample of the mixture to be separated is brought into contact with the surface of the stationary phase, over which a liquid or gas mobile phase is allowed to flow. Each component of the mixture will be adsorbed differently onto the stationary phase and be moved along at different rates by the mobile phase. This will result in different rates of movement. Components that are strongly adsorbed onto the surface of the stationary phase and weakly bonded with the mobile phase will hardly move. The components that are more soluble in the mobile phase will move further along the surface for a given time. For Reverse phase the more polar components bond weakly with the non-polar stationary phase and hence move more freely through the surface of the stationary phase. The task for the chromatographer is to choose the appropriate adsorbent and solvent system to give the best separation.
Different types of Chromatography Column Chromatography The Column consists of a vertical glass tube a little wider than a burette packed with small uniform sized particles of the stationary phase. A concentrated sample to be separated is carefully placed as a layer on the top of the stationary phase and this is washed down (eluted ) by the mobile phase introduced from a reservoir at the top of the column. The components of the sample will separate into bands. Given enough eluent, the components can eventually be washed out of the column in turn and collected for identification and further analysis. Thin Layer Chromatography (TLC) In TLC the stationary phase is usually a very thin layer (.02 m) of finely powdered alumina or silica, which has been applied evenly applied to a thin glass plate or plastic sheet. A small concentrated spot of the mixture to be separated is made on or near one end of the plate on a base line. The solvent is allowed to evaporate and the spotting procedure is repeated several times to increase the amount of the sample. The plate is then stood upright in a sealed vessel containing the liquid mobile phase to a shallow depth. Only the edge below the spot is submerged in the mobile phase. As the solvent moves up through the stationary phase by capillary action the components of the mixture separate. The components of the mixture can be identified by matching the distance that they have travelled with the distances that known standards have moved. This method can only be used if you use the same mobile and stationary phase.
In the above diagram a sample of an unknown mixture M is placed on the baseline. As are known compounds A and B. After a period of time M has separated into two components X and Y. X has moved the same distance as A so you can conclude that X and A are the same compound. Y has moved the same distance as B and therefore they are also the same compound. The Retardation Factor The movement of any component relative to the solvent front is a reproducible characteristic property. It is called the Retardation factor R f. It is defined as the ratio of the distance of a compound moves from the baseline relative to the distance the solvent front moves.
Gas- Solid Chromatography This form of adsorption chromatography can be used to separate mixtures of volatile organic compounds. The stationary phase consists of uniform sized solid particles either packed in a long narrow tube or coated on the inside. A very small quantity (0.5-20 L) of the mixture to be separated is injected by means of a syringe into a flash vaporiser at the head of the chromatography column which is heated in an oven. The temp. of the vaporiser must be 500C higher than the highest boiling point of the least volatile compound. The mobile phase is usually an inert gas such as helium or nitrogen. As the mixture sweeps through the column separation is achieved. On leaving the column each component passes through a detector that sends an electrical signal to a recorder that produces a peak The series of peaks representing all of the components of the mixture is called a chromatogram. Qualitative identification of the components can be done through comparing all the retention time values with those of known compounds. Confirming analysis can then be carried out using techniques such as mass spectroscopy or Infra -red spectroscopy.
The diagram below shows how a 2 component mixture has been separated. Quantitative analysis of the mixture is based on a comparison of either peak heights or peak areas for the components with those of standards that have been passed through the column. The ratio of the two peak areas is equal to the to the ratio of amounts of A and B in the mixture. High Pressure Liquid-Solid Chromatography(HPLC) For mixture with high boiling points gas chromatography is not applicable. HPLC is used instead. The solid stationary phase is packed or coated inside a column of 1-5mm in diameter and 30cm in length. The mobile phase is a high-pressure liquid forced through by a pump. The mixture gets separated after it is injected into the column and quantitative and qualitative analysis is performed by the use of retention times.
Atomic Spectroscopy When we heat or burn elements they have emit characteristic colours or frequencies of light. These are similar to fingerprints and can be used to identify metals. The emitted light is actually made of a number of frequencies of light that can observed using a prism or diffraction grating. The series of frequencies is called a line emission spectrum where the energy of the light corresponds to the energy level differences within an atom. The energy is emitted when electron fall from higher energy levels to lower ones emitting the energy as light.
Absorption Spectra When white light is passed through a prism it produces a continuous spectrum (ROYGBIV). White light is made up of a continuos spectrum of frequencies. When this light is shone through the vapour of a metal the metal will absorb certain frequencies of the white light. The frequencies absorbed will have energies that correspond to the energy level differences of the atoms or ion. By observing the white light that goes through the vapour certain frequencies would have been absorbed and when viewed you get the spectrum ROYGBIV minus certain frequencies which appear as black lines. This is called a line absorption spectrum. Atomic Absorption Spectroscopy (AAS) AAS involves the absorption of electromagnetic radiation of specific frequencies of light by atoms in a sample being analysed. It can be used for both qualitative and quantitative analysis. It requires the use of a spectrophotometer, which measures the intensity of the radiation passing or being transmitted through a relative sample. AAS is used to determine the concentration of metal ions in aqueous solutions, particularly in low concentrations (ppm). Eg the analysis of metal ions in blood or urine, checking for industrial waste. Trace amount of metals can also be found in solids by dissolving the solid in water acid or an alkali. Gases can be analysed if liquids can be found that absorb the compounds to be investigated. The requirements are as follows: A source of radiation: A hollow cathode lamp is used. The cathode should be made of the same metal being tested for. So if you are looking for zinc use a zinc cathode lamp. When electricity is passed through the cathode it emits light which can be absorbed by the metal being analysed.
Placing the sample in the path of the incident radiation: The radiation from the cathode must be focussed by optical devices through a flame into which the sample is aspirated (sprayed). In the flame the sample is reduced to the metal state creating a cloud of metal ions. The fraction of radiation absorbed is proportional to the concentration of the element in the sample. The presence of other metals will not affect the results because these atoms can t absorb the frequencies emitted by the cathode. The monochromator: The transmitted radiation is sent to a monochromator, which picks up on one of the key frequencies. The monochromator sends the key frequency to a detector, which measures the intensity and records it as an absorbance value. To convert this absorbance value to a concentration a calibration value must be used. This is constructed by placing samples of known concentrations in the spectrophotometer and placing the absorbances of the known samples on a graph