BST 152-1 Analytical Techniques for Biosystems Technology PDF

BST 152-1 Analytical Techniques for Biosystems Technology Volumetric methods Volumetric or titrimetric analyses are quantitative analytical techniques which employ a titration in comparing an unknown with a standard. Precise measurement of volume is the important to many an...

BST 152-1 Analytical Techniques for Biosystems Technology Volumetric methods Volumetric or titrimetric analyses are quantitative analytical techniques which employ a titration in comparing an unknown with a standard. Precise measurement of volume is the important to many analytical methods. Common units of volumes used is L, mL, uL. Volume measurements are affected by the temperature. Therefore, most of the apparatus which measures volumes indicate the temperature that the apparatus gives more precise measurement. Apparatus for Precisely Measuring Volume Volume may be measured reliably with a pipet, a buret, or a volumetric flask. Pipets Pipets are used to transfer volumes between containers/ flasks. The error associated with each measurement is given in the instrument (such as ± 0.001, ± 0.02) Calibrated pipets can be used to transfer 0.1 mL to 25.0 mL. If the pipet is not calibrated, only the designated volumes can be transferred. All volumetric and measuring pipets are first filled to a calibration mark, but the manner in which the transfer is completed depends on the particular type. Because most liquids are attracted to glass, a small amount of liquid tends to remain in the tip after the pipet is emptied. This residual liquid is never blown out of a volumetric pipet or from some measuring pipets, but it is blown out of other types of pipets. Buret Burets, like measuring pipets, make it possible to deliver any volume up to the maximum capacity of the device. The precision attainable with a buret is substantially greater than the precision with a pipet. A buret consists of a calibrated tube to hold titrant plus a valve arrangement by which the flow of titrant is controlled. A buret equipped with a glass stopcock for a valve relies on a lubricant between the ground-glass surfaces of stopcock and barrel for a liquid-tight seal. Volumetric flasks Volumetric flasks are manufactured with capacities ranging from 5 mL to 5 L and are usually calibrated to contain a specified volume when filled to a line etched on the neck. They are used for the preparation of standard solutions and for the dilution of samples to a fixed volume prior to taking aliquots with a pipets. Volumetric cylinders/ Graduated cylinders Volumetric cylinders are in different capacities to measure liquid volumes. Also, they are calibrated to measure liquids in different volumes. The error associated with the instrument is indicated on the glass. Widely using volumetric apparatus Beaker Conical flasks/ Erlenmeyer flasks Micropipette Titration flasks Measuring volumes Meniscus Correct way to measure the volume. The volume is read where the meniscus is residing. Wrong Wrong Volumetric determination-Titrations Titrations are used to determine the concentration of an unknown solution from a known solution Based on the chemical reaction involves, the titration can be classified as, Acid base titrations Redox titrations Gravimetric titrations Coulometric titrations etc. E.g- Acid base titration NaOH + HCl → NaCl + H2O Unknown Known solution- solution- The The concentration is concentration is known unknown Standard solution Volumetric determination-Titrations NaOH + HCl → NaCl + H2O Concentration C1 1.0 mol dm-3 Volume V1 25.00 mL Moles = C1 x V1 = 25 x 10-3 mol Stoichiometry 1 1 No of moles for =25 x 10-3 mol =25 x 10-3 mol complete reaction From the titration V1 will be determined Volumetric determination Then the C1 can is important be determined as follows C1= 25 x 10-3/V1 https://www.youtube.com/watch?v=RI14t0R1wMY Volumetric determination-Titrations E.g- Acid base titration Find the concentration of HCl. A laboratory volumetric determination needs to be done with a standard base HCl solution. NaOH + HCl → NaCl + H2O According to above reaction, 1 HCl molecule (mol) tracts with 1 molecule (mol) of NaOH. NaOH To find out unknown concentration of 1 mol/dm3 HCl solution, it needs to be titrated with a 25 mL NaOH solution with known concentration and find out the volume consumed. Volumetric determination-Titrations In a titration, a volume of a standardized solution containing a known concentration of reactant "A" is added incrementally to a sample containing an unknown concentration of reactant "B". The titration proceeds until reactant "B" is just consumed (stoichiometric completion). A This is known as the equivalence point. At this point the number of equivalents of "A" added to the unknown equals the number of equivalents of "B" originally present in the unknown. Volumetric methods have the potential for a precision of up to 0.1%. B There are different types of reactions/titrations are done. Acid base titration is the simplest form of the titration. Volumetric determination-Titrations For volumetric methods to be useful, the reaction must reach 99%+ completion in a short period of time. In almost all cases, a buret is used to meter out the titrant. When a titrant reacts directly with an analyte (or with a reaction the product of the analyte and some intermediate compound), the procedure is termed a direct titration. An indicator is always used to detect the equivalence point. Most common are the internal indicators, compounds added to the reacting solutions that undergo an abrupt change in a physical property (usually absorbance or color) at or near the equivalence point. Sometimes the analyte or titrant will serve this function (auto indicating). External indicators, electrochemical devices such as pH meters, may also be used. The titrations should be stopped precisely at the equivalence point. However, the ever-present random and systematic error, often results in a titration endpoint, the point at which a titration is stopped, that is not quite the same as the equivalence point. The systematic error, or bias may be estimated by conducting a blank titration. To improve accuracy, the titrant must be standardized (usually by volumetric analysis) against a compound that is available in a stable, highly pure form (i.e., a primary standard). Volumetric determination-Titrations The basic requirements or components of a volumetric method are 1. A standard solution (i.e., titrant) of known concentration which reacts with the analyte with a known and repeatable stoichiometry (i.e., acid/base, precipitation, redox, complexation) 2. A device to measure the mass or volume of sample (e.g., pipet, graduated cylinder, volumetric flask, analytical balance) 3. A device to measure the volume of the titrant added (i.e., buret) 4. If the titrant-analyte reaction is not sufficiently specific, a pretreatment to remove interferents 5. An internal indicator (e.g., phenolphthalein) or an external indicator (e.g., pH meter). Volumetric determination- Important terms Titration- volumetric method to determine the quantity of a reagent of known concentration that is required to react completely with the analyte Analyte-The solution with unknown concentration Equivalence point - point in a titration when the amount of added standard reagent is equivalent to the amount of analyte. It is a volume End point -point in a titration when a physical change occurs that is associated with the condition of chemical equivalence. I is a volume Indicators – Substance that changes the color based on the pH of the medium Primary standard - an ultrapure compound that serves as the reference material for a titration or for another type of quantitative analysis Volumetric determination-Titrations E.g- Acid base titration Standard base is needed- 1 mol/dm3 NaOH solution 1 mol/dm3 NaOH Experimental procedure: Titrate HCl with the 1M NaOH 25 mL. And determine the volume it consumes. Observation 12.50 mL of NaOH consumed from HCl the buret. 25 mL Analysis NaOH moles consumed = 1 mol/dm3 × 12.50/1000 dm3 = 1.250 × 10-2 mol HCl moles that needs = 1.250 × 10-2 mol Volumetric determination-Titrations Analysis NaOH moles consumed = 1 mol/dm3 × 12.50/1000 dm3 = 1.250 × 10-2 mol HCl moles that need to reacts with NaOH = 1.250 × 10-2 mol Therefore, HCl concentration = 1.250 × 10-2 mol /25.00 ×1000 dm3 = 0.5 mol/dm3 In volumetric methods, the titration error, Et, is given by Et = Vep - Veq where Vep is the actual volume of reagent required to reach the end point and Veq is the theoretical volume necessary to reach the equivalence point. References 1. All the materials, notes, descriptions and figures are taken from Fundamentals of Analytical Chemistry, Ninth Edition Douglas A. Skoog, Donald M. West, F. James Holler, Stanley R. Crouch 2. http://www.ecs.umass.edu/cee/reckhow/courses/572/572bk16/572BK16.html