ANALY CHEM I
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Date Created: 09/17/15
SPECTROPHOTOMETRIC DETERMINATION OF Fe IN DRINKING WATER Background Reading Harris 739 ed Chap 18 Skoog West Holler and Crouch 739 ed Chap 23A pp 594 601 Introduction Several contaminants in drinking water can be determined spectrophotometrically including iron Although iron is easily determined in contaminated water containing gt1 ppm 1 mgL federal and state regulations limit the iron content of drinking water to lt1 ppm Thus an intensely colored complex must be formed to detect the presence of these low levels of iron spectrophotometrically As is the case whenever trace quantities of an analyte are being measured cleanliness of equipment glassware etc is essential to prevent positive determinate errors false positives due to laboratory contamination One widely used iron complex is ironllophenanthroline which is orangered and easy to detect Like most metal complexation reactions the metal ion must compete with H30 ions and thus the metal complex will not form in strongly acidic solutions On the other hand most metals form insoluble metal hydroxides in basic solutions For these reasons the iron determination using 0 phenanthroline is carried out in a slightly acidic solution In most water samples iron exists in its oxidized form Felll due to the presence of oxygen Since it is the Fell species that forms the complex with ophenanthroline a reduction must first be carried out This can be accomplished by the addition of hydroxylamine In the presence of an excess of hydroxylamine the Fellophenanthroline complex is quite stable Your unknown is representative of a contaminated water sample It has Feophenanthroline complex been spiked to produce an Fe concentration greater than 1 ppm Fe Procedure Preparation of Stock and Standard Solutions To prepare a 50 ppm stock solution of Fe accurately weigh 175 mg of ferrous ammonium sulfate into a clean 500 mL volumetric flask Dissolve in 100 mL of H20 and then add 1 mL of concentrated H2804 Dilute to the mark and mix thoroughly Calculate the concentration of your stock solution which should be close to 50 ppm To determine the number of drops of sodium citrate needed to bring the pH of standards to a value between 3 and 4 use a piper to transfer a 5 mL aliquot of the iron stock solution to a beaker and add a drop of bromophenol blue indicator Add sodium citrate dropbydrop until the indicator turns blue noting the number of drops required Discard this sample and calculate the number of drops of citrate needed per milliliter of solution Obtain an unknown sample from the TA Exchange a 100mL volumetric flask for one that contains the unknown Dilute to volume with deionized water Record the unknown code from the label Find the number of drops of citrate needed to bring the sample to pH 34 Transfer a 10 mL aliquot of the sample to a beaker and add a drop of bromophenol blue Add either 25 sodium citrate if the water is acidic or 01 F H2804 if the solution is basic dropwise until the indicator reaches its transition color Calculate the number of drops required per milliliter of sample and then discard this sample All standard and sample solutions should be prepared at the same time so that when the spectrophotometric measurement is made all solutions will have had equal time for color development which is quite slow for this procedure It is recommended that the actual absorbance measurement not be made until the following lab period To prepare colored Fellophenanthroline standards transfer 1 2 5 and 10 mL aliquots of the POLYMER ENDGROUP ANALYSIS THE DETERMINATION OF AVERAGE MOLECULAR WEIGHT Background reading This exercise uses a strong acid titration by a strong base With a visual indicator See Harris 7quot7 ed Chap 11 Skoog West Holler and Crouch 7quot7 ed Chap 14 Introduction Polymers Polymers are a special form of macromolecules They are compounds of high molecular weight formed by combining a large number of small molecules The small molecules called monomers may all be of one type as in the compound used in this experiment or may be of different types Polymers are very important in biological systems For example proteins are composed of intricate sequences of amino acids and polysaccharides contain repeating units of simple sugar molecules Our everyday lives are also greatly influenced by polymers that are not obtained from natural sources Synthetic polymers are known to us as various bers eg Dacron plastics eg polyvinyl chloride abbreviated PVC and polystyrene and elastomers See Fig 1 Like PVC and polystyrene the compound involved in this experiment is synthesized from only one kind of monomer molecule ethylene glycol The monomers are joined by elimination of a molecule of water to form a series of ether linkages n2 HOCHgCHgOH gt HO39CH239CH239O39CH239CH2n39O39CH239CH239OH n H20 The polymer is called either poyethyene glycol PEG for short poyethyene oxide or more properly poyoxyethylene PEG is commonly found in the list of ingredients in hair preparations and cosmetics It is a good lubricant and also has the desirable property of being soluble in water Therefore PEG is heavily used as a base for therapeutic ointments and it is used industrially as a O lubricant in the formation of textile fibers and metal products ll 0 Ho c b o0H20H2 0H Molecular Weight The physical properties of polymers 0 depend heavily on their molecular weights which vary according Dacron 0 eth lens I coltere hthalate p W y M p to the number of repeating monomer units the n s In the CI CI structures in Fig 1 per polymer molecule For example the CHCH2 R 0HCH2JHR39 PEG molecules to be studied in this experiment have molecular vinyl chloride polyvinyl chloride weights less than 1000 and are viscous liquids As the molecular I weight increases the compound is more greaselike and if the CH39CHZ RECH39CH hR MW is greater than about 6000 PEG is a white solid The use of the term molecular weight is somewhat misleading because it implies that a sample of a polymer has a uniform styrene polystyrene formula weight ie that n is the same for all the polymer molecules in a sample For many biopolymers most notably Figure 1 Examples of proteins the sequence of the monomers is dictated by the synthetic polymers and their speci c biological role of the compound and the structure and monomers molecular weight is the same for all molecules of that type However whenever a synthetic polymer is made chains of varying length varying n s are produced and the product has a range of formula weights Therefore the molecular weight is really an average for the sample To complicate the situation there are several types of averages depending on how the molecular weight is measured In this experiment the number average molecular weight A1 of a sample of poyethyene glycol will be determined Conceptually A7 is the simplest of the molecular weights because it corresponds to the usual notion of an average The weights of all the molecules are added together and then the sum is divided by the total number of molecules present In symbolic form A7 is given Polymer End Group Analysis Page 2 by Z N M Mquot Ni where N is the number of moles of polymer with molecular weight M In the numerator each contributing molecular weight in the sample called M is multiplied by the corresponding number of moles N The sum is then divided by the total number of moles in the sample Unfortunately it is usually not possible to count up the number of moles of each chain length and the value of M7 must be obtained indirectly You should recognize that the numerator is actually the total mass of the sample in grams and as stated above this is divided by the total number of moles in the sample Therefore the number average molecular weight may be determined experimentally by measuring the total number of moles of polymer in a known weight of sample Determining Number of Moles To determine the number of moles of polymerthe analysis method must respond equally to each molecule without regard to its chain length As the structures given above show the monomer molecules on the ends of the chain must in some way be different from the chain itself For a linear polymer ie one in which there are no bonds called crosslinks between the chains every molecule has two end groups which may be identical as in PEG or different as in Dacron For polymers such as PVC and polystyrene the identities of R and R depend on the choice of reagent used to initiate the polymerization reaction Thus if the end groups can be analyzed the number of polymer molecules may be calculated using simple stoichiometric relationships This method for determining A7 is called end group analysis 0 Some Specifics The ends amp of PEG are alcohol groups 20 g 0 HOCl lgCHgEOClQCHZEhOCHZCHTOH gt which may be analyzed by a E reaction known as o PMDA 0 PEG esteri cation In an esteri cation the OH reacts O O O O with an organic acid or more 3 6 quot3 H commonly with a more 0 OCWCH2OCHZ0H2 QCHZCHTOCQ O reactive derivativeof the acid 9 QPOH HO 9 In this experiment the o o 3 o anhydride of 1245 Figure 2 Reaction of PMDA with PEG to produce ester benzenetetracarboxylic add known commonly as pyromellitic dianhydride or PMDA for short will be used The reaction with PEG is shown in Fig 2 The reaction must be conducted in a nonaqueous solvent and because of its low volatility and desirable solvating properties NNdimethyl formamide DMF is a good quot DMF h39 Atlt39l 39d d39 t39ft39 d HCNCH32 c oice caays is aso require an in many es en ica ion proce ures pyridine a rather foulsmelling organic base is used However for the reaction of alcohols with PMDA it has been found that imidazole IMDA a very soluble solid with no odor has at least equal catalyzing power The N IMDA formulas of DMF and IMDA are shown below N The analysis involves an indirect titration procedure An excess of PMDA is H mixed with the weighed PEG sample and the catalyst is added After the reaction is complete at least onehalf hour reaction time water is added to convert the unreacted anhydride groups to the acid form and the acid is titrated with standard NaOH The equations in Fig 3 show that each ester linkage to a PEG end group replaces one of the four Polymer End Group Analysis Page 3 acids formed 39om the PMDA and as a result the number of moles of titratable protons decreases as the number of end groups increases If the total number of moles of PMDA added at the start is knovm the number of moles ofOH end groups may be calculated 39om the volume of base needed forthe titration A standard solution of PMDA in DMF cannot be prepared directly by weighing and dilution The PMDA solution must be standardized by reacting a kn volume with water and titrating with base It should be noted that the imidazole catalyst is itself a base but it is so weak that it does not cause error in the titration For consistent results however an equal amount of the catalyst should be added to all samples 9 9 if R ZHZO o OiCH2cH2foiCH2Cl li0704201270 o 4 cioH H043 c o o o o o 0 HO o H CH 0 70H 7 7 it o CHZCH2fo CH2CH2H 2 2 fort HtH CioH o H O o o 0 3 l H07 40H 2H 2c o o 4 3 H043 GOH ll ll 0 pyromellltl and Figure 3 Formation of acid by reaction with water Another Molecular Weight As discussed above there is more than one expression for the average molecular weight of a polymer The weight average molecular weight 17 is given by where Wis the weight of polymer with molecular weight M Here each Mis multiplied by the weight present in the sample and the sum of all these contributions is divided by the total weight of the sample Unless the sample consists of only one chain length 17 is always greater than 17 The ratio of 17 to 171 is called the poydispersity and is an indication ofthe breadth ofthe distribution of molecular weights in the sample Procedure g 39 39 quot39 39 39 vupul and youshould wash DMF offimmedianely if you get it on yourskin Weigh a suf cient quantity of PMDA FW21812 to prepare 100 mL of 02 M solution and dry at Polymer End Group Analysis Page 4 160 C for 2 hours Dissolve the solid in 100 mL of DMF Your glassware must be dry before you start Do not add any water to this solution or to any of the samples until specified in the procedure If it is necessary to store the solution for a week obtain a small bottle from the stockroom and line the cap with aluminum foil Obtain a sample of PEG and accurately weigh 020024 g samples into each of four dry 125 mL Erlenmeyer flasks Note With a viscous liquid sample it is not feasible to weigh by difference from a weighing bottle Instead weigh the dry flask add the appropriate amount of polymer from a dropper carefully so as to deliver the sample to the bottom of the flask and not on the wall and reweigh the flask The top door ofthe analytical balance is very convenient for this weighing operation Pipet 10 mL aliquots of the 02 M PMDA into each of the Erlenmeyers Wash any PEG on the flask wall to the bottom as the pipet drains Avoid adding extra solvent because the mixture may become too dilute for the reaction to go to completion Use a Mohr pipet to add one mL of 3 M IMDA prepared ahead by the laboratory staff to each flask Mix the contents and allow onehalf hour for the reaction While waiting proceed to the PMDA standardization described below To standardize the PMDA solution pipet 10 mL aliquots into three Erlenmeyer flasks and add 1 mL of the IMDA solution to each Add 30 mL deionized water and phenolphthalein to each no waiting period required and titrate with the standard 02 M NaOH After waiting 30 minutes add 30 mL deionized water to each flask Add phenolphthalein indicator and titrate with standard 02 M NaOH prepared and standardized by laboratory staff Place all organic waste in the designated container Report For each standardization titration calculate and report the result as molarity of titratable hydrogens Calculate and report the average molarity and the standard deviation Calculate and reportll7n for each aliquot of PEG the average value of A7 and the standard deviation Some Notes on Calculations This experiment provides a good example of the usefulness of a back titration The reaction between PEG and PMDA is slow It would not be practical to titrate PEG end groups directly with PMDA Adding an excess of PMDA and waiting 30 minutes drive the reaction to completion The excess PMDA actually the excess reactive hydrogens of pyromellitic acid is then titrated with standard base This reaction is rapid and thus suited to a titration with a visual end point You have not been provided with acid dissociation constants for pyromellitic acid How do you know if titration to a phenolphthalein end point involves reaction of l 2 3 or 4 acid hydrogens You do not know but you do not need to know because of the way the standardization and determination are handled
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