CHE130L Analytical Chemistry Laboratory I. OBJECTIVES II.MALAYAN COLLEGES LAGUNA EXPERIMENT NO. 6 GRAVIMETRIC ANALYSIS OF IRON Upon completion of the experiment, the student should be able to: Define the principles and proper techniques involved in precipitation and gravimetric analysis perform properly the relevant techniques in precipitation and gravimetric analysis; i.e. washing, ignition and digestion; and, calculate the amount of iron in an unknown sample using gravimetry. LABORATORY EQUIPMENT / INSTRUMENTS / REAGENTS Equipment/Apparatus Crucible and cover Glass funnel Wire triangle Glass rod Rubber policeman Dessicator 250 mL beaker 400 mL beaker Tongs/test tube holder Iron ring Clamps Hot hands Heating pad Bunsen burner Analytical balance Quantity 2 1 1 1 1 1 2 2 1 1 1 1 1 1 1 Chemical/ Reagents/ Materials 3M HCl (corrosive) Litmus paper 6M HNO3 (corrosive, oxidizing agent) 3M NH3 (corrosive) 12M HCl (corrosive, releases fumes) Filter paper (regular & ashless) Unknown sample with iron Distilled water in wash bottle Experiment 6: Gravimetric Determination of Iron 1|Page Though in a way. gravimetry is limited in such a way that the experimental process of determining the values should be undergoing a ‘gravimetric analysis’. Under this classification would be one of the most important aspects of analytic chemistry. from an unknown compound with an iron composition. weighing and drying/ignition. It is ignited to lose the water on it. which is gravimetry.CHE130L Analytical Chemistry Laboratory III. The analysis is based on the fact that iron as Fe3+ species forms an insoluble precipitate as iron (III) hydroxide. precipitating. One of the methods acquainted with analytical chemistry would be classical methods. Gravimetric analysis steps will be used. In such a case. and the precipitates in a mixture include iron (III) hydroxide. Using stoichiometric relations is not hard typically if you understand how it moves around back and forth. which is the simplified counterpart of stoichiometry. as the unknown compound will be prepared to precipitate to gelatinous rust ( ). quantitatively or qualitatively. Experiment 6: Gravimetric Determination of Iron 2|Page . Continuous constant weighing will in the end get the percentage iron of the sample. The formation of this precipitate is pH dependent (above pH 5). usually represented as Fe2O3xH2O. Fe (OH)3 and FeO(OH). To make up for this kind of lengthy solving. This experiment will focus on the gravimetric determination of iron. gravimetry is born. GF is used to convert substances that are specifically related to one another. the iron should be completely precipitated in a form that is useful for gravimetric analysis. gravimetry only uses a single ratio that converts one another but still follows Dalton’s law of mass and proportions. which involves the various measures of solidstate substances. digesting. It includes preparing the solution. Gravimetric analysis is an analytical method which uses mass measurements and knowledge of reaction stoichiometry to determine the amount of analyte/s in a sample. which is a principle of mass conservation. forming rust. filtering. which is called the gravimetric factor. Stoichiometry is the relation between quantities of substances that take part in a reaction or form a compound (typically a ratio of whole integers). The latter is a gelatinous precipitate which contains appreciable amount of water that makes quantitative determination of iron difficult. Alas. Introduction Discussion MALAYAN COLLEGES LAGUNA EXPERIMENT NO. but writing it down includes some length of time. Such procedures are exemplified in the gravimetric determination of iron. 6 GRAVIMETRIC ANALYSIS OF IRON DISCUSSION OF FUNDAMENTALS Analytic chemistry is concerned with the critical measurements of anything that involves chemicals and substances and such. both have arsenic. and a second precipitation step by addition of fresh ammonia. iron (III) oxide. The samples can be heated with a mild oxidizing agent such as bromine water or nitric acid to convert Fe2+ to Fe3+.e. it can be easily removed from the final sample such that the sample is free from impurities. The use of ammonia not only controls the pH of the solution but also prevents coprecipitation of any other insoluble metallic ions into metal hydroxides. Gravimetric analysis provides information as to the determination of percentages of metallic substances in a sample. Ignition is then done to convert the hydroxide into a well-defined form of iron. This second precipitation step helps to further lower the amount of metal ions aside from Fe3+ to precipitate. and from what we are discussing in the lecture. the sample undergoes filtration. Since ammonia is highly volatile. 6 GRAVIMETRIC ANALYSIS OF IRON The first part should ensure that iron in a sample mixture is converted to Fe3+ form (Hage and Carr 2011). Gravimetry has many practical applications. Fe2O3. as to which the sample contains any compound that is molecularly related to the metal in query. dissolution of the precipitate with HCl to lower the pH.CHE130L Analytical Chemistry Laboratory Application MALAYAN COLLEGES LAGUNA EXPERIMENT NO. Experiment 6: Gravimetric Determination of Iron 3|Page . i. After precipitation. though one is a sulphate whereas one is a nitrate. This final precipitate is then washed several times with ammonium nitrate until the gelatinous iron (III) hydroxide is formed. gravimetry involves the conversion of a compound to another compound that is related by their molecular formulas. The pH is controlled by precipitating the iron in ammonia instead of NaOH. Figure 2. The precipitate was digested by boiling. the set up was let to cool in the open for 20-30 minutes. The supernatant liquid was decanted through the filter paper. The filter paper was allowed to drain thoroughly. 3M ammonia was added with constant stirring until the solution was basic. The precipitate and crucible were weighed and the steps were repeated until constant weight was seen. 15-mL water and 10mL of 3M HCl. When ignition was completed. the crucible was let to cool in the open and was transferred to the desiccator. 5-mL of 6M HNO3 was added. 6 GRAVIMETRIC ANALYSIS OF IRON The crucible was heated until it started to glow orange. Figure 1. The filter paper containing the precipitate was heated while inside the crucible previously heated to constant mass. The steps were repeated until constant weigh was observed. Weight percent of iron was calculated in every sample. Gravimetric determination of iron Experiment 6: Gravimetric Determination of Iron 4|Page . After the crucible has been cooled. it was then placed in the desiccator and was weighed in the analytical balance. Constant mass of the crucible The filtration set up was assembled while waiting for the precipitate to settle. After heating. METHODOLOGY About 0. MALAYAN COLLEGES LAGUNA EXPERIMENT NO.CHE130L Analytical Chemistry Laboratory IV. It was then boiled until it was clear yellow. Average and average deviation were also reported. The sample was diluted to 200-mL with distilled water.60 g of the sample was weighed into a 400-mL beaker. so in all doing getting the pure precipitate will lead you to more accurate results as pertained to the gravimetric determination of a certain substance.UP Figure 1. Experiment 6: Gravimetric Determination of Iron 5|Page . This is very important to do. especially since we didn’t know how much hydrate molecules have been stuck up in the precipitate. MALAYAN COLLEGES LAGUNA EXPERIMENT NO. which is usually in its gelatinous form. 6 GRAVIMETRIC ANALYSIS OF IRON DESCRIPTION OF THE APPARATUS / SET .CHE130L Analytical Chemistry Laboratory V. Ignition Set-up The ignition set-up is done to primarily dry the washed filtrate. 241 Table 2. Weight of unknown sample Weight of unknown iron sample (g) 0.598 Table 3.4171 42.4266 42.4173 42.4172 Experiment 6: Gravimetric Determination of Iron 6|Page . 6 GRAVIMETRIC ANALYSIS OF IRON Table 1. DATA SHEET MALAYAN COLLEGES LAGUNA EXPERIMENT NO. Constant weight of Crucible + Sample Trial 1 2 3 Ave Weight 0. Constant weight of Crucible Trial 1 2 Mass (g) 42.CHE130L Analytical Chemistry Laboratory VI.1761 Iron Mass (g) 42.241 42. CHE130L Analytical Chemistry Laboratory VII. As a result. Experiment 6: Gravimetric Determination of Iron 7|Page . and proved to be very important.61 % 20. Since weight can be measured better and more precisely than any other state. SAMPLE CALCULATIONS VIII. which proves it to be systematic error related. it is the most accurate to be measured among any other fundamental properties.60% ( ( ) ) The experiment is all about the gravimetric determination of iron. and their measuring instruments prove to be quite estimate-friendly. This branch of the classical method amongst analytical chemistry is one of oldest techniques. 6 GRAVIMETRIC ANALYSIS OF IRON Table 4.e. gravimetric methods in experiments are not popularly used amongst environmental analysis (i.58 % 20. gravimetric analysis is one of the most accurate classes of analytical methods available. Percentage iron Trial 2 3 Average % Iron 20. since samples are extensively treated to remove interfering substances (or the matrix). since liquids are always measured by their volume. And another. Gravimetric analyses rely on some final determination of weight as to getting on quantifying an analyte. With this. petroleum gathering in the depths of the Earth. RESULTS AND DISCUSSIONS MALAYAN COLLEGES LAGUNA EXPERIMENT NO.). The experimental process may be quite lengthy and tedious. etc. total suspended solids or colloids) and its volatility (evaporating readily at normal temperatures in pressures.e. it might pick up water molecules from vapor. for most). this proved to be a great method in the environmental field. it will be cooled by putting it in a rubber mat with aluminum sheets. the light blue portion inside. even the containers. and the simultaneous deposition of the ions on the cathode itself. The differences between these types is on the acquisition of the sample before weighing the analyte. If exposed in the air. 6 GRAVIMETRIC ANALYSIS OF IRON The gravimetric analysis process is subdivided among four different types. Lastly. electrodeposition. Non-Newtonian fluids). Physical gravimetry is the one that is commonly used in environmental engineering. It involves the physical separation and classification of the matter that is found in the samples in the environment. thermogravimetric analysis samples with hydrates or something that can be evaporated is heated. Volatile solids are of major concern to this type of gravimetric analysis. This process is continued for ten minutes. This can be done in a faster way when the crucible is placed in the hottest position of the flame. adding weight to the object. physical gravimetry. water is a viable medium Figure 3. citing a few exceptions. or in simple terms. which the combined process of thermogravimetric and precipitate gravimetric analysis.CHE130L Analytical Chemistry Laboratory MALAYAN COLLEGES LAGUNA EXPERIMENT NO. from which the separation and classification would be differed on their particle size (i. which can also contribute to the mass of the object. The crucible is to be heated strongly with a Bunsen burner as shown. Precipiate gravimetric analysis is pretty self explanatory. letting the heat in and out to avoid cracking of the crucible (since it cannot sustain that much heat at a prolonged period of time). to room temperature. Also. specifically pertaining to sulfite (since sulfite is generally insoluble to any other ion you will mix it with. as it acquires its analyte from precipitating it from a sample through a series of chemical reactions. for liquids or for those who behave like liquids i. The experiment started with the determination of the constant weight of the crucible. we are going to gravimetrically determine the percentage of iron through an unknown iron sample. precipitate gravimetric analysis and thermogravimetry.e. Electrodeposition is the electrochemical reduction of a cation (a metal ion. The crucible is then heated to become orange. After heating. even under laboratory conditions. To further ensure that the heat is evenly spread among the crucible. the subject must be either heated or be put in a dessicator. how to prepare it. to grow bacteria. This process is done since all solids have a certain affinity to water. and the changes in mass are also recorded. to be specific) at a cathode. Since this is the easiest methods amongst all four. The crucible Experiment 6: Gravimetric Determination of Iron 8|Page . in this flame’s case. With this. Heating set-up. In this experiment. the clay triangle that supports the crucible should be glowing. CHE130L Analytical Chemistry Laboratory MALAYAN COLLEGES LAGUNA EXPERIMENT NO. the dessicator is used to get rid of water molecules to take part in the weighing process. the switching from a hot temperature to a cooler temperature. however. filter through normal filter paper. After this. Experiment 6: Gravimetric Determination of Iron 9|Page . This step will cause the solution change from yellow to a yellowish white. This step is to be done under no confusion that we are going to dilute it to a total of 200 mL. While the determination of the constant weight of the crucible is in progress. By putting the beaker to the rubber mat. Moreover. the precipitate that is formed is digested to the solution by letting it boil for another 30 minutes. it can now be transferred to a dessicator. This is done primarily because of the missed out ions in the solution. and condensate. This should be done under the fume hood since there might be unnecessary reactions that might bring out fumes that are harmful if released to the working environment. This process is repeated until the mass of the crucible is constant to 0. for 5 minutes. The precipitate that can be seen is in now in the formula Fe2O3·xH2O.60 g of the unknown sample in a 400mL beaker. The basicity of the solution can be determined with the litmus paper. meaning its hydrate molecules are not specifically known. By replacing the crucible to various positions in the mat and touching its previous position. the aluminum in the mat provides conduction of the heat from the crucible and absorbing it. making the cooling process faster. nevertheless its basicity is what is only important. or the colloidal form of the precipitate. If the solution contains impurities that cannot be further dissolved. and by digesting it any other ions that were not precipitated out (hence called the coprecipitates) will now form and join the precipitate. not dilute it with 200 mL of distilled water. and there would be coagulation of the precipitate. by the time that all heat has been dissipated from the crucible. we dilute the solution to 200 mL. After this. the water vapor that is in the air can project into the crucible. the original purpose of the experiment can be started. By getting about 0. the heat that the triangle received can be transferred to the crucible. After such time that is in the dessicator. 6 GRAVIMETRIC ANALYSIS OF IRON can be cooled in the triangle after heating. It need not be on a certain pH. then the rubber under regulated that conducted heat. This event is what we are trying to avoid. regardless of temperature. By continuing the process. Since it is just previously heated. the crucible is now weighed with the cover. so in this case. as this will bring up errors in the final result. the solid is diluted with water and 3M HCl.3 mg. 3M ammonia (NH3) is qualitatively added until such time that the solution is basic. put 5mL of nitric acid (HNO3) to the solution and heat it to a boil. 6 GRAVIMETRIC ANALYSIS OF IRON While the precipitate has not yet precipitated. splatter or pour the liquid 1 cm higher than the funnel. making the error bigger. then re-filter. or a similar event that would rise to that. as shown. the heat from the solution helps since the solution is less dense. and therefore its viscosity (or the resistance to the relative flow speed) is low. After all or little of the supernatant liquid is drained from the filter paper. wash with pre-prepared hot ammonium chloride (NH4NO3) in the sides of the filter paper to further center the filtrate for lesser work in the next step. carefully lift the paper from the funnel and fold it. The excess solution (or the supernatant) is to be decanted first.CHE130L Analytical Chemistry Laboratory MALAYAN COLLEGES LAGUNA EXPERIMENT NO. wherein the ashless paper is. reform the solids that was all over in the beaker. using the stirring rod. account the burnt weight of the filter paper. Filtration set-up. as to Experiment 6: Gravimetric Determination of Iron 10 | P a g e . The net effect of this is when you will be using a regular filter paper. and then the weight of the ignited substance will Figure 4. fixated on the stirring rod. for the liquid might bring out some of the precipitate and be left out when spilled or when anything above happens. and putting it to the crucible that has been weighed to constant mass. When there is still precipitate on the Figure 5. The filter paper must be carefully removed from the funnel where it has been stuck. Regardless of its temperature. one can already filter even though it just came off from heating. Decantation. transfer the precipitate quantitatively using a rubber policeman. therefore the solution will be filtered up faster than what it should be when it is at room temperature. We are using an ashless filter paper since an ordinary filter paper will combusted but its remains will not be disintegrated into the atmosphere. the filtration process can be started. After the precipitate has been settled. Be careful not to splash. wet it with distilled water. 110-mm diameter). After the entire solid has been transferred. When the supernatant liquid is decanted to a close. beaker. To make the filter paper stick into the funnel. spill. ashless filter paper (Whatman 41. This is not the same as the regular filter paper that is used when filtering impurities. though not that accurate since some of its ashes might be disintegrated into the air. the filtration set-up is then assembled. pour the supernatant liquid on it all over again. Also. The filter paper used is a coarse. which the percentage is computed to be 20. Precipitate ignition set-up. Heat the set-up to low flame. Errors should be in particular be very much avoided. which is hence known as rust. this experiment concluded the concepts that involves on this classical analysis method. and putting a small error to a small range would by ratio give you a bigger ratio than what you expected. even the slightest of errors would cause a dramatic change since the values that we are talking of here is significantly small. and the water that has not been drained will evaporate. which is within the range of how the theoretical weight will be in this experiment. it was then allowed to cool at room temperature via the same conditions when determining the constant mass of the crucible. ashless filter paper. inflaming the crucible for 15 minutes to ensure complete ignition of the iron oxide. MALAYAN COLLEGES LAGUNA EXPERIMENT NO. 6 GRAVIMETRIC ANALYSIS OF IRON where the filter paper must not be torn down during this step. By repeating ignition. SUMMARY AND CONCLUSIONS By following the steps involving gravimetric analysis. the weight of the iron oxide can be computed through weighing by difference and finally getting the percentage iron of the sample by the formula: ( ) . the final form. By Figure 6. After digesting it. or Fe2O3.CHE130L Analytical Chemistry Laboratory IX. as shown in Figure 2. is now used as to get the gravimetric determination of iron.3 mg. By continuous ignition and getting a constant weight. Do not cover the crucible as the ignition of the precipitate inside will require a substantial amount of oxygen. since every step in the experiment will provide crucial inconsistency with the final form of the sample when errors are introduced for they are very sensitive. Experiment 6: Gravimetric Determination of Iron 11 | P a g e . Position the crucible to the heating process again. cooling and weighing until the successive weighing that comply agree to 0.60%. After such time that the filter paper is burned. When the precipitate and crucible are already dry (by inspection or by checking). which is rust. as to avoid any irrelevant weight being in the final procedure. it was prepared with an acid-base reaction to form a gelatinous precipitate which is reddish. it will be filtered by a coarse. turn the burner to full heat to burn the filter paper. then weighing it. By having an unknown compound with an iron composition. the gelatinous red precipitate now turned into a black powdery substance. therefore the solution would have practically less viscosity (even though it is similar to water but is basic) and there would be greater flow in the filtration process. Also. therefore flocculation occurs. 2. Calculate the percent Fe in this iron (III) oxide. then we could get all the other ions that we missed out (or the coprecipitates) that is in the solution that is unprecipitated.CHE130L Analytical Chemistry Laboratory X. swelling the existing crystals. which is undesirable because it will introduce a systematic error in the process. since the solution is less dense. an event that we call as recrystallization. 4. 6 GRAVIMETRIC ANALYSIS OF IRON What is the effect of prolonged digestion? Answer: Digestion is referred to as the need to dissolve or dissolute a colloid. POST LAB QUESTIONS 1. An ashless filter paper burns out to become tiny ashes that deteriorate in contact with air (which is proved by our group as to which we burned an ashless filter paper. ( ( ( ) ) ) Experiment 6: Gravimetric Determination of Iron 12 | P a g e . so we could get a lower percentage difference. which is very much needed for the colloidal formation of the precipitate. During rest comes the completion of coagulation for solids with colloidal reduction of the surface and thus there will be less adsorption than it would normally be. therefore leaving out only the filtrate alone. Why is filtration of a gelatinous precipitate done while the solution is still hot? Answer: The filtration of a gelatinous precipitate that is hot will be faster than a cold one. Answer: MALAYAN COLLEGES LAGUNA EXPERIMENT NO. What is the purpose of using an ashless filter paper? Answer: We have to use an ashless filter paper since the weight of an ordinary filter paper will contribute to the overall weight of the dried precipitate. that disintegrated in the air leaving no clumps of ashes behind).6094 g of FeCl3 is used. and subject it to reform again in a much bigger state than what it was before. heat helps in the coagulation of the substance. 3. If this process is prolonged. Calculate the expected amount of Fe2O3 (in g) that would be precipitated when 0. Analytical chemistry and quantitative analysis. 2004.). 2003. as there are certain environmental disturbances that cannot be accounted for. John Wiley and Sons Inc. and certain estimations to the measurements would arise to the discrepancy of the theoretical to the experimental values. Using the expected results from #4. Are your errors systematic or random? Explain the sources of errors. calculate your percent relative error. al. Christian. 2011. Daniel C. H. Freeman and Company. and James D. Gary D. Quantitative chemical analysis. Fundamentals of Analytical Chemistry (8th ed. Skoog.CHE130L Analytical Chemistry Laboratory Answer: XI. Experiment 6: Gravimetric Determination of Iron 13 | P a g e . Singapore: Thomson Learning. Hage. 6 GRAVIMETRIC ANALYSIS OF IRON 5. David S. Douglas et. New Jersey: Pearson Prentice Hall.). (6th ed). New York: W. Carr. REFERENCES MALAYAN COLLEGES LAGUNA EXPERIMENT NO. 2004. Analytical chemistry (6th ed. Any errors that were made would be random. Harris.
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