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Home , Colorimetric analysis, Crucible tongs, Thiocyanate

College of Science

Determination of by thiocyanate colorimetry

Use this method if you have access to a

Safety • 1 mol L−1 (see below for preparation) Lab coats, safety and enclosed footwear must • 1 mol L−1 be worn at all times in the . • 1 mol L−1 hydrochloric acid Concentrated acids are highly corrosive − wear rubber −1 gloves and take care when handling. • 0.15 mol L potassium permanganate solution (see below for preparation) Introduction • 100 mL beaker • 100, 200, 250 and 500 mL volumetric flasks Iron is one of the many minerals required by the human body. It is used in the manufacture of the • 5 mL -carrying proteins, haemoglobin and myoglobin. • 10 mL measuring cylinder A deficiency of iron in the body can leave a person • 100 mL conical flask feeling tired and listless, and can lead to a disorder • at least 6 boiling tubes (or large test tubes) called anemia. Many of the foods we eat contain small quantities of iron. • distilled In this analysis the iron present in an iron tablet (dietary supplement) or a sample of food is extracted to form Method a solution containing Fe3+ (ferric) . To make the Preparation of Fe3+ standard presence of these ions in solution visible, thiocyanate ions (SCN−) are added. These react with the Fe3+ ions to NB: It may take several days to dissolve the Fe3+ salt form a blood-red coloured complex: used here, so carry out this preparation well in advance of the rest of the experiment. Weigh out about 3.0 g Fe3+ + SCN− → [FeSCN]2+ (aq) (aq) (aq) of ferric ammonium sulfate (FeNH4(SO4)2•12H2O). Use By comparing the intensity of the colour of this solution a mortar and pestle to grind the salt to a fine powder. with the colours of a series of standard solutions, with Accurately weigh 2.41 g of the powder into a 100 mL known Fe3+ concentrations, the concentration of iron beaker and add 20 mL of concentrated sulfuric acid (see in the tablet or food sample may be determined. This safety notes). Leave powder to soak in acid overnight. technique is called colorimetry. The next day, carefully pour the acid/powder slurry into a 500 mL , rinsing the beaker into the flask a few times with water, then make up to the Equipment and Materials Required mark with distilled water. Let this solution stand for

• ferric ammonium sulfate FeNH4(SO4)2•12H2O several days until the ferric ammonium sulfate powder standard solutions: has fully dissolved. If possible, insert a 2, 4, 6, 8 and 10 × 10−5 mol L−1 bar and stir the solution to speed up this dissolving (see below for preparation) process. • iron tablet (dietary supplement) or sample of iron- containing food (e.g., silverbeet, spinach, raisins) Use a pipette to transfer 20 mL of ferric solution to a 200 mL volumetric flask and make up to the mark with − usually no more than about 2 mL of 0.15 mol L−1 distilled water. This gives a solution with [Fe3+] = 0.001 permanganate solution will be required. −1 −5 −1 mol L . To prepare a 2 × 10 mol L standard solution 3. Transfer the iron solution to a 250 mL volumetric −1 pipette 10 mL of the 0.001 mol L solution into a 500 mL flask, rinsing the beaker with distilled water a −1 volumetric flask, add 10 mL of 1 mol L sulfuric acid, and few times and transferring the washings to the then make up to the mark with distilled water. Repeat volumetric flask. Make up to the mark with distilled this procedure in separate 500 mL volumetric flasks, water, stopper the flask and mix well. pipetting in 20, 30, 40 and 50 mL of 0.001 mol L−1 Fe3+ solution in turn, to obtain 4, 6, 8 and 10 × 10−5 mol L−1 4. Use a pipette to transfer 5 mL of iron solution to a solutions respectively. 100 mL volumetric flask and make up to the mark with distilled water. This diluted solution will be used (NB: if you do not have five 500 mL volumetric flasks for colorimetric analysis. you can use one flask to prepare each standard in turn. After preparing each standard, pour the solution into a Preparation of food sample for analysis labelled vessel which has a lid (eg: a glass bottle). 1. Accurately weigh a few grams (typically 2 − 5 g is Then rinse your 500 mL volumetric flask thoroughly required, depending on iron content of sample) of with distilled water before using it to prepare your next your food sample into a . standard solution.) 2. Heat the crucible over a (see Figure Preparation of 1 mol L−1 ammonium thiocyanate 1) until the sample is reduced completely to ash, solution or (preferably) combust the sample directly in the Weigh 38 g of solid ammonium thiocyanate into a bunsen flame (as shown in Figure 2), reducing it to 500mL volumetric flask and make up to the mark with ash. NB: be very careful with the bunsen flame while distilled water. heating/combusting your sample. Also beware that the crucible will become very hot during this process, Preparation of 0.15 mol L−1 potassium permanganate so handle it only with crucible tongs − or preferably solution not at all − until it has cooled. Only required for analysis of iron tablet. Weigh 2.4 g 3. When the sample and crucible have cooled, use a of solid potassium permanganate into a 100 mL stirring rod to crush the ash to a fine powder (see volumetric flask and make up to the mark with Figure 3). Use a measuring cylinder to add 10 mL of distilled water. 1 mol L−1 hydrochloric acid and stir for 5 minutes, Preparation of iron tablet for analysis making sure that all the ash is soaked. 1. Place iron tablet in a 100 mL beaker and use a 4. Add 5 mL of distilled water and then filter the measuring cylinder to add 20 mL of 1 mol L−1 sulfuric solution into a 100 mL conical flask to remove the acid (see safety notes). Allow the tablet’s coating ash. This filtered solution will be used for colorimetric to break down and its contents to dissolve. You may analysis. help this process by using a stirring rod to carefully Colorimetric analysis crush the tablet and stir the solution. (NB: iron 1. Accurately measure 10 mL of your sample solution tablets sometimes contain filler materials that may into a clean, dry boiling tube. (NB: a boiling tube, or not fully dissolve in acid) large , is ideal; however any small vessel − 2. Once the iron tablet is dissolved, add 0.15 mol L−1 eg: a beaker, flask or glass vial may also be used). This potassium permanganate solution dropwise, swirling measurement is most accurately made using a 10 mL the beaker after each addition. Iron tablets usually pipette; however, it is possible to do this accurately contain ferrous sulfate, with iron present as Fe2+ ions. enough (and with less hassle) using a clean 10 mL Since Fe2+ does not form a coloured complex with measuring cylinder if you measure carefully. thiocyanate, permanganate ions are added to oxidise 3+ 2+ 3+ 2. Next, measure 10 mL of each Fe standard solution all the Fe to form Fe ions. For the first few drops into separate boiling tubes (one standard per tube) in of permanganate, the purple colour will disappear order of increasing concentration, beginning with the immediately upon addition to the iron solution; 2 × 10−5 mol L−1 standard. It is a good idea to first rinse however, as further drops are added the colour your pipette or measuring cylinder with a few mLs of will begin to linger for a little longer. Stop adding the 2 × 10−5 mol L−1 standard. potassium permanganate drops when the purple NB: Make sure you label each boiling tube colour persists for several seconds after addition Figure 5. The same boiling tubes as in Figure 4 (prepared using Fe3+ standard solutions with concentrations of 2 − 12 × 10−5 mol L−1) viewed Figure 5. The same boiling tubes as in Figure 4 (prepared using Fe3+ from above, looking directly down the length of the tube. This view standard solutions with concentrations of 2 − 12 × 10−5 mol L−1) viewed provides the most accurate “eyeball” comparison of the solutions’ colour from above, looking directly down the length of the tube. This view intensities. provides the most accurate “eyeball” comparison of the solutions’ colour Calculationsintensities. 1.Calculations Using only the absorbance results obtained for your1. Fe Using3+ standard only the solutions absorbance (not resultsyour unknown obtained iron for sample),your Fe3+ preparestandard a solutionsgraph with (not [Fe 3+your] (in unknownmol L−1) as iron the Figure 5. The same boiling tubes as in Figure 4 (prepared using Fe3+ 3+ −1 standard solutions with concentrationshorizontalsample), of 2 − 12 × 10−5 prepare molaxis L−1) viewedand a graphabsorbance with [Fe (at ]490 (in molnm) L as) asthe the from above, looking directly down the length of the tube. This view Figure 1. Set up used for heating a silverbeet sample in aprovides crucible. the most accurate “eyeball”verticalhorizontal comparison axis.of the axissolutions’ and colour absorbance (at 490 nm) as the intensities. Figure 1. Set up used for heating a silverbeet sample in a crucible. vertical axis. Calculations 2. Draw a line of best fit for your data points that goes through the origin (because absorbance must be 1. Using only the absorbance2. results Draw obtained a line for of best fit for your data points that 3+ your Fe3+ standard solutionszerogoes (not whenyour through unknown Fe the concentration iron origin (because is zero. absorbance must be sample), prepare a graph withzero [Fe 3+when] (in mol Fe L−1)3+ as concentration the is zero. horizontal axis and absorbance3. (at Now 490 nm) identify as the the point on your line of best fit Figure 1. Set up used for heating a silverbeet sample in a crucible. vertical axis. which3. Now corresponds identify the to the point absorbance on your line measured of best fitfor 2. Draw a line of best fityour for your unknown data points thatiron sample. By drawing a vertical line goes through the origin (becausewhich absorbance corresponds must be to the absorbance measured for zero when Fe3+ concentrationtoyour isthe zero. unknown horizontal iron axis sample. you will By be drawing able to adetermine vertical line the 3+ 3. Now identify the pointconcentrationto on the your horizontal line of best of fit Feaxis in you your will unknown be able to solution. determine the which corresponds to the absorbanceconcentration measured of for Fe 3+ in your unknown solution. your unknown iron sample.4. By drawing Use thisa vertical concentration line to calculate the mass of to the horizontal axis you williron4. be (inable Use mg) to thisdetermine in concentrationyour the original tablet to calculate or food the sample mass (NB: of appropriately with the name of the sample or concentration of Fe3+ in your unknown solution. theiron molecular (in mg) in weightyour original of iron tablet is 55.8 or g foodmol−1 sample). Remember (NB: standard it contains. A is very 4. Use this concentration to calculate the mass of −1 useful for holding and transporting your tubes iron (in mg) in your originaltothe tablet take molecular or foodinto sample Figureaccount weight 5. (NB:The same any boilingof irondilutions tubes asis in 55.8 Figure that 4 g (prepared mol you using). performed RememberFe3+ −1 standard solutions with concentrations of 2 − 12 × 10−5 mol L−1) viewed the molecular weight of ironwhileto is take 55.8 preparing g intomol ).from accountRemember above, your looking anysample directly dilutions down solution. the length that of the tube.you This performed view (see Figure 4). Alternatively you can use a large to take into account any dilutions that you providesperformed the most accurate “eyeball” comparison of the solutions’ colour beaker to hold them. while preparing your sample5.while solution. If preparing the absorbanceintensities. your sample value yousolution. measured for your 3. Using a 10 mL measuring cylinder, add 10 mL of 5. If the absorbance valueunknown5. you measured If the iron absorbanceCalculations for sampleyour is value greater you than measured the absorbance for your Figure 2. Set up used for combusting a silverbeet sampleunknown directly ironin the sample is greater than the absorbance1. Using only the absorbance results 3+obtained for −1 Figure 2. Set up used for combusting a silverbeet sample directly in the 3+ 1molL ammonium thiocyanate solution to each iron value for your highest concentrationvalueunknown for Fe your ironstandard highest sample 3+you concentrationis greater than Fe the standard absorbance you bunsenFigure burner burner 1. flame.Set flame.up used for Figure 2. Set up used for your Fe standard solutions (not your unknown iron Figure 2. Set up used for combusting a silverbeet sample directly in the 3+ solution in sequence, with 2 minutes between each combusting awill silverbeet need to modify the abovewillvalue procedure. need for toyour In sample), modifythe highest case prepare the concentration abovea graph with procedure. [Fe3+] (in Fe mol standardIn L−1) theas the case you bunsenheating burner a silverbeet flame. sample in of an iron tablet, you should repeat the analysis with a addition. These additions must be carefully timed so a crucible. sample directly in the bunsen ofwill an need iron totablet,horizontal modify you axis the should and above absorbance repeat procedure. (at the 490 nm)analysis In as the casewith a Figure 1. Set up used formore heating dilute a silverbeet solution sample of in the a crucible. dissolved iron tablet.vertical In axis.the that all samples react for the same period of time. burner flame.case of a food sample, youmoreof should an diluterepeatiron tablet,the solution analysis you of should the dissolved repeat the iron analysis tablet. withIn the a using a smaller mass of your food. 2. Draw a line of best fit for your data points that 4. Mix the solutions by swirling. A stable red colour will casemore of dilute a foodgoes solution sample, through ofthe you the origin should dissolved (because repeat absorbance iron the tablet. must analysis be In the 3+ appear over the next few minutes. Contact Us usingcase of a smallera foodzero sample, whenmass Fe of concentration you your should food. is zero. repeat the analysis using a smaller3. Nowmass identify of your the point food. on your line of best fit If you have any questions or comments relatingwhich to corresponds this to the absorbance measured for 5. As near as possible to 15 minutes after adding Figure 3. Left photo: ash remaining after combustion of a silverbeet sample. experiment, please contact us: Right photo: silverbeet ash which has been ground to a fine powder using Contact Usyour unknown iron sample. By drawing a vertical line thiocyanate, measure the absorbance at a wavelength a stirring rod. Outreach to the horizontal axis you will be able to determine the of 490 nm for each coloured solution using your College of Science IfContact you have anyUsconcentration questions of Fe3+ or in yourcomments unknown solution. relating to this colorimeter. These measurements will be made in Figure 3. Left photo: ash remaining after combustion ofUniversity a silverbeet of Canterburysample. experiment,4. please Use this contact concentration us: to calculate the mass of RightFigure photo: 3. Left silverbeet photo: ash ash which remaining has been after ground combustion to a Privatefine powder Bag of 4800 a using If you have any questions or comments relating to this sequence − one sample every two minutes − reflecting Figure 3. Left photo: ash remaining after combustion of a silverbeet sample. iron (in mg) in your original tablet or food sample (NB: asilverbeet stirring rod. sample. Right photo: silverbeet ash whichChristchurch has been experiment,the please molecular contact weight of us: iron is 55.8 g mol−1). Remember Right photo: silverbeet ash which has been ground to aNew fine Zealand powder using Outreach the timing of the thiocyanate additions above. The aground stirring torod. a fine powder using a stirring rod. to take into account any dilutions that you performed measured absorbance of light is a direct measure of Phone: +64 3 364 2178 CollegeOutreach of Sciencewhile preparing your sample solution. Fax: +64 3 364 2490 UniversityCollege of ofScience5. Canterbury If the absorbance value you measured for your the intensity of the solution’s colour. Figures 4 and 5 Figure 4. Blood-red coloured solutions produced by the ferric thiocyanate Email: [email protected] complex ion (Fe[SCN]2+). These solutions were prepared using the following PrivateUniversity Bag of 4800unknown Canterbury iron sample is greater than the absorbance illustrate the range of colour intensities that you can range of Fe3+ standards: 2, 4, 6, 8, 10, 12 × 10−5 mol L−1. Figure 2. Set up used for combusting a silverbeet sample directly in the value for your highest concentration Fe3+ standard you bunsen burner flame.www.outreach.canterbury.ac.nz expect from your set of Fe3+ standards. 3 ChristchurchPrivate Bag will4800 need to modify the above procedure. In the case NewChristchurch Zealandof an iron tablet, you should repeat the analysis with a New Zealandmore dilute solution of the dissolved iron tablet. In the Calculations Phone: +64case 3 364 of a 2178food sample, you should repeat the analysis Fax:Phone: +64 +64using 33 364364 a smaller 24902178 mass of your food. 1. Using only the absorbance results obtained for your FigureFigure 4. 4.Blood-red Blood-red coloured coloured solutions solutions produced produced by the ferric by thiocyanate the ferric Email:Fax: [email protected] +64 3 364 2490 3+ 2+ Contact Us Fe standard solutions (not your unknown iron complexFigurethiocyanate 4. ion Blood-red (Fe[SCN]2+). complex coloured These ion solutions (Fe[SCN] solutions produced were). These prepared by solutions the usingferric thiocyanatethe were following Email: [email protected] 3+ −1 rangecomplexprepared of Fe3+ ion using (Fe[SCN]2+).standards: the following 2, These 4, 6, 8,solutions 10, range 12 × 10−5were of Femol prepared3+ standards:L−1. using the 2, 4,following 6, 8, www.outreach.canterbury.ac.nzIf you have any questions or comments relating to this sample), prepare a graph with [Fe ] (in mol L ) as the Figure 3. Left photo: ash remaining after combustion of a silverbeet sample. experiment, please contact us: 3 range10, 12 of× 10Fe3+−5 mol standards: L−1. 2, 4, 6, 8, 10, 12 Right× 10−5 photo: mol silverbeet L−1. ash which has been ground to a finewww.outreach.canterbury.ac.nz powder using horizontal axis and absorbance (at 490 nm) as the a stirring rod. Outreach vertical axis. 3 College of Science University of Canterbury 2. Draw a line of best fit for your data points that goes Private Bag 4800 through the origin (because absorbance must be zero Christchurch New Zealand when Fe3+ concentration is zero). Phone: +64 3 364 2178 FigureFigure 5. 5.The The same same boiling boiling tubes tubesas in Figure as in 4 Figure(prepared 4 (preparedusing Fe3+ using Fax: +64 3 364 2490 3. Now identify the point on your line of best fit which 3+ −5 standardFe standard solutions solutions with concentrations with concentrations ofFigure 2 − 4. 12 Blood-red × 10−5 of coloured mol 2 − L−1) 12solutions × viewed 10 produced by the ferric thiocyanate Email: [email protected] corresponds to the absorbance measured for your frommol above,L−1) viewed looking from directly above, down looking the lengthcomplex directly of ion the (Fe[SCN]2+). tube. down TheseThis the solutionsview length were prepared using the following unknown iron sample. By drawing a vertical line to provides the most accurate “eyeball” comparisonrange of Fe3+ standards:of the solutions’ 2, 4, 6, 8, 10, 12 colour × 10−5 mol L−1. www.outreach.canterbury.ac.nz intensities.of the tube. This view provides the3 most accurate “eyeball” the horizontal axis you will be able to determine the comparison of the solutions’ colour intensities. concentration of Fe3+ in your unknown solution. Calculations 4. Use this concentration to calculate the mass of Contact Us 1. Using only the absorbance results obtained for iron (in mg) in your original tablet or food sample yourIf you Fe have3+ standard any questions solutions or comments (not your relatingunknown to thisiron (NB: the molecular weight of iron is 55.8 g mol−1). sample),experiment, prepare please a contactgraph with us. Please [Fe3+] (innote mol that L−1 this) as servicethe is Remember to take into account any dilutions that for senior school chemistry students in horizontal axis and absorbance (at 490 nm) as the you performed while preparing your sample solution. New Zealand only. We regret we are unable to respond to Figure 1. Set up used for heating a silverbeet sample in a crucible. verticalqueries axis.from overseas. 5. If the absorbance value you measured for your unknown iron sample is greater than the absorbance 2.Outreach Draw a line of best fit for your data points that College of Science value for your highest concentration Fe3+ standard you goes through the origin (because absorbance must be zeroUniversity when ofFe Canterbury3+ concentration is zero. will need to modify the above procedure. In the case Private Bag 4800 of an iron tablet, you should repeat the analysis with a 3.Christchurch Now identify the point on your line of best fit more dilute solution of the dissolved iron tablet. In the whichNew Zealand corresponds to the absorbance measured for case of a food sample, you should repeat the analysis yourPhone: unknown +64 3 364 iron 2178 sample. By drawing a vertical line using a smaller mass of your food. Fax: +64 3 364 2490 toEmail: the [email protected] axis you will be able to determine the 3+ concentrationwww.outreach.canterbury.ac.nz of Fe in your unknown solution. 4. Use this concentration to calculate the mass of iron (in mg) in your original tablet or food sample (NB: the molecular weight of iron is 55.8 g mol−1). Remember to take into account any dilutions that you performed while preparing your sample solution. 5. If the absorbance value you measured for your unknown iron sample is greater than the absorbance Figure 2. Set up used for combusting a silverbeet sample directly in the value for your highest concentration Fe3+ standard you bunsen burner flame. will need to modify the above procedure. In the case of an iron tablet, you should repeat the analysis with a more dilute solution of the dissolved iron tablet. In the case of a food sample, you should repeat the analysis using a smaller mass of your food.

Contact Us If you have any questions or comments relating to this Figure 3. Left photo: ash remaining after combustion of a silverbeet sample. experiment, please contact us: Right photo: silverbeet ash which has been ground to a fine powder using a stirring rod. Outreach College of Science University of Canterbury Private Bag 4800 Christchurch New Zealand Phone: +64 3 364 2178 Fax: +64 3 364 2490 Figure 4. Blood-red coloured solutions produced by the ferric thiocyanate Email: [email protected] complex ion (Fe[SCN]2+). These solutions were prepared using the following range of Fe3+ standards: 2, 4, 6, 8, 10, 12 × 10−5 mol L−1. www.outreach.canterbury.ac.nz 3