College of Science

Determination of Vitamin C Concentration by Titration

(Redox Titration Using Iodine Solution) Safety Solutions Needed Lab coats, safety glasses and enclosed footwear must Iodine solution: (0.005 mol L−1). Weigh 2 g of potassium be worn at all times in the laboratory. iodide into a 100 mL beaker. Weigh 1.3 g of iodine and add it into the same beaker. Add a few mL of distilled Introduction water and swirl for a few minutes until iodine is dissolved. Transfer iodine solution to a 1 L volumetric This method determines the vitamin C concentration flask, making sure to rinse all traces of solution into the in a solution by a redox titration using iodine. Vitamin volumetric flask using distilled water. Make the solution C, more properly called ascorbic acid, is an essential up to the 1 L mark with distilled water. antioxidant needed byDetermination the human body (see additional of Vitamin C Concentration notes). As the iodine isby added Titration during the titration, the Starch indicator solution: (0.5%). Weigh 0.25 g of ascorbic acid is oxidised to dehydroascorbic acid, while soluble starch and add it to 50 mL of near boiling water the iodine is reduced to(Redox iodide Titration ions. Using Iodine Solution)in a 100 mL conical flask. Stir to dissolve and cool before using. − ascorbic acid + I2 →Introduction 2 I + dehydroascorbic acid Due to this reaction, theThis iodinemethod determinesformed is the immediately vitamin C concentration reduced to iodide as longin a solution as there by a is redox any titration ascorbic using acid iodine. Vitamin C, more properly called ascorbic acid, is an essential Burette containing present. Once all the ascorbicantioxidant acid needed has by thebeen human oxidised, body (see additional iodine solution the excess iodine is freenotes). to reactAs the iodinewith isthe added starch during the titration, the ascorbic acid is oxidised to dehydroascorbic acid, while indicator, forming the theblue-black iodine is reduced starch-iodine to iodide ions. complex. This is the endpoint of the titration. − ascorbic acid + I2 → 2 I + dehydroascorbic acid The method is suitableDue for to use this reaction,with vitamin the iodine C formed tablets, is immediately fresh or packaged fruitreduced juices to and iodide solid as long fruits as there and is any ascorbic acid vegetables. present. Once all the ascorbic acid has been oxidised, the excess iodine is free to react with the starch NB: This method is moreindicator, straight forming forward the blue-black than starch-iodine the complex. alternative method usingThis is potassium the endpoint of iodate, the titration. but as the potassium iodate Thesolution method is suitablemore forstable use with than vitamin the C tablets, fresh or packaged fruit juices and solid fruits and iodine as a primary standard,vegetables. the alternative method is more reliable. NB: This method is more straight forward than the alternative method using potassium iodate, but as the potassium iodate solution is more stable than the Equipment Needediodine as a primary standard, the alternative method is more reliable. burette and stand Conical flask 100 mL or 200 mL volumetricEquipment flask Needed burette and stand 20 mL pipette Vitamin C sample solution 100 mL or 200 mL volumetric flask 10 mL and 100 mL measuring cylinders 20 mL pipette 250 mL conical flasks 10 mL and 100 mL measuring cylinders 250 mL conical flasks

Solutions Needed Iodine solution: (0.005 mol L−1). Weigh 2 g of potassium iodide into a 100 mL beaker. Weigh 1.3 g of iodine and add it into the same beaker. Add a few mL of distilled water and swirl for a few minutes until iodine is dissolved. Transfer iodine solution to a 1 L volumetric flask, making sure to rinse all traces of solution into the volumetric flask using distilled water. Make the solution up to the 1 L mark with distilled water.

1 Method 4. Calculate the concentration in mol L−1 of ascorbic acid in the solution obtained from fruit/vegetable/ SampleStarch Preparation indicator solution: (0.5%). Weigh 0.25 g of 3.juice. Using Also, thecalculate equation the ofconcentration, the titration (below)in mg/100mL Forsoluble vitamin starch C tablets: and add Dissolve it to 50 a mLsingle of near tablet boiling in 200 water mL determineor mg/100g the of number ascorbic of acid, moles in theof ascorbic sample acidof fruit/ of distilledin a 100 mLwater conical (in a flask.volumetric Stir to flaskdissolve if possible).and cool before reacting.vegetable/juice. using. − For fresh fruit juice: Strain the juice through ascorbic acid + I2 → 2 I + dehydroascorbic acid cheesecloth to remove seeds and pulp which may block Additional Notes −1 Method 4. Calculate the concentration in mol L of ascorbic pipettes. 1. acidIodine in thestains solution both skinobtained and clothing from fruit/vegetable/juice. so proper care is ForSample packaged Preparation fruit juice: This may also need to be Also,advised. calculate If staining the concentration, does occur, alcohol in mg/100mL may remove or mg/ strained through cheesecloth if it contains a lot of pulp 100gskin stainsof ascorbic and cleanersacid in the are sample available of fruit/vegetable/ for fabric stains. For vitamin C tablets: Dissolve a single tablet in 200 mL or seeds. juice. of distilled water (in a volumetric flask if possible). 2. Vitamin C, or ascorbic acid, is a water soluble For fruits and vegetables: Cut a 100 g sample into small antioxidant that plays a vital role in protecting the piecesFor freshand grind fruit injuice: a mortar Strain andthe juicepestle. through Add 10 cheese mL Additionalbody from infection Notes and disease. It is not synthesised portionscloth to of remove distilled seeds water and several pulp timeswhich whilemay block grinding by the human body and therefore must be 1. Iodine stains both skin and clothing so proper care thepipettes. sample, each time decanting off the liquid extract acquired from dietary sources – primarily fruits and is advised. If staining does occur, alcohol may remove intoFor a 100packaged mL volumetric fruit juice: flask. This mayFinally, also strain need the to beground vegetables. The chemical structure and antioxidant skin stains and cleaners are available for fabric stains. fruit/vegetablestrained through pulp cheese through cloth cheesecloth, if it contains rinsing a lot theof pulp (reducing) action of ascorbic acid are illustrated in pulpor withseeds. a few 10 mL portions of water and collecting 2.the redox Vitamin half C, equation or ascorbic below: acid, is a water soluble all filtrate and washings in the volumetric flask. Make antioxidant that plays a vital role in protecting the body For fruits and vegetables: Cut a 100 g sample into small 3. The concentration of the prepared iodine solution the extracted solution up to 100 mL with distilled water. from infection and disease. It is not synthesised by pieces and grind in a mortar and pestle. Add 10 mL can be more accurately determined by titration with the human body and therefore must be acquired from Alternativelyportions of the distilled 100 g watersample several of fruit times or vegetable while grinding a standard solution of ascorbic acid or a standard dietary sources – primarily fruits and vegetables. The maythe be sample, blended each in a time food decanting processor off together the liquid with extract solution of potassium thiosulfate using a starch chemical structure and antioxidant (reducing) action of aboutinto 50 a 100mL mLof distilled volumetric water. flask. After Finally, blending, strain strainthe ground indicator. This should be done if possible as iodine ascorbic acid are illustrated in the redox half equation thefruit/vegetable pulp through cheesecloth,pulp through washing cheese cloth, it with rinsing a few the solutions can be unstable. below: 10mLpulp portions with a fewof distilled 10 mL portions water, and of water make and the collecting extracted HO HO solutionall filtrate up to and 100 washings mL in a volumetricin the volumetric flask. flask. Make oxidation O O O O the extracted solution up to 100 mL with distilled water. HO HO + 2H+ + 2e- reduction TitrationAlternatively the 100 g sample of fruit or vegetable may HO OH O O ascorbic acid dehydroascorbic acid 1. Pipettebe blended a 20 in mL a foodaliquot processor of the sample together solution with about into 50 mL of distilled water. After blending, strain the a 250 mL conical flask and add about 150 mL of 3. The concentration of the prepared iodine solution pulp through cheese cloth, washing it with a few 10 distilled water and 1 mL of starch indicator solution. 4. canThe beaverage more accuratelytitre volume determined should ideally by titration be in the with a mL portions of distilled water, and make the extracted 2. Titrate the sample with 0.005 mol L−1 iodine solution. standardrange of 10solution – 30 mL. of ascorbicIf the titre acid required or a standard for a 20 solution mL solution up to 100 mL in a volumetric flask. The endpoint of the titration is identified as the first ofaliquot potassium of sample thiosulfate solution using is well a starch outside indicator. this range This permanent trace of a dark blue-black colour due to shouldthen a belarger done or ifsmaller possible aliquot as iodine volume solutions should can be be theTitration starch-iodine complex. unstable.chosen. If the volume of the titre is too low, dilute the standard. If the titre volume is too high, dilute 3. Repeat1. Pipette the titration a 20 mL with aliquot further of the aliquots sample of solution sample into 4.the sample. The average titre volume should ideally be in solutiona 250 mL until conical you flask obtain and concordant add about 150results mL of (titres distilled the range of 10 – 30 mL. If the titre required for a 20 agreeingwater and within 1 mL of 0.1 starch mL). indicator solution. 5. mLAscorbic aliquot acid of sampleis susceptible solution to is oxidation well outside by this atmospheric oxygen over time. For this reason, the 2. Titrate the sample with 0.005 mol L−1 iodine range then a larger or smaller aliquot volume should besamples chosen. should If the volumebe prepared of the immediately titre is too low, before dilute Calculationssolution. The endpoint of the titration is identified as the titrations. However, if the samples have to be the first permanent trace of a dark blue-black colour due the standard. If the titre volume is too high, dilute the 1. Calculate the average volume of iodine solution used prepared several hours earlier, oxidation can be to the starch-iodine complex. sample. from your concordant titres. minimised by the addition of a small amount of 5.oxalic Ascorbic acid (eg acid 1 g oxalicis susceptible acid per to 100 oxidation mL of sample by 2. Calculate3. Repeat the the moles titration of iodine with reacting.further aliquots of sample solution until you obtain concordant results (titres atmosphericsolution). oxygen over time. For this reason, the 3. Using the equation of the titration (below) determine samples should be prepared immediately before the agreeing within 0.1 mL). 6. Identification of the endpoint in this titration is the number of moles of ascorbic acid reacting. titrations. However, if the samples have to be prepared severalsignificantly hours earlier,affected oxidation by the cancolouration be minimised of the by the − sample solution used. If the solutions are colourless ascorbicCalculations acid + I2 → 2 I + dehydroascorbic acid addition of a small amount of oxalic acid (eg 1 g oxalic or are pale in colour, there is no problem identifying 1. Calculate the average volume of iodine solution acid per 100 mL of sample solution). used from your concordant titres. 6. Identification of the endpoint in this titration is 2. Calculate the moles of iodine reacting. significantly affected by the colouration of the sample solution used. If the solutions are colourless or are pale 2 in colour, there is no problem identifying the endpoint. For strongly coloured juices there can be a problem with thethe endpoint endpoint. and For it stronglyis advised coloured to carry outjuices a “rough” there can titrationbe a problem in order with to become the endpoint familiar and with it isany advised distinct in colour, therecolourto carryis no change problem out a which identifying“rough” occurs thetitration endpoint. at the in endpoint order to (it become may For stronglyjustfamiliar coloured be a juices darkeningwith there any can distinctof be the a problem colour) colour with This change will also which help by the endpointestablishing and it is advised an toapproximate carry out a “rough” volume of iodate solution FigureF igure1 Vitamin 1 Vitamin C tablet. C tablet. Left Left photo: photo: before before endpoint, endpoint, added added iodine titrationreacts in orderoccurs to become at the familiar endpoint, with any (it distinct may just be a darkening with ascorbic acid leaving the solution colourless. Centre photo: At the required. reactstitration with ascorbic endpoint acid all theleaving ascorbic the acid solution has reacted colourless. and the excess iodinecolour changeof whichthe colour) occurs at This the endpoint will also (it may help by establishing an in colour, there is no problem identifying the endpoint. Centrereacts photo: with At the the starch titration indicator endpoint to give all a palethe ascorbicblue colour. acid Right has photo: just If be a darkening7.approximate The of theabove colour) volume method This will of may also iodine help be usedby solution to carry required. out a addition of iodine is continued after the endpoint, further iodine-starchFor strongly coloured juices there can be a problem with reacted andFigure the 1 excess Vitamin Ciodine tablet. Left reacts photo: with before the endpoint, starch added indicator iodine reacts to giveestablishingin colour, numberthere an approximate is no of problem interesting volume identifying of investigations iodate the solution endpoint. regarding the a palecomplex bluewith colour. isascorbic formed. Right acid NB:leaving photo: in eachthe Ifsolution additionof these colourless. images of iodine Centre a flask photo: is continued showing At the the afterthe palerequired. endpoint 7. Theand it above is advised method to carry outmay a “rough” be used to carry out a blue colourtitration of endpoint the endpoint all the ascorbic is shown acid has for reacted comparison. and the excess iodine titrationFor strongly in orderconcentration coloured to become juices familiar of there vitamin with can any be C adistinct inproblem various with foods and drinks: the endpoint,reacts further with the starchiodine-starch indicator to give complex a pale blue is colour.formed. Right NB: photo: in If each of colour7.the endpoint Thechange abovenumber which and method itoccurs is of advisedmay interestingat bethe usedto endpoint carry to carry outinvestigations (it amayout “rough” a regarding the these imagesaddition a flask of iodine showing is continued the after pale the endpoint,blue colour further ofiodine-starch the endpoint is complex is formed. NB: in each of these images a flask showing the pale justnumbertitration be a darkening of in •interestingconcentration Vitaminorder of to the become investigationsC colour) content familiarof This vitamin ofwill regarding differentwith also anyhelpC the in distinct by typesvarious of fruits/foods and shown for bluecomparison. colour of the endpoint is shown for comparison. concentrationvegetables/juices. of vitamin C in various foods and drinks: Figure 1 Vitamin C tablet. Left photo: before endpoint, added iodine reacts establishingcolour change andrinks: approximate which occurs volume at the of endpoint iodate solution (it may with ascorbic acid leaving the solution colourless. Centre photo: At the required.•just Vitamin be a Cdarkening content of of different the colour) types This of fruits/ will also help by titration endpoint all the ascorbic acid has reacted and the excess iodine • Vitamin C content of different types/brands of vitamin Figure 1 Vitamin C tablet. Left photo: before endpoint, added iodine reacts establishing • anVitamin approximate C content volume of iodate different solution types of fruits reacts with the starch indicator to give a pale blue colour. Right photo: If 7.vegetables/juices. The abovetablets. method may be used to carry out a withaddition ascorbic of iodine acid leavingis continued the solution after the colourless. endpoint, further Centre iodine-starch photo: At the required. titrationcomplex endpoint is formed. all NB: the in ascorbic each of these acid hasimages reacted a flask and showing the excess the paleiodine number• Vitamin of interestingC contentvegetables/juices. of investigations different types/brands regarding ofthe vitamin reactsblue colour with the of thestarch endpoint indicator is shown to give for a comparison. pale blue colour. Right photo: If concentrationtablets.7. The above• Vitamin of vitamin method C C contentin may various be used offoods tablets to and carry drinks: orout food/drink a in the addition of iodine is continued after the endpoint, further iodine-starch • Vitamin C content of different types/brands of number ofpresence interesting and investigations absence of regarding added theoxalic acid or metal ions complex is formed. NB: in each of these images a flask showing the pale • Vitamin• Vitamin C Ccontent contentvitamin of of different tablets tablets. ortypes food/drink of fruits/ in the blue colour of the endpoint is shown for comparison. vegetables/juices.presenceconcentration andover absence ofvarious vitamin of added periods C in oxalicvarious of acid time.foods or metal and drinks:ions over• Vitamin various C• periods contentVitamin of of time. different C content types of of fruits/tablets or food/drink in the Figure 2 Commercial fruit juice. Left flask: before the endpoint, the• colour Vitamin C content• Vitamin of different C content types/brands of food/drink of vitamin before and after tablets.•vegetables/juices. Vitamin C content of food/drink before and after of theFigure solution 2 Commercial reflects fruit the juice. pale Left yellow flask: colourbefore the of endpoint, the fruit the juice. colourFigure Centre flask: subjection presence to andcooking absence conditions. of added oxalic acid or At theof titration the solution endpoint reflects the all pale the yellow ascorbic colour acid of the has fruit reacted juice. Centre and flask: any excess• Vitaminsubjection• Vitamin C content toC content cookingmetal of tablets ofconditions. ions different or over food/drink types/brands various in the periods of vitamin of time. 2 CommercialiodineAt reacts the fruit titration with juice. endpoint the Left starch all flask: the indicator ascorbic before acid to formhasthe reacted endpoint, a dark and blue-black any theexcess colour complex. of • Vitamin C content of fruits/vegetables at different iodine reacts with the starch indicator to form a dark blue-black complex. presence•tablets. Vitamin and C content absence of of fruits/vegetables added oxalic acid at or different metal ions the solutionIn this case reflects the result the ispale a darkening yellow colour of the solution’sof the fruit colour juice. from yellow stages• Vitamin of ripeness. C content of food/drink before and after Centreto flask: brown-grey.In this At case the the Right titration result flask: is a darkening endpointThis illustrates of theall solution’s the the ascorbic effect colour of from addingacid yellow has just aover stagesmL variousor of ripeness. periods of time. to brown-grey. Right flask: This illustrates the effect of adding just a mL or • Vitamin C content of tablets or food/drink in the reactedtwo and moretwo any more of excess iodine of iodine solutioniodine solution reacts after after the the withendpoint endpoint the is reached,starch is reached, resulting indicator resulting in the to • inVitamin the C content subjection of food/drink to before cooking and after conditions. formationFigureformation 2 ofCommercial further of further fruitiodine-starch iodine-starch juice. Left flask:complex. complex. before the endpoint, the colour presence and absence of added oxalic acid or metal ions form a darkof the blue-black solution reflects complex. the pale yellow In this colour case of the the fruit result juice. Centre is a darkeningflask: subjectionContactover various to ContactUs• cooking periodsVitamin conditions. of Ustime. C content of fruits/vegetables at different of the solution’sAt the titration colour endpoint from all theyellow ascorbic to acid brown-grey. has reacted and Right any excess flask: iodine reacts with the starch indicator to form a dark blue-black complex. • Vitamin C content of fruits/vegetables at different This illustrates the effect of adding just a mL or two more of iodineIf • youVitamin have If Cany youcontentstages questions have of of food/drinkany or ripeness. comments questions before relating orand comments toafter this relating to this FigureIn this 2 case Commercial the result fruit is a darkening juice. Left of flask: the solution’sbefore the colour endpoint, from theyellow colour stagesexperiment, of ripeness. please contact us: solutionof afterto the brown-grey. solution the endpoint reflects Right flask: the is Thispale reached, illustrates yellow colour theresulting effect of the of fruit addingin juice.the just formationCentre a mL orflask: of subjectionexperiment, to cooking conditions. please contact us: further iodine-starchAttwo the more titration of iodine endpoint complex. solution all theafter ascorbic the endpoint acid has is reached, reacted resulting and any inexcess the Outreach iodineformation reacts of with further the iodine-starch starch indicator complex. to form a dark blue-black complex. • Vitamin OutreachC content of fruits/vegetables at different In this case the result is a darkening of the solution’s colour from yellow ContactCollege ofContact Us Science Us stages of Collegeripeness. of Science to brown-grey. Right flask: This illustrates the effect of adding just a mL or If Universityyou have any of Canterbury questions or comments relating to this two more of iodine solution after the endpoint is reached, resulting in the Private BagIf Universityyou 4800 have anyof Canterbury questions or comments relating to this formation of further iodine-starch complex. experiment, please contact us: ChristchurchContactexperiment,Private Us Bag please4800 contact us. Please note that this OutreachNew Zealand service is for senior school chemistry students in If you haveChristchurch any questions or comments relating to this Figure 3 Freshly squeezed orange juice. Left flask: before the endpoint, the College ofNew Science Zealand only. We regret we are unable to respond colour of the solution reflects the bright orange colour of fresh orange juice UniversityPhone:experiment, +64 ofNew Canterbury3 364 please Zealand2178 contact us: Figureand 3 Freshlyis unaffected squeezed by addition orange of iodine. juice. Centre Left flask: flask: Once before all the theascorbic endpoint, PrivateFax: the +64 Bag to3 4800364 queries 2490 from overseas. colouracid of hasthe been solution oxidised, reflects a slight theexcess bright of added orange iodine colour complexes of withfresh the orange Outreach juice Phone: +64 3 364 2178 starch indicator, giving the solution a green colour in this case. This is the ChristchurchEmail: [email protected] and isendpoint unaffected of the bytitration. addition Right offlask: iodine. If further Centre iodine flask: solution Once were allto bethe ascorbicCollege Outreach ofFax: Science +64 3 364 2490 Newwww.outreach.canterbury.ac.nz Zealand acid hasadded, been the oxidised,solution’s green a slight colour excess would becomeof added darker iodine as shown. complexes withUniversity the College of Canterbury of Science starchFigure indicator, 3 Freshly squeezedgiving the orange solution juice. Left a green flask: before colour the in endpoint, this case. the This is the Email: [email protected] Figure 3 Freshlycolour of thesqueezed solution reflects orange the brightjuice. orange Left colourflask: of freshbefore orange the juice endpoint, Phone:Private +64 UniversityBag 3 364 4800 2178 of Canterbury endpointand is unaffected of the titration. by addition Right of iodine. flask: Centre If further flask: Once iodine all the solution ascorbic were to be the colouradded, of the the solution’s solution green reflects colour the would bright become orange darker colour as shown.of freshFax: Christchurch +64 3 364www.outreach.canterbury.ac.nz 2490 acid has been oxidised, a slight excess of added iodine complexes with the Email: [email protected] Bag 4800 orange juicestarch and indicator, is unaffected giving the solution by additiona green colour of iniodine. this case. CentreThis is the flask: New ZealandChristchurch Once allFigure theendpoint ascorbic 3 Freshly of the squeezedtitration. acid has Right orange been flask: juice. Ifoxidised, further Left flask: iodine abefore slightsolution the excess wereendpoint, to be of the added www.outreach.canterbury.ac.nz colouradded, of the the solution’s solution greenreflects colour the brightwould orangebecome colour darker ofas freshshown. orange juice Phone: +64 3 364 2178 iodine complexes with the starch indicator, giving the solution a New Zealand and is unaffected by addition of iodine. Centre flask: Once all the ascorbic Fax: +64Phone: 3 364 2490 +64 3 364 2178 green colouracid has in been this oxidised, case. This a slight is excessthe endpoint of added iodine of the complexes titration. with the Email: [email protected] Right flask:starch If indicator, further giving iodine the solutionsolution a green were colour to bein thisadded, case. Thisthe is solution’s the endpoint of the titration. Right flask: If further iodine solution were to be Fax: +64 3 364 2490 green colouradded, would the solution’s become green darker colour would as shown. become darker as shown. www.outreach.canterbury.ac.nzEmail: [email protected] Figure 4 Red capsicum. Left flask: Before the endpoint, the solution www.outreach.canterbury.ac.nz retains its original colour. Centre flask: Once all the ascorbic acid has been oxidised, a slight excess of added iodine forms a dark complex with starch indicator giving a purple colour. This is the titration endpoint. Right flask: If a further mL or two of iodine were to be added after the endpoint, the solution would develop the dark purple colour shown here. Figure 4 Red capsicum. Left flask: Before the endpoint, the solution retains3Figure its 4 original Red capsicum. colour. Left Centreflask: Before flask: the Onceendpoint, all thethe solution ascorbic acid has been retains its original colour. Centre flask: Once all the ascorbic acid has been oxidised,oxidised, a slighta slight excess of of added added iodine iodine forms forms a dark complex a dark withcomplex starch with starch indicatorindicator giving giving a a purple colour. colour. This This is the is titration the titration endpoint. endpoint. Right flask: Right flask: If a furtherIf a further mL mL or or twotwo of of iodine iodine were were to be to added be addedafter the after endpoint, the theendpoint, the solutionsolution would would develop develop the the dark dark purple purple colour colour shown here.shown here. Figure 4 Red capsicum. Left flask: Before the endpoint, the solution Figure3 43retains Red capsicum. its original colour. Left Centreflask: flask: Before Once allthe the endpoint, ascorbic acid the has solutionbeen retains itsoxidised, original a slight colour. excess Centre of added flask:iodine forms Once a darkall thecomplex ascorbic with starch acid has indicator giving a purple colour. This is the titration endpoint. Right flask: been oxidised,If a further a mLslight or two excess of iodine of were added to be iodine added after forms the endpoint, a dark complexthe with starchsolution indicator would develop giving the a dark purple purple colour. colour shown This here.is the titration endpoint.3 Right flask: If a further mL or two of iodine were to be added after the endpoint, the solution would develop the dark purple colour shown here.