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Determination of Vitamin C Concentration by Titration

(Redox Titration Using Iodine Solution) Safety

Lab coats, safety glasses and enclosed footwear must be worn at all times in the laboratory.

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.

Introduction

This method determines the vitamin C concentration in a solution by a redox titration using iodine. Vitamin C, more properly called ascorbic acid, is an essential antioxidant needed by the human body (see additional Starch indicator solution: (0.5%). Weigh 0.25 g of notes). As the iodine is added during the titration, the soluble starch and add it to 50 mL of near boiling water ascorbic acid is oxidised to dehydroascorbic acid, while in (Redox Titration the iodine is reduced to iodide ions. Using Iodine Solution) a 100 mL conical flask. Stir to dissolve and cool before using. ascorbic acid + I2 2 I- + dehydroascorbic acid Introduction This method formed the vitamin C concentration Due to this reaction, the iodine determinesis immediately in a redox titration using reduced to iodide as longsolution by acalled ascorbic acid, isiodine. Vitamin as there is any ascorbic acid C, more properly an essential Burette containing present. Once all the ascorbic acid has the human body (see additional iodine solution antioxidant needed by been oxidised, notes). As the with the starch the excess iodine is free to react iodine is added during the titration, the is indicator, forming theascorbic acidreduced to iodide ions. blue-black oxidised to dehydroascorbic acid, while starch-iodine complex. the iodine is This is the endpoint of the titration. -

Determination of Vitamin C Concentration by Titration

The method is suitable for usereaction, the iodineC tablets, Due to this with vitamin formed is immediately fresh or packaged fruit juices and solid fruits and ascorbic acid reduced to iodide as long as there is any present. Once all the ascorbic acid has been oxidised, vegetables.

the excess iodine is free to react with the starch

ascorbic acid + I2 2 I + dehydroascorbic acid

indicator, forming the blue-black the NB: This method is more straight forward thanstarch-iodine complex. This the endpoint iodate, but alternative method usingispotassiumof the titration. as the potassium iodate The method issuitable for use with vitamin C tablets, solution is more stable than the fresh or packaged fruit juices and solid fruits and iodine as a primary standard, the alternative method is vegetables. more reliable. NB: This method is more straight forward than the

Equipment Needed

burette and stand 20 mL pipette

alternative method using potassium iodate, but as the potassium iodate solution is more stable than the iodine as a primary standard, the alternative method is more reliable.

Conical flask

Equipment Needed 100 mL or 200 mL volumetric flask

burette and stand 100 mL or 200 mL volumetric flask 20 mL pipette

Vitamin C sample solution

10 mL and 100 mL measuring cylinders

250 mL conical flasks

250 mL conical flasks 10 mL and 100 mL measuring cylinders

Solutions Needed

Iodine solution: (0.005 mol L-1). Weigh 2 g of potassium

Method

4. Calculate the concentration in mol L-1 of ascorbic acid in the solution obtained from fruit/vegetable/ Sample Preparation 3. Using the equation of the titration (below) Starch indicator solution: (0.5%). Weigh 0.25 g of juice. Also, calculate the concentration, in mg/100mL determine the number of moles the sampleacid soluble starch and add it to 50 mL of near boiling water or mg/100g of ascorbic acid, in of ascorbic of fruit/ For vitamin C tablets: Dissolve a single tablet in 200 mL in a 100 water (in a volumetric dissolve and cool vegetable/juice. of distilledmL conical flask. Stir to flask if possible). before reacting. using. ascorbic acid + I2 2 I- + dehydroascorbic acid For fresh fruit juice: Strain the juice through cheesecloth to remove seeds and pulp which may block Additional Notes 4. Calculate the concentration in mol L-1 of ascorbic Method pipettes. 1. acid in the solution skin and clothing so proper care is Iodine stains both obtained from fruit/vegetable/juice. advised. If staining does occur, alcohol may remove ForSample Preparation This may also need to be packaged fruit juice: Also, calculate the concentration, in mg/100mL or mg/ skin of ascorbic acid in the sample of for fabric stains. strained through cheesecloth if it contains a lot of pulp 100g stains and cleaners are availablefruit/vegetable/ For vitamin C tablets: Dissolve a single tablet in 200 mL or seeds. 2. juice. C, or ascorbic acid, is a water soluble Vitamin of distilled water (in a volumetric flask if possible). antioxidant that plays a vital role in protecting the For fruits and vegetables: Cut a 100 g sample into small For and fruit juice: Strain the pestle. Add 10 mL body from infection and pieces freshgrind in a mortar and juice through cheese Additional Notes disease. It is not synthesised cloth to remove seeds and pulp times whileblock which may grinding by the human body and therefore must be portions of distilled water several 1. Iodine stains bothsources ­ clothing so proper care pipettes. acquired from dietary skin and primarily fruits and the sample, each time decanting off the liquid extract is advised. If The chemical structure and antioxidant vegetables. staining does occur, alcohol may remove into a 100 mL volumetric flask. Finally, strain the ground For packaged fruit juice: This may also need to be (reducing) action of ascorbic acid are illustrated in fruit/vegetable pulpcheese cloth if it contains a lot thepulp skin stains and cleaners are available for fabric stains. strained through through cheesecloth, rinsing of the Vitamin C, or ascorbic acid, 2. redox half equation below: is a water soluble pulp with a few 10 mL portions of water and collecting or seeds. antioxidant that playsthe prepared iodine solutionbody a vital role in protecting the all filtrate and washings in the volumetric flask. Make For fruits and vegetables: Cut a 100 g sample into small 3. The concentration of frombe more accurately determinedsynthesised by the extracted solution up to 100 mL with distilled water. can infection and disease. It is not by titration with pieces and grind in a mortar and pestle. Add 10 mL the human body andof ascorbic acid be a standard a standard solution therefore must or acquired from Alternatively the 100 g water several times while grinding portions of distilled sample of fruit or vegetable dietary sources ­ primarily fruits and vegetables. The solution of potassium thiosulfate using a starch may be blended in atime decanting off the liquid extract the sample, each food processor together with indicator. This should antioxidant (reducing) action about 50100 mLdistilled water. After blending, strain mL of volumetric flask. Finally, strain the ground chemical structure andbe done if possible as iodine of into a ascorbic acid are illustrated in the redox half equation solutions can be unstable. the pulp through cheesecloth, washingcloth, rinsing the fruit/vegetable pulp through cheese it with a few below: 10mL portionsfew 10 mL portionsandwater and collecting of distilled water, of make the extracted pulp with a HO HO solution up to 100 mL in a volumetric flask. flask. Make all filtrate and washings in the volumetric oxidation O O O O HO + 2H+ + 2ethe extracted solution up to 100 mL with distilled water. HO

Titration the 100 g sample of fruit or vegetable may Alternatively

be blended mL aliquot of the together with about 1. Pipette a 20in a food processorsample solution into 50 mL of conical water. After blending, strain the a 250 mL distilledflask and add about 150 mL of pulp through and 1 cloth, washing it with solution. distilled watercheesemL of starch indicator a few 10 mL portions of distilled water, and make the extracted 2. Titrate the sample with 0.005 mol L-1 iodine solution. solution up to 100 mL in a volumetric flask. The endpoint of the titration is identified as the first permanent trace of a dark blue-black colour due to Titration the starch-iodine complex.

HO

OH

reduction

O

O

ascorbic acid

dehydroascorbic acid

3. The concentration of the prepared iodine solution 4. can be more accurately determined by titration with a The average titre volume should ideally be in the range of 10 ­ 30 mL. ascorbic acid or a standard solution standard solution of If the titre required for a 20 mL aliquot of sample solution is well outside this range of potassium thiosulfate using a starch indicator. This then a larger or smaller aliquot volume should be should be done if possible as iodine solutions can be chosen. If unstable. the volume of the titre is too low, dilute the standard. If the titre volume is too high, dilute 3. Repeat the titration with further aliquots of sampleinto 4. sample. 1. Pipette a 20 mL aliquot of the sample solution the The average titre volume should ideally be in solution until you obtain concordant150 mL of distilled a 250 mL conical flask and add about results (titres the range of 10 ­ 30 mL. If the titre required for a 20 5. mL aliquot of sample solution is well outside this Ascorbic acid is susceptible to oxidation by agreeing withinof starch indicator solution. water and 1 mL 0.1 mL). atmosphericlarger or over time. For this reason, the range then a oxygen smaller aliquot volume should 2. Titrate the sample with 0.005 mol L-1 iodine samples should be prepared immediately before be chosen. If the volume of the titre is too low, dilute Calculations solution. The endpoint of the titration is identified as the titrations. However,volume is too high, dilute if the samples have to be the first permanent trace of a dark blue-black colour due the standard. If the titre earlier, oxidation can be the prepared 1. Calculate the average volume of iodine solution used sample. several hours to the starch-iodine complex. minimised by the addition of a small amount of from your concordant titres. 5. Ascorbic acidoxalic acid perto oxidation by 3. Repeat the titration with further aliquots of sample oxalic acid (eg 1 g is susceptible 100 mL of sample 2. Calculate the moles of iodine reacting. atmospheric oxygen over time. For this reason, the solution). solution until you obtain concordant results (titres 3. Using thewithin 0.1 mL). titration (below) determine equation of the samples should be prepared immediately before the agreeing 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 significantly affected by the colouration of the several hours earlier, oxidation can be minimised by the - sample solution used. If the solutions are colourless ascorbic acid + I2 2 I + dehydroascorbic acid Calculations addition of a small amount of oxalic acid (eg 1 g oxalic or are pale in colour, there is no problem identifying acid per 100 mL of sample solution). 1. Calculate the average volume of iodine solution used from your concordant titres. 6. Identification of the endpoint in this titration is significantly affected by the colouration of the sample 2. Calculate the moles of iodine reacting. solution used. If the solutions are colourless or are pale

in colour, there is no problem identifying the endpoint. For strongly coloured juices there can be a problem with the endpoint. For is advised to carry juices there can the endpoint and it strongly coloured out a "rough" be a problem with the endpoint and it any distinct titration in order to become familiar with is advised in colour, therecarry out awhich occurs at the endpoint (it may to is no problem "rough" titration in colour change identifying the endpoint. order to become For stronglyfamiliarjuices there can be a problem with will also help by coloured with any of the colour) This just be a darkeningdistinct colour change which the endpoint and it is advised to carry out a "rough" establishing anendpoint, (it may Figure 1 Vitamin C tablet. occurs at the approximate volume of iodate solution Figure 1 Vitamin C tablet. LeftLeft photo: before endpoint,added iodinetitration in order to become familiar with any distinctjust be a darkening photo: before endpoint, added iodine reacts with ascorbic acid leaving the solution colourless. Centre photo: At the required. of the colour) the endpoint (it help reacts with ascorbic acid leaving the solution colourless. colour change which occurs atThis will alsomay by establishing an

titration endpoint all the ascorbic acid has reacted and the excess iodine

colour, darkening of the colour) This will also help Centre photo: Atthe starch indicator to give a pale ascorbic acid hasphoto: If be athere is no problem identifying the iodineby to carry out a reacts with the titration endpoint all the blue colour. Right injust approximate volume of endpoint. solution required. The above method may be used For strongly 7.an approximate volumebe a problem with coloured juices there can of iodate solution addition of excess C continued after the endpoint, further iodine-starch establishing reacted and the1iodine isiodineLeft photo:with the starch indicator to give colour, there is no problem identifying the endpoint. Figure Vitamin tablet. reacts before endpoint, added iodine reacts inendpoint and it is advised to carry out a "rough" number of interesting investigations regarding the 7. required. The above method may a palecomplex is formed. NB: in each additionimages a flaskcontinued after strongly coloured juices there can be be used to carry out a bluewith ascorbic acid leaving theIf of these of iodinephoto: At the the pale colour. Right photo: solution colourless. Centre is showing the For titration endpoint all the ascorbic acid has reacted and the excess iodine blue colour of the endpoint is shown for comparison. concentration of vitamin Cdistinct titration in order to become familiar with any a problem with the endpoint, further starch indicator to give a pale blue colour. RightNB: inIfeach of endpoint and it isof interestingout in various foods and drinks: iodine-starch complex is formed. photo: reacts with the number may the endpoint (it maya 7. change which occurs at be to carry investigations regarding the out the colour The above method advised used to carry a "rough" addition of iodine is continued after the endpoint, further iodine-starch these images a flask showing the pale blue colour of the endpoint is number ofin Vitamininvestigationswill alsoanyin types of fruits/ and interestingbecome of vitamin helpdistinct titration ·concentration familiar differentby orderof the colour) Thisof with Cthe various foods to C content regarding complex is formed. NB: in each of these images a flask showing the pale just be a darkening shown for blue colour of the endpoint is shown for comparison. comparison. concentration approximate in various foods and drinks: colour change which occurs at of iodate solution establishingvegetables/juices.the endpoint (it may an of vitamin C volume Figure 1 Vitamin C tablet. Left photo: before endpoint, added iodine reacts drinks: with ascorbic acid leaving the solution colourless. Centre photo: At the just be darkening of the colour) Thisfruits/ required. aC·content of different types ofdifferenthelp by · Vitamin Vitamin C content of will also types/brands of vitamin titration endpoint all the ascorbic acid has reacted and the excess iodine · Vitamin C content of out a establishing an approximateused to carrydifferent types of fruits Figure 1with the starch indicatorphoto: before blue colour. Right photo: If reacts Vitamin C tablet. Left to give a pale endpoint, added iodine reacts 7.vegetables/juices. 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Left flask: before the endpoint, the colour presence cooking conditions. vegetables/juices. of food/drink before and after tablets. Csubjection to and absence of added oxalic acid or · of the solution reflectsfruit juice. Left flask: colour ofendpoint, the colour Centre flask: Figure 2 Commercial the pale yellow before the the fruit juice.Figure Vitamin content At the titration endpoint all the ascorbic acid has reacted flask: · Vitamin C content of tablets or food/drink in the subjection tocontent conditions. various periods of of the solution reflects the of the Centre metal ions over · C cooking C content of fruits/vegetables at different 2 Commercial fruit juice. Leftallpale yellow colourthe reactedjuice.any and any excess flask: before form fruit andblue-black complex. endpoint, the colour ofVitamin· Vitaminof different types/brands of vitamin time. At the titration the starch indicator to excess iodine reacts withendpoint the ascorbic acid has a dark ·tablets. C absence of added oxalic acid or metal ions Vitamin the solutioncasereactsresultpaledarkeningto formofdark blue-black complex. yellow reflects the is a yellow colour solution's colour iodine the with the starch indicator of thea the fruit juice.from presence andcontent of fruits/vegetables at different In this · Vitamin C content of food/drink before and after In At the titrationaendpoint all solution's result is darkening of the the ascorbic acid over variousstages of ripeness. periods Centre flask:this case the Right flask: This illustrates the effecteffect of justyellowjust a stages of ripeness. of time. to brown-grey. Right flask: This illustrates the colour from has adding mL or to brown-grey. of adding a mL or · the Vitamin C content of tablets or food/drink in the subjection to cooking conditions. two more of iodine iodine reactsthe endpoint is reached, in the reacted and any excess solutionafter thewith the reached, resultingresulting in two more of iodine solution after endpoint is starch indicator to · Vitamin C content of food/drink before and after presence and absence of added oxalic acid or metal ions Figure 2 Commercial iodine-starch complex. formation of further fruit juice. Left flask: before the endpoint, the colour formation of further iodine-starch this case the result is a form a dark blue-black complex. In complex.of the fruit juice. Centre darkening subjection toUs Vitamintime. of the solution reflects the pale yellow colour flask: ContactContact Uscontent of fruits/vegetables at different ·cooking conditions. over various periods of C At the titration endpoint all yellow to brown-grey. Right flask: of the solution's colour from the ascorbic acid has reacted and any excess · If you haveIf you of fruits/vegetables at different iodine reacts with the starch indicator anystages any questions and after questions or comments · Vitamin C content of food/drink beforeor comments relating to this This illustrates Commercial fruitdarkening ofto formmL thetwo from theof iodineVitamin C content haveof ripeness. relating to this the effect of juice. Left flask: a a dark blue-black complex. or endpoint, yellow Figure 2case the result is a adding just solution's colour more colour stages of ripeness. In this the before pale yellow colour of the in the Centre flask: experiment, please contact us: solutionof the solution reflects theis reached, the effect offruit juice.formation of subjection to cooking conditions. after the endpoint This illustrates resultingadding just a mL or to brown-grey. Right flask: experiment, please contact us:

two more of iodine solution after At the titration endpoint all the the endpoint is reached, and any excess further iodine-starch complex. ascorbic acid has reactedresulting in the formation of further iodine-starch complex.

· VitaminOutreach fruits/vegetables at different C Contact Contact Uscontent ofUs College of Science stages of College of Science ripeness. IfUniversityanyCanterburyor comments relating comments relating to this you have of questionsany questions or to this If you have Private Bagplease contact us: 4800 experiment, University of Canterbury us. Please note that this experiment, 4800 ChristchurchUs Bag please contact Contact Private Outreach service is for senior school chemistry students in New Zealand Christchurch If you have any questions or comments relating to this Science Figure 3 Freshly squeezed orange juice. Left flask: before the endpoint, the College ofNew Zealand only. We regret we are unable to respond Phone: +64 3 364Zealand us: New 2178 colour of the solution reflects the bright orange colour of fresh orange juice University of Canterbury experiment, please contact and is unaffected by addition of iodine. Centre flask: Once all the ascorbic Fax: +64 3 364 2490 Figure 3 Freshly squeezed orange juice. Left flask: before the endpoint, the Bag 4800 Private to queries from overseas. acid a slight excess of added iodine complexes OutreachPhone: +64 3 364 2178 colour of has been oxidised, the solution bright colour in this case.of withtheorange [email protected] the solution reflects the a green orange colour This is the fresh Email: starch indicator, giving Christchurch Science College of and is unaffected by addition of iodine. Centre flask: Once allbe ascorbic Outreach 3 364 2490 Fax: +64 endpoint of the titration. Right flask: If further iodine solution were to the New Zealand www.outreach.canterbury.ac.nz acid has been oxidised, a slight excess become darker as shown. added, the solution's green colour would of added iodine complexes with the College of Science University of Canterbury Email: [email protected] Figure 3 Freshly giving the starch indicator, squeezed orange juice. Leftgreen colour in this case. This is the a flask: before the endpoint, the Private University Bag 4800 colour of squeezed orange bright Left flask: before the endpoint, the juice. of fresh orange juice Figure 3 Freshly the solution reflects solution orange colouriodine solution werePhone: +64 3 364 2178 of Canterbury endpointunaffected by addition iodine. Centre flask: Once all the ascorbic of to be and isthethe titration. Right flask: If further 364 2490 www.outreach.canterbury.ac.nz the colour ofhas been oxidised,greenofexcess ofbright orange colour of fresh Christchurch Bag 4800 solution reflects would iodine complexes with the Fax: +64 3Private added, the solution's a slight the added become darker as shown. colour acid Email: [email protected] orange juice and is unaffected by addition of iodine. Centrethe starch indicator, giving the solution a green colour in this case. This is flask: New Zealand Christchurch endpoint of the acid has been oxidised, before the endpoint, Once allFigureascorbictitration. Right flask: If further iodineslight excessbe the www.outreach.canterbury.ac.nz the 3 Freshly squeezed orange juice. Left flask:a solution were to of added +64 3 Zealand added, the solution's reflects the would become darker as shown. colour of the solution green colour bright orange colour of fresh orange juice Phone:New 364 2178 iodine complexes with the starch indicator,flask: Once all the ascorbica giving the solution and is unaffected by addition of iodine. Centre Fax: +64 3 364 2490 3 364 2178 Phone: +64 green colour inbeen oxidised,This isexcess of added iodinethe titration. the acid has this case. a slight the endpoint of complexes with Email: [email protected] starch further iodine solution a were to be this case. This solution's Right flask: If indicator, giving thesolution green colour inadded, the is the Fax: +64 3 364 2490 endpoint of the titration. Right flask: If further iodine solution were to be www.outreach.canterbury.ac.nz green colour would become darker as shown. darker as shown. Email: [email protected] added, the solution's green colour would become www.outreach.canterbury.ac.nz Figure 4 Red capsicum. Left flask: Before the endpoint, the solution

iodine reacts with the starch indicator to form a dark blue-black complex. In this case the result is a darkening of the solution's colour from yellow to brown-grey. Right flask: This illustrates the effect of adding just a mL or two more of iodine solution after the endpoint is reached, resulting in the formation of further iodine-starch complex.

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.

Outreach

Figure 4 Red capsicum. Left flask: Before the endpoint, the solution Figure 3 4 Red capsicum. Left flask: Before the endpoint, the ascorbic retains its original colour. Centre flask: Once all the solution acid has been retains its original colour. Centre flask: Once all the ascorbic acid has been oxidised, a slight excess of added iodine formscomplex with starch with starch a dark complex oxidised, a slight excess of added iodine forms a dark indicator giving a purple colour. This is the titration endpoint. Right flask: indicator giving a purple colour. This is the titration endpoint. Right flask: If a further mL or twoof iodine were to beto be added after the the If a further mL or two of iodine were added after the endpoint, endpoint, the solution would develop the dark purple colour shown shown solution would develop the dark purple colour here. here.

Figure 4retainscapsicum. LeftCentre flask: Oncethethe ascorbic acid has been 3 3 Red its original colour. flask: Before all endpoint, the solution oxidised, a slight excess of added iodine Once dark complex with starch retains its original colour. Centre flask: forms a all the ascorbic acid has indicator giving purple colour. This is titration endpoint. Right been oxidised, a mL oratwo of iodine were to theadded after the endpoint,flask: If a further slight excess of added iodine forms a dark complex be the with starch indicator giving adark purple colour shown here. titration solution would develop the purple colour. This is the endpoint. Right flask: If a further mL or two of iodine were to be 3 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

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