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Home , Iodised salt, Potassium iodate, Potassium iodide, Sodium thiosulfate

College of Science

Determination of Iodate in Iodised by

Safety • 0.6 M solution (10 g KI made up to 100 mL with distilled ) • 0.5% indicator solution Lab coats, safety glasses and enclosed footwear must (see below for preparation) be worn at all times in the laboratory. • 250 mL volumetric flask Introduction • 50 mL pipette (or 20 and 10 mL pipettes) • 250 mL conical flasks New Zealand is low in and hence New Zealand is low in iodine. Until was • 10 mL measuring cylinder commonly used (starting in 1924), a large proportion • burette and stand of school children were reported as being affected • distilled water by – as high as 60% in Canterbury schools, and averaging 20 − 40% overall. In the worst cases this deficiency can lead to disorders such as Method , and impaired physical and mental development. 1. Preparation of 0.002 mol L−1 In earlier times salt was “iodised” by the addition of solution: Accurately weigh about 2.5 g of solid ; however, nowadays iodine is more (NaS2O3•5H2O) and dissolve in commonly added in the form of 100 mL of distilled water in a volumetric flask. (This gives a 0.1 mol L−1 solution). Then use a pipette to (KIO3). The Australia New Zealand Food Standards Code specifies that iodised salt must contain: “equivalent to transfer 10 mL of this solution to a 500 mL volumetric no less than 25 mg/kg of iodine; and no more than 65 flask and dilute by adding distilled water up to the mg/kg of iodine”. mark; you will use this diluted thiosulfate solution In this method we determine the amount of iodate in your . The concentration of the diluted (IO3−) in iodised salt by first reacting the iodate with thiosulfate solution may be calculated as follows: added iodide (I−), under acid conditions, to produce 2− −1 [S2O3 ] (mol L ) = mass of NaS2O3•5H2O used (g) iodine: ÷ 248.2 (g mol−1) ÷ 0.1 (L) ÷ 50 − − + IO3 + 5 I + 6 H → 3 I2 + 3 H2O Alternatively, the concentration of thiosulfate Then the resulting iodine is titrated with thiosulfate may be determined more accurately by titration as follows: with a standard solution of iodate or (if either is available). I + 2 S O 2− → 2 I− + S O 2− 2 2 3 4 6 2. Preparation of 0.5% starch indicator solution: Weigh 0.25 g of soluble starch into a 100 mL conical Equipment and Materials Required flask or beaker and add 50 mL of distilled water. Heat • iodised salt solution with stirring at 79 ° C for 5 minutes, being careful not to exceed the stated temperature. Allow −1 • 0.002 mol L sodium thiosulfate solution solution to cool to room temperature. (see below for preparation) 3. Accurately weigh about 50 g of iodised salt into a −1 • 1 mol L hydrochloric acid 250mL volumetric flask and add distilled water up to the mark. Shake to dissolve salt. 4. Use a pipette to transfer a 50 mL aliquot of salt 4. Calculate the amount, in moles, of iodate in the salt solution into a 250 mL conical flask. Then add 5 mL solution. −1 −1 of 1 mol L hydrochloric acid and 5 mL of 0.6 mol L 5. Calculate the concentration, in mol L−1, of iodate in potassium iodide solution. The solution will turn a the salt solution. yellow/brown colour as iodine is produced, as shown in Figure 1. 6. Calculate the number of grams of iodate in the salt solution. 5. Titrate the solution with your 0.002 mol L−1 sodium thiosulfate solution until the yellow/brown colour 7. Based on the weight of the iodised salt you used to of iodine becomes very pale (see Figure 1). Then add make your salt solution, calculate the iodate content of your salt, in mg of iodate (IO3−) per kg of salt. NB: 1 mL of starch indicator solution, which will produce − −1 a dark blue-black coloured complex with iodine − as the molecular weight of IO3 is 174.9 g mol . shown in Figure 2 − and continue your titration until 8. In order to see if your salt satisfies the Australia New this colour completely disappears (see Figure 3). Zealand Food Standards Code, calculate the iodine 6. Repeat the titration with further aliquots of your content (in mg of iodine (I) per kg of salt) from your salt solution until concordant results (titres agreeing result above as follows: within 0.1 mL) are obtained. iodine (I) content = iodate (IO3−) content x 126.9/174.9

solution used from your concordant titres. solutionAdditional3. used Calculate from your the concordantamount, Notes in titres. moles, of thiosulfate 3. reacting. Calculate the amount, in moles, of thiosulfate reacting.solution used from your concordant titres. Figure 1. Right flask: yellow/ 1. 4.According Calculate the amount,to the in moles,specified of iodate inlimits the for iodate in iodised 3. Calculate the amount, in moles, of thiosulfate 4. salt Calculate solution. the amount, in moles, of iodate in the −1 brown colour of iodine formed saltreacting. solution.salt, the volume of 0.002 mol L sodium thiosulfate 5. Calculate the concentration, in mol L−1, of iodate in from reaction of iodate from 5.4. Calculate Calculate the the concentration, amount, in moles, in mol of Liodate−1, of iodate in the in therequired salt solution. in the above titration should lie between salt with acidic iodide solution. thesalt salt solution. solution. 6.5. 6. Calculate5.9 Calculate Calculate mL the the andnumber concentration, the number 15.4of grams ofinmL. ofgramsmol iodate L −1Therefore,, of in iodate the salt in in the salt a “rougher” method Left flask: pale yellow colour the salt solution. solution.solution.for quickly determining whether or not your salt left when nearly all iodine has 7.6. 7. Based Calculate Based on the the on weight number the weight of of the grams iodisedof the of iodisediodatesalt you in saltused the yousalt used tosolution. maketosample make your saltyour solution, saltconforms solution, calculate calculate the to iodate thethese content iodate limitscontent is to prepare the reacted with added thiosulfateFigure Figure 1. 1.Right Right flask: flask: yellow/brown yellow/brown colour colour of of iodine iodine formed formed from from reaction reaction of your salt, in mg of iodate (IO −) per kg− of salt. NB: the 7. of Based your onsalt, the in weight mg of of iodate the3 iodised (IO3 ) saltper youkg of used salt. NB: the of ofiodate iodate from from salt salt with with acidic iodideiodide solution. solution. Left Left flask: flask: pale pale yellow yellow colour colour − − −1. −1. molecularto makemolecularsample your weight salt weight of solution, solutionIO3 of is 174.9IO calculate is g 174.9mol asthe g iodate molabove content (adding hydrochloric acid during titration. leftleft when when nearly nearly all all iodine iodine has reactedreacted with with added added thiosulfate thiosulfate during during 3 titration.Figure 1. Right flask: yellow/brown colour of iodine formed from reaction − titration. 8.of your In order salt, into mg see of if youriodate salt (IO satisfies3 ) per kg the of Australiasalt. NB: the of iodate from salt with acidic iodide solution. Left flask: pale yellow colour molecular8.and In weight order potassium ofto IO see− is if 174.9 your g salt moliodide satisfies−1. theas Australia described), but instead left when nearly all iodine has reacted with added thiosulfate during NewNew Zealand Zealand Food StandardsFood3 Standards Code, calculate Code, calculate the iodine the iodine Figure 2. Right flask: yellow/ titration. content8. of In order(in titrating mg to of see iodine if your (I) perthesalt kg satisfies ofsolution salt) the from Australia your simply add 1 mL of starch resultcontent above as (in follows: mg of iodine (I) per kg of salt) from your Newresult Zealand above Food as Standards follows: Code, calculate the iodine brown solution containing contentiodineindicator, (in(I) content mg of iodine = iodate followed (I) per (IO kg−) content of salt) byfromx 126.9/174.9 your5.9 mL of thiosulfate solution, result iodineabove as (I) follows: content = iodate3 (IO −) content x 126.9/174.9 last trace of iodine. Left flask: the blue-black colour3 of starch-iodine should persist. iodine (I) content = iodate (IO −) content x 126.9/174.9 dark blue-black colour formed Additional Notes 3 1. Additional AccordingBut when to the Notes specified a further limits for iodate 9.5 inmL of thiosulfate solution is when starch indicator is added −1 iodisedAdditional1.added salt, According the Notes volume the to ofthe colour 0.002 specified mol L shouldlimits sodium for iodate disappear. in to solution containing iodine thiosulfate1. iodised According required salt, to the the in volume thespecified above of limits titration 0.002 for mol shouldiodate L−1 sodium inlie betweeniodisedthiosulfate salt, 5.9 mLthe and requiredvolume 15.4 mL.of in 0.002 Therefore,the abovemol L a−1 titrationsodium“rougher” should lie method for quickly determining whether or not your near endpoint. thiosulfatebetween required 5.9 mL in and the 15.4above mL. titration Therefore, should a “rougher” lie salt sample conforms to these limits is to prepare the Figure 2. Right flask: yellow/brown solution containing last trace of iodine. betweenmethod 5.9 mLfor andquickly 15.4 mL.determining Therefore, whethera “rougher” or not your Left flask: dark blue-black colour formed when starch indicator is added to sampleContactmethod solution for quickly as above determining Us (adding whether hydrochloric or not acid your Figuresolution 2. Right containing flask: yellow/browniodine near endpoint. solution containing last trace of iodine. and saltpotassium sample iodide conforms as described), to these but limits instead is to of prepare the Figure 2. Right flask: yellow/brown solution containing last trace of iodine. salt sample conforms to these limits is to prepare the Left flask: dark blue-black colour formed when starch indicator is added to titratingsample the solution simply as above add (adding1 mL of starch hydrochloric indicator, acid solutionLeft flask: containing dark blue-black iodine colour near endpoint. formed when starch indicator is added to Ifsample you solution have as above any (adding questions hydrochloric acid or comments relating to this Figure 3. A series of flasks solution containing iodine near endpoint. followedandand potassium potassiumby 5.9 mL iodide of iodidethiosulfate as described), as described), solution, but instead the but blue- insteadof of blackexperiment,titratingtitrating colour the of solution the starch-iodine solution simply please simply shouldadd 1 mL add persist. contact of 1 starch mL But of indicator, whenstarch us. a indicator, Please note that this showing the colour change furtherfollowedfollowed 9.5 by mL 5.9 byof mL thiosulfate5.9 of mL thiosulfate of thiosulfatesolution solution, is added solution, the the blue- colour the blue- as the last remaining iodine shouldserviceblackblack colour disappear. colour of is starch-iodine offor starch-iodine senior should should persist.school persist. But when chemistry But a when a students in Newfurtherfurther 9.5 Zealand mL 9.5 of mL thiosulfate of thiosulfate only. solution solutionWe is added regret is the added colour thewe colour are unable to respond (with added starch indicator) Contactshouldshould disappear. Us disappear. is titrated with thiosulfate. Ifto you queries have any questions from or comments overseas. relating to this The dark blue-black colour experiment,ContactContact Us please Us contact us: OutreachIf you have any questions or comments relating to this OutreachIf you have any questions or comments relating to this disappears, leaving a colourless Collegeexperiment, of Science please contact us: Collegeexperiment, of please Science contact us: Figure 3. A series of flasks showing the colour change as the last remaining University of Canterbury solution at the endpoint. iodine (with added starch indicator) is titrated with thiosulfate. The dark Outreach blue-black colour disappears, leaving a colourless solution at the endpoint. PrivateUniversityCollegeOutreach Bag of Science4800 of Canterbury Figure 3. A series of flasks showing the colour change as the last remaining ChristchurchUniversityCollege of of Canterbury Science iodine (with added starch indicator) is titrated with thiosulfate. The dark Figure 3. A series of flasks showing the colour change as the last remainingNew PrivateUniversity Zealand Bag 4800 Bag of Canterbury 4800 Calculations iodineCalculationsblue-black (with addedcolour disappears,starch indicator) leaving is a titratedcolourless with solution thiosulfate. at the endpoint. The dark blue-black colour disappears, leaving a colourless solution at the endpoint.Phone:ChristchurchPrivate +64 Bag3 364 4800 2178 1. From the redox equations above, determine the Fax:New Christchurch Zealand +64 3 364 2490 1. From the redox equationsnumberCalculations above, of moles determine of thiosulfate required the for reaction Email:NewPhone:New [email protected] +64Zealand 3 364 2178 with each mole of iodate in the original salt solution. number of moles of thiosulfateCalculations1. From the required redox equations for above, reaction determine the www.outreach.canterbury.ac.nzPhone:Fax:Phone: +64 3 +64 364+64 3 2490 364 3 2178 364 2178 Email: [email protected] 1. 2.number From Calculate of the moles redox the of average thiosulfateequations volume requiredabove, of thiosulfate determine for reaction the Fax: +64 3 364 2490 with each mole of iodate in the original salt solution. Fax: +64 3 364 2490 with each mole of iodatenumber2 in the of moles original of thiosulfate salt required solution. for reaction www.outreach.canterbury.ac.nzEmail: [email protected] with2. each Calculate mole theof iodate average in volume the original of thiosulfate salt solution. Email: [email protected] 2. Calculate the average volume of thiosulfate solution www.outreach.canterbury.ac.nz 2.2 Calculate the average volume of thiosulfate www.outreach.canterbury.ac.nz used from your concordant2 titres. 3. Calculate the amount, in moles, of thiosulfate reacting.