Determination of Iodate in Iodised Salt by Redox Titration

College of Science Determination of Iodate in Iodised Salt by Redox Titration Safety • 0.6 M potassium iodide solution (10 g solid KI made up to 100 mL with distilled water) • 0.5% starch 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 soil is low in iodine and hence New Zealand food is low in iodine. Until iodised salt 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 iodine deficiency – 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 goitre, and impaired physical and mental development. 1. Preparation of 0.002 mol L−1 sodium thiosulfate In earlier times salt was “iodised” by the addition of solution: Accurately weigh about 2.5 g of solid potassium iodide; however, nowadays iodine is more sodium thiosulfate (NaS2O3•5H2O) and dissolve in commonly added in the form of potassium iodate 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 titrations. 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 potassium permanganate (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. usedCalculate 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. saltCalculate 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. CalculateCalculate 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.Calculate5.9Calculate 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.BasedCalculate 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. ofBased 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 yourIn 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. ofIn 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. AdditionalAccordingBut 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. iodisedAccording 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.

Load more