N by Titration (Mohr's Method)
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College of Science Determination of Chloride Ion Concentration by Titration (Mohr’s Method) Safety Solutions Needed Lab coats, safety glasses and enclosed footwear must Silver nitrate solution: (0.1 mol L−1) If possible, dry 5 g of be worn at all times in the laboratory. AgNO3 for 2 hours at 100°C and allow to cool. Accurately The chromate solution needs to be prepared and used weigh about 4.25 g of solid AgNO3 and dissolve it in with care as chromate is a known carcinogen. 250 mL of distilled water in a conical flask. Store the Silver nitrate solution causes staining of skin and solution in a brown bottle. fabric (chemical burns). Any spills should be rinsed with water immediately. Determination of ChloridePotassium Ion chromate indicator solution: (approximately 0.25 molL-1) Dissolve 1 g of K CrO in 20 mL distilled Concentration by Titration (Mohr’s Method)2 4 Introduction water. This method determines the chloride ion concentration of a solution byIntroduction titration with silver nitrate. As the silver nitrate solution is slowly added, a precipitate of silver chloride forms.This method determines the chloride ion concentration of a solution by titration with silver nitrate. As the silver nitrateAg+ solution + Cl– is slowly → AgCladded, a precipitate of silver Burette containing chloride(aq) forms. (aq) (s) silver nitrate solution The end point of the titration occurs when all the Ag+ + Cl– → AgCl chloride ions are precipitated. Then(aq) additional(aq) silver(s) The end point of the titration occurs when all the ions react withchloride the chromate ions are precipitated. ions of the Then indicator, additional chloride potassium chromate,ions react to with form the achromate red-brown ions ofprecipitate the indicator, of silver chromate.potassium chromate, to form a red-brown precipitate of silver chromate. 2– 2 Ag+(aq) + CrO4 + (aq) → Ag2– 2CrO4(s) 2 Ag (aq) + CrO4 (aq) → Ag2CrO4(s) This method canThis be method used canto determinebe used to determine the chloride the chloride ion ion concentration ofconcentration water samples of water from samples many from sources many sources such as seawater,such stream as seawater, water, stream river water, water river and water estuary and estuary water. Seawaterwater. is used Seawater as the is used example as the examplehere. here. The pH of the sampleThe pH of solutions the sample should solutions be should between be between 6.5 and 10. (Refer to the additional notes (3) for the 6.5 and 10. (Referexplanation). to the additional If the solutions notes are (3) acidic, for the gravimetric explanation). Ifmethod the solutions or Volhard’s are method acidic, should the gravimetric be used. method or Volhard’s method should be used. Equipment Needed burette and stand Equipment Needed Conical flask burette and stand10 and 20 mL pipettes 10 and 20 mL pipettes100 mL volumetric flask Seawater and chromate 100 mL volumetric250 mL flask conical flasks indicator 250 mL conical10 flasks mL and 100 mL measuring cylinders 10 mL and 100 mL measuring cylinders Solutions Needed Silver nitrate solution:(0.1 mol L−1) If possible, dry 5 g of AgNO3 for 2 hours at 100°C and allow to cool. Accurately weigh about 4.25 g of solid AgNO3 and dissolve it in 250 mL of distilled water in a conical flask. Store the solution in a brown bottle. Potassium chromate indicator solution: (approximately -1 0.25 molL ) Dissolve 1 g of K2CrO4 dissolved in 20 mL distilled water) 1 Method and a stronger red-brown colour results. NB: The titration should be stopped when the first trace of red-brown colour is observed. Using an incompletely titrated reference flask for comparison is a helpful way to Sample Preparation Method identifyand a stronger the first red-brown appearance colour of red-brown results. NB: colouration. The titration should be If the seawater contains traces of solid matter such stopped when the first trace of red-brown colour is observed. Using an incompletely titrated reference flask for comparison is a helpful way to asSample sand or Preparation seaweed, it must be filtered before use. Result Calculations Otherwise they will end up being weighed along with identify the first appearance of red-brown colouration. theIf the silver seawater chloride contains precipitate. traces of solid matter such 1. Determine the average volume of silver nitrate as sand or seaweed, it must be filtered before use. usedResult from Calculations your concordant titres. MethodTitrationOtherwise they will end up being weighed along with 4. Repeat the titration with further aliquots of diluted the silver chloride precipitate. 2.1. CalculateDetermine the the moles average of silver volume nitrate of silver reacting. nitrate 1. Dilute seawater by pipetting a 20 mL sample into seawater until concordant results (titres agreeing 3.used Use from the your following concordant reaction titres. equation to determine Samplea 100 mL Preparation volumetric flask and making it up to the mark within 0.1 mL) are obtained. Titration the moles of chloride ions reacting. Ifwith the distilled seawater water. contains traces of solid matter such as 2. Calculate the moles of silver nitrate reacting. 1. Dilute seawater by pipetting a 20 mL sample into sand or seaweed, it must be filtered before use. 3. Use the followingAg+ + reaction Cl– equation AgCl to determine 2.a 100Pipette mL volumetric a 10 mL aliquot flask and of dilutedmaking seawater it up to the into mark a Result Calculations(aq) (aq) → (s) the moles of chloride ions reacting. Titrationconicalwith distilled flask and water. add about 50 mL distilled water and 1 4.1. DetermineCalculate the the averageconcentration volume of of chloride silver nitrate ions in used the mL of chromate indicator. diluted seawater. + – 1. 2.Dilute Pipette seawater a 10 mL by aliquotpipetting of diluteda 20 mL seawater sample intointo aa from your concordantAg (aq) + Cl titres.(aq) → AgCl(s) 3.conical 100Titrate mL flask volumetric the and sample add flask about with and 0.150 mLmolmaking distilled L−1 silver it up water nitrate to the and mark1 5.2.4. CalculateCalculateCalculate the thethe moles concentrationconcentration of silver nitrate ofof chloridechloride reacting. ionsions inin the solution.mLwith of chromatedistilled Although water. indicator. the silver chloride that forms is a original undiluted seawater. 3.diluted Use the seawater. following reaction equation to determine the white precipitate, the chromate indicator initially gives 2.3. Pipette Titrate a 10 the mL sample aliquot with of diluted 0.1 mol seawater L−1 silver nitrate into a 6. molesCalculate of chloride the concentration ions reacting. of sodium chloride in the cloudy solution a faint lemon-yellow colour (figure 5. Calculate the concentration of chloride ions in the solution.conical flaskAlthough and theadd silver about chloride 50 mL thatdistilled forms water is a and the seawater in molL−1, gL−1 and g/100 mL (%). 1). The endpoint of the titration is identified as the first original undiluted seawater. white1 mL precipitate,of chromate the indicator. chromate indicator initially gives Ag+(aq) + Cl–(aq) → AgCl(s) appearance of a red-brown colour of silver chromate 6. Calculate the concentration of sodium chloride in 3.the Titrate cloudy the solution sample a with faint 0.1 lemon-yellow mol L−1 silver colour nitrate (figure 4. Calculate the concentration of chloride ions in the (figure 2). Additionalthe seawater inNotes molL−1, gL−1 and g/100 mL (%). 1).solution. The endpoint Although of the the titration silver chlorideis identified that as forms the first is diluted seawater. 4. Repeat the titration with further aliquots of diluted 1. Silver nitrate solution will stain clothes and skin. appearancea white precipitate, of a red-brown the chromate colour of indicator silver chromate initially 5. Calculate the concentration of chloride ions in the seawater until concordant results (titres agreeing within Any spills should be rinsed with water immediately. (figuregives 2).the cloudy solution a faint lemon-yellow colour Additionaloriginal undiluted Notes seawater. 0.1(figure mL) are 1). obtained. 4. Repeat the titration with further aliquots of diluted 2.6.1. CalculateResiduesSilver nitrate the containing concentration solution silver will stainionsof sodium are clothes usually chloride and saved skin. in the seawater until concordant results (titres agreeing within forAny seawaterlater spills recovery should in molL of be silver −1rinsed, gL−1 metal. and with g/100 waterCheck mL immediately. this (%). with your 0.1 mL) are obtained. teacher.2. Residues containing silver ions are usually saved 3.Additionalfor laterThe recoveryMohr titrationNotes of silver should metal. be Check carried this out with under your conditionsteacher. of pH 6.5 – 9. At higher pH silver ions may be removed3. The byMohr precipitation titration should with hydroxide be carried ions, out underand at low1.conditions Residues pH chromate of containing pH 6.5ions – may9. silver At behigher ionsremoved pHare silver usually by an ions acid-base saved may forbe reactionremovedlater recovery to by form precipitation hydrogenof silver metal.with chromate hydroxide Check ions this ions, or with dichromate and your at ions,lowteacher pH affecting chromate or the the laboratory ionsaccuracy may beof supervisor. theremoved end point. by an acid-base FigureFigure 1 1 Before Before the the addition addition of any of anysilver silver nitrate nitrate the chromate the chromate indicator 4.2.reaction TheIt Mohris ato good form titration idea hydrogen to should first chromate carry be carried out ionsa “rough” out or underdichromate titration indicatorgives the clear gives solution the clear a lemon-yellow solution a lemon-yellowcolour. colour. inions, orderconditions affecting to become of the pH accuracyfamiliar 6.5 – 9. with Atof higherthe the end colour pH point. silver change ions at may thebe end removed point. by precipitation with hydroxide ions, Figure 1 Before the addition of any silver nitrate the chromate indicator 4.