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Home , Silver chromate, Silver nitrate

College of Science

Determination of Concentration by (Mohr’s Method) Safety Solutions Needed

Lab coats, safety glasses and enclosed footwear must 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 . 250 mL of distilled water in a conical flask. Store the solution causes 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 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 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 ,ions react to with form the achromate red-brown ions ofprecipitate the indicator, of .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 ,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. Calculate Determine 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 100 Pipette 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. Determine Calculate 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 100 Titrate 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. Calculate Calculate Calculate 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. moles Calculate 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. Calculate Residues Silver 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 later The 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 The It 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. It is a good idea to first carry out a “rough” titration gives The the endpoint clear solution of the a lemon-yellow titration colour.is identified as the first 5.in andorder The at toMohr low become pH titration chromate familiar is sensitive ionswith maythe to colour bethe removed presence change by at an appearance of a red-brown colour of silver chromate ofthe bothacid-base end chloride point. reaction and to form ions hydrogen in solution chromate and ions (figure 2). willor not dichromate be too accurate ions, affecting when there the is aaccuracy significant of the 5. end The point. Mohr titration is sensitive to the presence concentrationof both chloride of andbromide bromide present ions as in wellsolution as the and chloride.3.will It notis a begoodHowever, too idea accurate in to most first when cases,carry there suchout is a a as“rough” significant seawater, titration theconcentrationin bromide order to concentration become of bromide familiar present will withbe negligible.as the well colour as the For change this at reason,chloride.the end the However, point.method in can most also cases, be used such to asdetermine seawater, either4.the The bromide the Mohr total titrationconcentration concentration is sensitive will of be chloride to negligible. the presence and bromideFor thisof both in solution, or the concentration of bromide when the + reason,chloride the and method bromide can alsoions be in usedsolution to determine and will not be Figure 2 Left flask: before the titration endpoint, addition of Ag ions chloride concentration is known to be negligible. leads to formation of silver chloride precipitate, making the solution eithertoo accuratethe total concentrationwhen there is aof significant chloride and concentration bromide cloudy. The chromate indicator gives a faint lemon-yellow colour. in ofsolution, bromide or presentthe concentration as well as theof bromide chloride. when However, the + + FigureCentreFigure flask:2 2 Left Left at flask:flask: the endpoint, before before the theall titration the titration Cl− ionsendpoint, haveendpoint, precipitated.addition addition of Ag The ionsof Ag chloridein most concentration cases, such as is knownseawater, to bethe negligible. bromide ionsleads leads to formation to formation of+ silver of chloride silver chloride precipitate, precipitate, making the making solution slightest excess of Ag precipitates with the chromate indicator giving a concentration will be negligible. For this reason, the theslightcloudy. solution red-brown The chromate cloudy. colouration. The indicator chromate Right gives flask: aindicator faint If addition lemon-yellow gives of Ag a +faint is colour. continued lemon- − yellowpastCentre the colour. flask:endpoint, at Centre the further endpoint, flask: silver allat chromate thethe Cl endpoint, ions precipitate have allprecipitated. the is formed Cl− ions The method can also be used to determine either the total + haveslightest precipitated. excess of AgThe precipitates slightest excess with the of chromate Ag+ precipitates indicator givingwith a concentration of chloride and bromide in solution, + 2theslight chromate red-brown indicator colouration. giving Right a slight flask: Ifred-brown addition of colouration. Ag is continued or the concentration of bromide when the chloride past the endpoint, further silver chromate precipitate is formed Right flask: If addition of Ag+ is continued past the endpoint, concentration is known to be negligible. 2further silver chromate precipitate is formed 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 identify the first appearance of red-brown colouration.