College of Science Determination of Chloride Ion Concentration by Titration (Volhard’s Method) neutral or basic, Mohr’s method or the gravimetric Safety method should be used. The method is illustrated Lab coats, safety glasses and enclosed footwear must below by using the procedure to determine the be worn at all times in the laboratory. concentration of chloride (from sodium chloride) Silver nitrate solution causes staining of skin and in cheese. fabric (chemical burns). Any spills should be rinsed with water immediately. Equipment Needed Concentrated nitric acid is very corrosive: take great boiling chips care using the 6molL-1 solution. 500 mL volumetric flask Introduction 10 mL and 100 mL measuring cylinders conical flasks This method uses a backDetermination titration with potassium of Chloride Ion Concentration thiocyanate to determine the concentration of chloride Bunsen burner, tripod and gauze ions in a solution. Beforeby the Titration titration an excess (Volhard’s volume buretteMethod) and stand of a silver nitrate solution is added to the solution 50 mL pipette (if possible) containing chloride ions, forming a precipitate of silver chloride. The term ‘excess‘Introduction is used as the moles of silver nitrate added are knownThis to method exceed uses the a back moles titration of with sodium potassium chloride present in the thiocyanatesample so to determinethat all the concentration chloride of chloride ions in a solution. Before the titration an excess volume Burette containing ions present will react. of a silver nitrate solution is added to the solution potassium thiocyanate containing chloride ions, forming a precipitate of silver solution + – chloride. The term ‘excess‘ is used as the moles of silver Ag (aq) + Cl (aq) → AgCl(s) nitrate added are known to exceed the moles of sodium 3+ The indicator Fe (ferricchloride ion) is present then in added the sample and so thatthe all the chloride solution is titrated withions the present potassium will react. thiocyanate + – solution. The titrate remains pale yellowAg (aq) + Clas(aq) the → excessAgCl(s) (unreacted) silver ions reactThe indicator with Fe the3+ (ferric thiocyanate ion) is then added ions and the solution is titrated with the potassium thiocyanate to form a silver thiocyanatesolution. precipitate. The titrate remains pale yellow as the excess + (unreacted)– silver ions react with the thiocyanate ions to Ag (aq) + SCNform a(aq) silver → thiocyanate AgSCN precipitate.(s) + – Once all the silver ions have reacted,Ag (aq) the+ SCN slightest(aq) → AgSCN (s) excess of thiocyanate reactsOnce all with the silver Fe ions3+ to have form reacted, a dark the slightest red complex. excess of thiocyanate reacts with Fe3+ to form a dark red complex. 3+ – 2+ 3+ – Fe (aq) + SCN 2+(aq) → [FeSCN] (aq) Fe (aq) + SCN (aq) → [FeSCN] (aq) The concentration of chloride ions is determined by The concentration of chloridesubtracting ions the titration is determined findings of the by moles of silver subtracting the titrationions findings that reacted of with the the thiocyanatemoles of silverfrom the total Conical flask moles of silver nitrate added to the solution. ions that reacted with the thiocyanate from the total This method is used when the pH of the solution after moles of silver nitrate addedthe sample to hasthe been solution. prepared is acidic. If the pH is neutral or basic, Mohr’s method or the gravimetric Cheese extract solution This method is used whenmethod the should pH ofbe used.the Thesolution, method isafter illustrated below the sample has been prepared,by using the is procedure acidic. to If determine the pH theis concentration of chloride (from sodium chloride) in cheese. Equipment Needed boiling chips 500 mL volumetric flask 10 mL and 100 mL measuring cylinders conical flasks Bunsen burner, tripod and gauze burette and stand 50 mL pipette (if possible) 1 Solutions Needed and addition of 5 mL of potassium permanganate should be continued, checking each time until there -1 4. Cool the solution and filter it. Wash the solid SolutionsConcentrated Needed nitric acid (see safety notes): (6 mol L ) is a satisfactory level of digestion. −1 residue with a few mL of distilled water. ConcentratedSilver nitrate solution: nitric acid (0.1 (see mol safety L ). If notes possible,): (6 mol dry L5-1 g) of 4. Cool the solution and filter it. Wash the solid residue AgNO for 2 hours at 100°C and allow to cool. Accurately 5. Make the filtrate up to 500 mL in a volumetric Silver 3nitrate solution: (0.1 mol L−1). If possible, dry 5 g of with a few mL of distilled water. weigh about 4.25 g of solid AgNO and dissolve it in flask. AgNO for 2 hours at 100°C and allow3 to cool. Accurately 5. Make the filtrate up to 500 mL in a volumetric flask. 250 mL3 of distilled water in a conical flask. Store the weigh about 4.25 g of solid AgNO and dissolve it in 250 solution in a brown bottle. 3 Titration mL of distilled water in a conical flask. Store the solution −1 1. Use a volumetric cylinder to measure 100 mL of the inPotassium a brown bottle.thiocyanate solution: (0.1 mol L ). Weigh 1. Use a volumetric cylinder to measure 100 mL of the 2.43 g of solid KSCN and dissolve it in 250 mL of cheese extract solution (be as precise as possible) and −1 cheese extract solution (be as precise as possible) Potassiumdistilled water thiocyanate in a volumetric solution: flask. (0.1 mol L ). Weigh pourand it pourinto ait conical into a conicalflask. flask. 2.43 g of solid KSCN and dissolve it in 250 mL of distilled 2. Add 1 mL of saturated ferric ammonium sulfate waterPotassium in a volumetric permanganate flask. solution: (5%) Add 1.5 g 2. Add 1 mL of saturated ferric ammonium sulfate solution as indicator. KMnO4 to 30 mL of distilled water. solution as indicator. Potassium permanganate solution: (5%) Add 1.5 g Ferric ammonium sulfate solution: (saturated) Add 8g 3. TitrateTitrate the the unreacted unreacted silver silver ions ions with with the the 0.1 0.1 mol mol L−1 KMnO4 to 30 mL of distilled water. −1 of NH4Fe(SO4)2.12H2O to 20 mL of distilled water and L potassium thiocyanate solution. The endend pointpoint isis thethe Ferricadd a ammoniumfew drops of sulfateconcentrated solution: nitric (saturated) acid (see Add safety 8 g firstfirst appearance appearance of a ofdark a dark red redcolour colour due dueto the to theferric ferric ofnotes). NH4Fe(SO4)2.12H2O to 20 mL of distilled water and add thiocyanatethiocyanate complex complex (figure (figure 1). 1). a few drops of concentrated nitric acid. 4. RepeatRepeat the the titration titration with with 100 100 mL mL samples samples of of the the Method cheesecheese extract extract solution solution until until you you obtain obtain concordant concordant Method resultsresults (titres (titres agreeing agreeing within within 0.1 0.1mL). mL). Sample Preparation SampleThe salt sodiumPreparation chloride is added during the Themanufacture salt sodium of cheddarchloride ischeese. added In during this method, the manufacturethe cheese is of‘digested’ cheddar to cheese. release In this this salt method, to obtain the cheeseconcentration is ‘digested’ of chloride to release ions. this To saltcarry to out obtain this thedigestion, concentration the cheese of chloride is reacted ions. with To nitric carry acidout this and digestion,potassium the permanganate. cheese is reacted The chloridewith nitric ions acid are and then potassium‘free’ to form permanganate. a precipitate with The chloridethe added ions silver are ions.then ‘free’1. Cut to or form grate a theprecipitate cheese intowith finethe added pieces silver and ions. 1. accuratelyCut or grate weigh the about cheese 6 ginto into fine a 500 pieces mL andconical accuratelyflask. weigh about 6 g into a 500 mL conical flask. 2. Precisely add 50 mL of 0.1 mol L−1 silver nitrate 2. Precisely add 50 mL of 0.1 mol L−1 silver nitrate solution (by pipette if possible), 20 mL of solution (by pipette if possible), 20 mL of concentrated concentrated nitric acid, (very carefully – see nitric acid, (very carefully – see safety notes), 100 mL − safety notes), 100 mL of distilled water and a few Figure 1 Left flask: before the titration endpoint, addition of SCN of distilled water and a few boiling chips, and heat the Figureions leads 1 Left to formation flask: before of silver the thiocyanate titration endpoint, precipitate, addition making theof boiling chips, and heat the solution to boiling in a − solution to boiling. SCNsolution ions cloudy. leads Here to formation the solution of also silver takes thiocyanate a faint yellow precipitate, colour due fumehood. makingto the colour the solutionof the cheese cloudy. extract. Here Centre the solutionflask: at the also endpoint takes a all the 3. As Asthe the solution solution boils boils add add 5 mL5 mL of of 5% 5% potassium potassium faintfree silver yellow ions colour have been due precipitatedto the colour by ofSCN the−. The cheese slightest extract. excess of permanganatepermanganate solution. solution. This This addition addition will will cause cause a very a CentreSCN− forms flask: a dark at thered colouredendpoint complex all the withfree thesilver Fe 3+ions ions havefrom thebeen smelly reaction and should be done in a fumehood. precipitatedferric ammonium by SCN sulfate−. The indicator, slightest giving excess the solutionof SCN− aforms slight aorange/ very smelly reaction so done in the fumehood. Keep − Keep boiling until the purple colour disappears, then darkred colouration. red coloured Right complex flask: If additionwith the of Fe SCN3+ ions is continued from the pastferric the boiling until the purple colour disappears, then add endpoint, further ferric thiocyanate complex is formed and a stronger add another 5 mL of potassium permanganate solution.