College of Science

Determination of Concentration by (Volhard’s Method) Safety neutral or basic, Mohr’s method or the gravimetric 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) solution causes of skin and in cheese. fabric (chemical burns). Any spills should be rinsed with water immediately. Equipment Needed Concentrated 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 of Chloride Ion Concentration to determine the concentration of chloride Bunsen burner, tripod and gauze in a solution. Beforeby the Titration titration an excess (Volhard’s volume buretteMethod) and stand of a solution is added to the solution 50 mL pipette (if possible) containing chloride ions, forming a precipitate of . 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 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 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 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 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. Titrate Titrate 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. Repeat Repeat 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. accurately Cut 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. ammonium sulfate indicator, giving the solution a slight another 5 mL of potassium permanganate solution. orange/reddark red colour colouration. results. NB: Right The titration flask: shouldIf addition be stopped of SCN when− is the ContinueContinue this this process process until until 30 30mL mL of potassiumof potassium continuedfirst trace of past dark thered colourendpoint, is observed. further Using ferric an thiocyanate incompletely complex titrated permanganatepermanganate solution solution has has been been added added and and the the isreference formed flask and for a stronger comparison dark is ared helpful colour way results. to identify NB: the The first cheesecheese particles particles are are completely completely digested digested (or (oras closeas close titrationappearance should of red becolouration. stopped when the first trace of dark red as aspossible). possible). To Tofind find out outwhen when digestion digestion is complete, is complete, colour is observed. Using an incompletely titrated reference removeremove the the flask flask from from heat heat and andallow allow it to it stand to stand for afor a flask for comparison is a helpful way to identify the first fewfew moments. moments. Undigested Undigested cheese cheese particles particles will will float float appearance of red colouration. uponupon the the surface surface of theof the clear clear liquid, liquid, while while the the white white precipitateprecipitate of silverof silver chloride chloride will will sink sink to theto the bottom. bottom. If If therethere is stillis still too too much much undigested undigested cheese, cheese, the the boiling boiling and addition of 5 mL of potassium permanganate should be continued, checking each time until there is a satisfactory level of digestion. 2 Result Calculations Contact Us 1. Determine the average volume of potassium If you have any questions or comments relating to this thiocyanate used from your concordant titres. experiment, please contact us. Please note that this 2. Calculate the moles of potassium thiocyanate used. service is for senior school chemistry students in New Zealand only. We regret we are unable to respond 3. Use the equation of the reaction between silver ions to queries from overseas. and thiocyanate ions Outreach + –( Ag (aq) + SCN aq) → AgSCN(s) College of Science to calculate the moles of unreacted silver nitrate in University of Canterbury 100 mL of cheese extract, and multiply the figure by Private Bag 4800 five to determine the total moles of unreacted silver Christchurch nitrate (the excess) in the 500 mL volumetric flask. New Zealand Phone: +64 3 364 2178 4. Calculate the moles of silver nitrate in the 50 mL Fax: +64 3 364 2490 of solution that was added during the sample Email: [email protected] preparation to the cheese. www.outreach.canterbury.ac.nz 5. Calculate the total moles of silver nitrate that reacted with the salt from the cheese by subtracting the moles of unreacted silver nitrate (the excess) from the total moles of silver nitrate added to the cheese. 6. Use the equation of the reaction between the silver ions and the chloride ions to calculate the moles of sodium chloride in the sample of cheese.

+ – Ag (aq) + Cl (aq) → AgCl(s) 7. Calculate the concentration of sodium chloride in the cheese as grams of salt per 100 g cheese (% salt).

Additional Notes 1. Residues containing silver ions and precipitate are usually saved for later recovery of silver metal. Check this with your teacher or the laboratory supervisor. 2. A ‘blank’ titration substituting sucrose (sugar) for the cheese should be carried out to see if there are any ‘contaminating’ chloride ions present in the reagent solutions used. If any are found, the figure should be subtracted from the titration results. 3. For greatest accuracy it is a good idea to standardise your thiocyanate solution by titrating several samples against your standardised silver nitrate solution (once again using ferric ammonium sulfate indicator). The concentration of SCN– determined by this titration should then be used in all calculations.