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Determination of Chloride Ion Concentration by Titration (Volhard’s Method) Safety Lab coats, safety glasses and enclosed footwear must be worn at all times in the laboratory. Silver nitrate solution causes staining of skin and fabric (chemical burns). Any spills should be rinsed with water immediately. Concentrated nitric acid is very corrosive: take great care using the 6molL-1 solution.
neutral or basic, Mohr’s method or the gravimetric method should be used. The method is illustrated below by using the procedure to determine the concentration of chloride (from sodium chloride) in cheese.
Equipment Needed boiling chips 500 mL volumetric flask 10 mL and 100 mL measuring cylinders
Introduction
conical flasks This method uses a back titration with potassium thiocyanate to determine the concentration of chloride Bunsen burner, tripod and gauze ions in a solution. Before the titration an excess volume burette 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‘ is used as the moles of silver Introduction nitrate added are known exceed moles ofwith sodium Thisto method usesthe a back titration potassium to determine chloride present in the thiocyanate sample so that all the theconcentration 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
Determination of Chloride Ion Concentration by Titration (Volhard’s Method)
solution
containing chloride ions, forming a precipitate of silver
Ag+(aq) +chloride. Cl–(aq)The→term AgCl ‘excess‘ (s) is used as the moles of silver
nitrate added are known to exceed the moles of sodium
The indicator Fe3+ (ferricchloride ion) ispresent theninadded and theall the chloride the sample so that ions present will react. solution is titrated with the potassium thiocyanate Ag+(aq) + Cl → excess AgCl(s) solution. The titrate remains pale yellow as–(aq)the 3+ The indicator ion) is then added (unreacted) silver ions react withFethe(ferric thiocyanate ionsand the solutionprecipitate. is titrated with the potassium thiocyanate to form a silver thiocyanate
Ag
+ (aq)
+
solution. The titrate remains pale yellow as the excess (unreacted) silver ions react with the thiocyanate ions to – SCN → AgSCNprecipitate. form a(aq) silver thiocyanate (s)
– Once all the silver ions have reacted, slightest Ag+(aq) the + SCN → AgSCN(s) (aq) 3+ excess of thiocyanate reacts Feionstohave form a dark red Once allwith the silver reacted, the slightest 3+ to form a dark red excess of thiocyanate reacts with Fe complex.
complex.
+ SCN 2+ → [FeSCN] (aq) (aq) Fe3+(aq) + SCN–(aq) →Fe[FeSCN] (aq) 3+
–
2+ (aq)
The concentration of chloride ions is determined by
The concentration of chloride ions is determined bymoles of silver subtracting the titration findings of the ions that reacted with the thiocyanate from the total subtracting the titration findings of the moles of silver 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 added tohas thebeen solution. the sample prepared is acidic. If the pH is neutral or basic, Mohr’s method or the gravimetric This method is used when the pH of the solution, after method should be used. The method is illustrated below the sample has been prepared, acidic.toIfdetermine the pH the is concentration by using theisprocedure of chloride (from sodium chloride) in cheese.
Equipment Needed boiling chips
Conical flask
Cheese extract solution
Solutions Needed ConcentratedNeeded nitric acid (see safety notes): (6 mol L-1) Solutions Silver nitrate solution: mol L−1). Ifnotes): possible, dry L5-1g) of Concentrated nitric acid(0.1 (see safety (6 mol AgNO3 for 2 hours at 100°C and allow to cool. Accurately −1 Silver nitrate solution: (0.1 mol L ). If possible, 5 g of weigh about 4.25 g of solid AgNO3 and dissolvedry it in for 2 hours at 100°C and allow to cool. Accurately AgNO 3 of distilled water in a conical flask. Store the 250 mL weigh about g ofbottle. solid AgNO3 and dissolve it in 250 solution in a 4.25 brown mL of distilled water in a conical flask. Store−1the solution Potassium thiocyanate solution: (0.1 mol L ). Weigh in a brown bottle. 2.43 g of solid KSCN and dissolve it in 250 mL of Potassium thiocyanate solution: (0.1 mol L−1). Weigh distilled water in a volumetric flask. 2.43 g of solid KSCN and dissolve it in 250 mL of distilled Potassium permanganate solution: (5%) Add 1.5 g water in a volumetric flask. KMnO4 to 30 mL of distilled water. Potassium permanganate solution: (saturated) (5%) Add 1.5Add g 8g Ferric ammonium sulfate solution: to 30 mL of distilled water. KMnO of NH 4Fe(SO ) .12H2O to 20 mL of distilled water and 4
42
add a ammonium few drops of sulfate concentrated nitric acid (seeAdd safety Ferric solution: (saturated) 8g notes). of NH4Fe(SO4)2.12H2O to 20 mL of distilled water and add a few drops of concentrated nitric acid.
Method Method Sample Preparation
and addition of 5 mL of potassium permanganate should be continued, checking each time until there 4. Cool the solution and filter it. Wash the solid is a satisfactory level of digestion. residue with a few mL of distilled water. 4. Cool the solution and filter it. Wash the solid residue 5. with Make themL filtrate up to 500 mL in a volumetric a few of distilled water. flask. 5. Make the filtrate up to 500 mL in a volumetric flask.
Titration Titration 1. UseUse a volumetric cylinder measure the 1. a volumetric cylinder toto measure 100100 mLmL ofof the cheese extract solution (be as precise as possible) and cheese extract solution (be as precise as possible) pour into aitconical flask. flask. andit pour into a conical 2. Add Add 1 mL saturated ferric ammonium sulfate 2. 1 mL of of saturated ferric ammonium sulfate solution as indicator. solution as indicator. 3. Titrate Titrate unreacted silver ions with mol 3. thethe unreacted silver ions with thethe 0.10.1 mol L−1 L−1 potassium thiocyanate solution. The end point point isisthe the first appearance of aofdark redred colour duedue to the ferric first appearance a dark colour to the ferric thiocyanate complex (figure 1). 1). thiocyanate complex (figure 4. Repeat thethe titration with 100100 mLmL samples ofof thethe 4. Repeat titration with samples cheese extract solution until obtain concordant cheese extract solution until youyou obtain concordant results (titres agreeing within 0.1 mL). results (titres agreeing within 0.1 mL).
The salt sodium chloride is added during the Sample Preparation
manufacture of cheddar cheese. this method, The salt sodium chloride is addedInduring the the cheese is ‘digested’ to release this to obtain manufacture of cheddar cheese. In thissalt method, the cheese concentration of chloride ions.this To salt carrytoout this the is ‘digested’ to release obtain digestion, the cheese is reacted with nitric acid and the concentration of chloride ions. To carry out this potassiumthe permanganate. The chloride ions areand then digestion, cheese is reacted with nitric acid ‘free’ to form a precipitate with the added silver ions. potassium permanganate. The chloride ions are then 1. Cuttoorform gratea the cheese into pieces silver and ions. ‘free’ precipitate withfine the added accurately weigh about 6 g into a 500 mL conical 1. Cut or grate the cheese into fine pieces and flask. accurately weigh about 6 g into a 500 mL conical flask. 2. Precisely add 50 mL of 0.1 mol L−1 silver nitrate −1 silver 2. solution Precisely add 50 mL of 0.1 mol L (by pipette if possible), 20 mL ofnitrate solution (by pipette if possible), mL of concentrated concentrated nitric acid, (very20carefully – see nitric acid, (very carefully – see safety notes), mL safety notes), 100 mL of distilled water and100 a few of distilled water and a few boiling chips, and heat boiling chips, and heat the solution to boiling inthe a solution to boiling. fumehood. 3. solution boils add 5 mL potassium 3. AsAs thethe solution boils add 5 mL ofof 5%5% potassium permanganate solution. This addition will cause a very permanganate solution. This addition will cause a smelly and should be done in fumehood. a fumehood.Keep very reaction smelly reaction so done in the Keep boiling until purple colour disappears, then boiling until thethe purple colour disappears, then add addanother another5 5mL mLofofpotassium potassiumpermanganate permanganatesolution. solution. Continue thisthis process until 30 30 mLmL of potassium Continue process until of potassium permanganate solution hashas been added andand thethe permanganate solution been added cheese particles are completely digested (or as cheese particles are completely digested (or close as close as as possible). To To find outout when digestion is complete, possible). find when digestion is complete, remove thethe flask from heat andand allow it toit stand for for a a remove flask from heat allow to stand fewfew moments. Undigested cheese particles willwill float moments. Undigested cheese particles float upon thethe surface of the clear liquid, while thethe white upon surface of the clear liquid, while white precipitate of silver chloride willwill sinksink to the bottom. If If precipitate of silver chloride to the bottom. there is still tootoo much undigested cheese, thethe boiling there is still much undigested cheese, boiling and addition of 5 mL of potassium permanganate should be continued, checking each time until there is a satisfactory level of digestion.
Figure 1 Left flask: before the titration endpoint, addition of SCN−
Figure 1 Left flask: before the titration endpoint, addition of ions −leads to formation of silver thiocyanate precipitate, making the SCN ions leadsHere to formation ofalso silver thiocyanate precipitate, solution cloudy. the solution takes a faint yellow colour due making the solution cloudy. Here the solution also takes aall the to the colour of the cheese extract. Centre flask: at the endpoint − cheese extract. faint yellow due precipitated to the colour the . The slightest excess of free silver ionscolour have been by of SCN − 3+ Centre flask: at the all the free a dark redendpoint coloured complex withsilver the Feions ionshave frombeen the SCN forms − precipitated by SCN . The slightest excess of SCN− aforms ferric ammonium sulfate indicator, giving the solution slightaorange/ 3+ − is continued red colouration. Right flask: If addition dark red coloured complex with theofFeSCN ions from thepast ferricthe endpoint, further ferricindicator, thiocyanate complex formed and a stronger ammonium sulfate giving the is solution a slight − dark red colour results. NB:Right The titration be stopped orange/red colouration. flask: Ifshould addition of SCNwhen is the first trace ofpast dark the red colour is observed. incompletelycomplex titrated continued endpoint, furtherUsing ferrican thiocyanate reference flask for comparison is a helpful way to identify the first is formed and a stronger dark red colour results. NB: The appearance of red be colouration. titration should stopped when the first trace of dark red colour is observed. Using an incompletely titrated reference flask for comparison is a helpful way to identify the first appearance of red colouration.
Result Calculations
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1. Determine the average volume of potassium thiocyanate used from your concordant titres.
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2. Calculate the moles of potassium thiocyanate used. 3. Use the equation of the reaction between silver ions and thiocyanate ions
Ag
+ (aq)
+ SCN
–( aq)
→ AgSCN(s)
to calculate the moles of unreacted silver nitrate in 100 mL of cheese extract, and multiply the figure by five to determine the total moles of unreacted silver nitrate (the excess) in the 500 mL volumetric flask. 4. Calculate the moles of silver nitrate in the 50 mL of solution that was added during the sample preparation to the cheese. 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.
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