PRECIPITATON TITRATION

Transcription

1 PRECIPITATON TITRATION

2 Precipitation titration is a special type of titrimetric procedure, which involves the formation of precipitates during the course of titration. The titrant react with the analyte forming an insoluble material and the titration continues till the very last amount of analyte is consumed. The first drop of titrant in excess will react with an indicator resulting in a colour change and announcing the termination of the titration.

3 PRECIPITATION TITRATION CAN BE CLASSIFIED ACCORDING TO THE TITRANTS: Argentometry titrant AgNO, Thiocyanatometry titrant NH 4 SCN, or (KSCN, NaSCN), MercurOmetry titrant Hg (NO ), Sulfatometry titrant H SO 4, (or Na SO 4 ) Hexacyanoferatometry titrant K 4 [Fe(CN) 6 ].

4 Requirements to chemical reaction used in a precipitation titration: 1. Reaction should be fast and the stoichiometry should be known and reproducible. No secondary reactions of interference.. The reaction should be complete. In general a precipitation titration is considered complete when the precipitate is hard soluble with Ksp <10-8. It should not be formed a supersaturated solution. 4. It should be an indicator to find the equivalence point which, for this type of titration, corresponds to when precipitation of the analyte is complete. 5. Phenomenon of co-precipitation should be avoided.

5 TITRATION CURVE Titration curves for precipitation reactions are derived in a completely analogous way to the methods described for titrations involving strong acids and strong bases. P-functions are derived for the preequivalence-point region, the postequivalence point region, and the equivalence point for a typical precipitation titraton. When calculating a precipitation titration curve, we can choose to follow the change in the titrant s concentration or the change in the analyte s concentration.

6 TITRATION CURVE Let s calculate the titration curve for the titration of ml of M NaCl with M AgNO. The reaction in this case is. AgNO NaCl AgCl NaNO AgNO Most indicators for argentometric titrations respond to changes in the concentration of silver or chloride ions. As a consequence, titraton curves for precipitation reactions usually consist of a plot of pag (or pcl) versus volume of AgNO. Before the titration NaCl pcl 1 c ( Cl ) 1 10 mol / l lgc( Cl ) lg

7 1 Before the equivalence point the analyte, NaCL, is in excess. We will calculate pcl before the equivalence point by determining the concentration of unreacted NaCl. The concentration of unreacted Cl after adding 90.0 ml of Ag+, for example, c0( Cl ) Vrest ( NaCl ) 0,1 10 c( Cl ) 5 10 mol / l V ( total) 190 pcl lgc( Cl ) lg5 10,

8 The concentration of unreacted Cl after adding 99.0 ml of Ag +, c0( Cl ) Vrest ( NaCl ) 0,1 1 4 c( Cl ) 5 10 mol / l V ( total) 199 pcl lgc( Cl ) lg5 10 4, 4 The concentration of unreacted Cl after adding 99.9 ml of Ag +, c0( Cl ) Vrest ( NaCl ) 0,1 0,1 5 c( Cl ) 5 10 mol / l V ( total) 199,9 pcl lgc( Cl ) lg ,

9 4 At the titration s equivalence point we know that the concentrations of Ag + and Cl are equal. To calculate the concentration of Cl we use the K sp expression for AgCl; thus 10 5 c( Cl ) c( Ag ) KSP mol / l pcl pag lg10 5 5

10 After the equivalence point, the titrant is in excess. We first calculate the concentration of excess Ag + and then use the K sp expression to calculate the concentration of Cl. 5 For example, after adding 100,10 ml of titrant : c0( Ag ) Vrest ( AgNO ) 0,1 0,1 5 c( Ag ) 5 10 mol / l V ( total) 00,1 pag lgc( Ag ) lg , pcl 10 pag 10 4, 5,7

11 6 After adding 101 ml of titrant : c0( Ag ) Vrest ( AgNO ) 0,1 1 4 c( Ag ) 5 10 mol / l V ( total) 01 pag lgc( Ag ) lg5 10 4, pcl 10 pag 10, 6,7 7 After adding 110 ml of titrant : c0( Ag ) Vrest ( AgNO ) 0,1 10 c( Ag ) 5 10 mol / l V ( total) 10 pag lgc( Ag ) lg5 10, pcl 10 pag 10, 7,7

12 Titration curve for the titration of ml of M NaCl with M AgNO. Black line a precipitation titration curve, calculated by the change in the AgNO concentration; Red line - a precipitation titration curve, calculated by the change in the NaCl concentration. V( AgNO) pcl pag , ,

13 Conclusions 1. As the concentrations of analyte and titrant decrease, the end point break decreases.the titration curve brake depends on K sp of the precipitate. As K sp becomes larger, the inflection near the equivalence point decreases, until the equivalence point becomes too shallow to detect. It is not practical to titrate an analyte when it forms too soluble precipitate (K sp >10-10 ).The titration curve brake depends on temperature also, and increases with the temperature decreasing.

14

15 ARENTOMETRY titrant AgNO

16 Precipitation titrimetry, which is based on reactions that yield ionic compounds of limited solubility, is one of the oldest analytical techniques. The slow rate of formation of most precipitates, however, limits the number of precipitating agents that can be used in titrations to a handful. The most widely used and important precipitating reagent, silver nitrate, which is used for the determination of the halogens, the halogen-like anions. Titrations with silver nitrate are sometimes called argentometric titrations.

17 Argentometry is the most usefull method of precipitatometric titration, as it caused of very low solubility product of halide (or pseudohalide) salts,i.e. K sp AgCl = 1, K sp AgCNS = 1, K sp AgBr = 5, K sp AgCN =, There are chemical techniques of end point determination method of Mohr (indicator : chromic potassium) method of Fajans (indicator : fluorosceince, eosin)

18 The Mohr method The Mohr method was first published in 1855 by Karl Friedrich Mohr. Potassium chromate (K CrO 4 ) can serve as an indicator for the direct argentometric determination of chloride, bromide, cyanide and thiocyanide ions by reacting with silver ion to form a reddish-brown silver chromate (Ag CrO 4 ) precipitate in the equivalence-point region. Mohr method is not feasible for I - as of the salt precipitate adsorption. Chromate is yellow in neutral and alkaline conditions and at endpoint is more soluble than silver halide. It will only form after all the analyte has been precipitated.

19 Equivalence point detection The reactions involved in the direct argentometric determination of chloride, bromide, thiocyanide (X - ) are Ag X white AgX precipitate, where Х = Cl -, Br -, CN-, SCN -. indicator K CrO 4 ( or Na CrO 4 ), Ag CrO4 Ag CrO4 reddish brown precipitate The change of precipitate colour occurs at the end point!!!!!

20 Conditions of argentometric titration (to overcome errors) 1) It is very important the order of the titration. Always to an analyzed solution (NaCl, KCl, KBr etc) titrant AgNO should be added, and not vice versa. Mohr s method cannot be used to titrate silver with a standard chloride solution. This is not possible in since chromate added to a silver solution would immediately cause a reddish-brown precipitate. ) Mohr titration has to be performed at a neutral or weak basic solution of ph 7-8. Above this ph, silver will form a precipitate with hydroxide. Below this ph chromate converts to dichromate, a bright orange colour thereby obscuring the endpoint. ) Cations which also can be precipitated with CrO 4 - (Ba +, Pb +, Bi +, Hg + ) and anions precipitated by Ag + (PO 4 -, AsO 4 -, C O 4 - etc.) must be absent in the analyzed solution.

21 Preparation of titrant AgNO Silver nitrate is available in pure form, but it is hygroscopic and therefore unable to be used as a primary standard. It can be standardized against dried sodium chloride. c AgNO NaCl AgCl NaNO n( AgNO ) n( NaCl ) ( AgNO ) V( AgNO ) c( NaCl ) V( NaCl ) Solution of silver nitrate should be protected from unnecessary exposure to light: a dark coloured bottle is recommended. Concentration of silver nitrate solution changes over time.

22 The Fajans method This method uses an adsorption indicator such of fluorescein and eosin. Fluorescein Eosin

23 An adsorption indicator is an organic compound that tends to be adsorbed onto the surface of the solid in a precipitation titration. Ideally, the adsorption occurs near the equivalence point and causes a change in the colour of the indicator.

24 AgNO Example: Consider a titration of KBr with AgNO titrant in the presence of eosin (НE). Before the end point KBr AgNO added to a dilute solution of KBr, the solution becomes turbid and if other electrolytes are absent, coagulation does not occur immediately AgNO KBr AgBr KNO H + K + Br - Ag + NO - E -

25 The colloidal sized AgBr particles adsorb Br - (because KBr in excess before the end point) and these attract silver ions. Colloidal particles are electrically charged and repel each other preventing coagulation. As titration continues, the amount of bromide decreases but there is still some surface charge which acts to repel the negatively charged indicator ion (E - ). AgBr Br,Ag AgBr Br Ag

26 After the end point AgBr NO, Ag AgBr Ag NO AgBr Ag NO E AgBr Ag E NO eosinium ion red violet colour Immediately after endpoint there is an excess of silver ions which will adsorb onto the surface of the precipitate. The charged surface has now changed polarity and attracts the negative indicator ion. A colour change will be observed.

27 Limitations of Fajan s method 1) Fluorescein can be used for titration of Cl -, Br -, I -, SCN -. Eosin only for I -, Br -, SCN -, no chlorides can be alalysed ) The ph must be well controlled for maintaining indication ionic concentration, in basic or acidic solution. For example, fluorescein (Ka =10-7 ) in a solution having ph > 7 will release small quantity of fluoreseinate ion, hence we can not observe colour change of the indicator. Fluorescein is used feasibly in a solution of ph 7-10, eosin (Ka=10-4 ) is at ph -7. ) Titration should be performed at steering of the solution.

28 THIOCYANATOMETRY (Volhards Method) titrant NH 4 SCN

29 Thiocyanatometric method uses a direct titration with ammonium thiocyanate for the determination of silver and mercury(i) salts. The titration uses iron alum NH 4 Fe(SO 4 ) 1H O as the indicator. AgNO NH 4SCN AgSCN NH 4NO white Fe n nscn ( 4 [ Fe( SCN ) n red solution AgNO ) n( NH SCN ) ] n At the equivalent point The iron alum indicator works by forming the coloured complex when an excess of thiocyanate occurs. The solution must be acidic, with a concentration of about 1 M nitric acid to ensure the complex formed is stable. The change of solution colour occurs at the end point!!!!!

30 Determination of mercury(i) salts Hg NO ) NH 4SCN Hg ( SCN ) ( NH NO 4 Fe nscn [ Fe( SCN ) n ] n n ( 4 1 Hg ( NO ) ) n( NH SCN )

31 Determination of halides using back titrarion This method also uses a back titration with ammonium thiocyanate for the determination of chlorides, bromides and iodides in acidic solutions. A known excess of silver nitrate solution is added to the sample and the excess is back titrated with standard thiocyanate solution. Determination of KBr KBr AgNO ( excess) AgBr KNO AgNO ( leftover) AgNO leftover) ( NH SCN AgSCN NH NO 4 4 Fe nscn [ Fe( SCN ) n ] n n( KBr ) n( AgNO ) n( NH 4SCN ) In a similar way KI and NaI can be determined

32 Determination of BaCl BaCl AgNO ( excess) AgCl Ba( NO ) AgNO ( leftover) AgNO leftover) ( NH SCN AgSCN NH NO 4 4 Fe nscn [ Fe( SCN ) n ] n n ( 4 1 BaCl ) n( AgNO ) n( NH SCN )

33 Determination of chloroform using indirect titration CHCl 4KOH KCl HCOOK HO KCl AgNO ( excess) AgBr KNO AgNO ( leftover) AgNO leftover) ( NH SCN AgSCN NH NO 4 4 Fe nscn [ Fe( SCN ) n ] n n ( 4 1 CHCl ) n( AgNO ) n( NH SCN )

34 Preparation of titrant NH 4 SCN Ammonium thiocyanate is unable to be used as a primary standard. Thiocyanate is standardized against a secondary standard silver solution, with the silver solution being in the titration flask and the thiocyanate in the burette. c AgNO NH 4SCN AgSCN NH 4NO n ( 4 AgNO ) n( NH SCN ) ( 4 AgNO ) V( AgNO ) c( NH 4SCN ) V( NH SCN )

35 MERCUROMETRY (titrant Hg (NO ) )

36 When halide ions are titrated with mercury(i) nitrate solution, precipitation of Hg X occurs during the titration process. After equivalent point, excess of Hg +, react with indicator. Hg X Hg X whrer Х = Cl -, Br -. ½Hg (NO ) equivalent formula There are indicators for end point determination: [Fe(SCN)]Cl diphenylcarbazone

37 1 Application of [Fe(SCN)]Cl as indicator. In this case excess of titrant leads to decomposition of [Fe(SCN)]Cl and red colour of the solution turns yellow. Hg [ Fe( SCN )] Hg ( SCN ) Fe

38 Application of diphenylcarbazone as indicator. In this case excess of Hg + reacts with diphenylcarbazone, oxidizes to Hg +, and forms complex compound of blue violet colour. Hg + + O = C NH NH C 6 H 5 +H + Hg + + O = C NH NH C 6 H 5 N = N C 6 H 5 diphenylcarbazone NH NH C 6 H 5 Hg + + O = C NH NH C 6 H 5 N = N C 6 H 5 - H + Hg N C 6 H 5 NH O = C N = N C 6 H 5 рн = 0 (HNO ) Blue violet colour

39 Preparation of titrant Hg (NO ) Mercury (I) nitrate is not a primary standard. It should be standardized against dried sodium chloride. Hg NO ) NaCl Hg Cl ( NaNO n ( 1 Hg ( NO ) ) n( NaCl ) Mercury (I) nitrate is toxic but less expensive then silver nitrate.

40 Advantages of mercurometry: 1. Not required expensive silver salts.. Mercury(I) precipitates are less soluble than analogous silver salts. Therefore, equivalence point is clearly marked.. Mercurometric determination are carried out with direct titration in acidic solution. 4. Chloride ion can be determined with reducing agents (S, SO ) and oxidising agents (MnO 4, Cr O 7 ) presence. 5. Titration can be carried out in turbid and coloured solutions.

41 SULPHATOMETRY titrant H SO 4 (Na SO 4 )

42 Determination of BaCl Ba HInd free indicator colour BaInd colour of complex H Before end point BaInd SO 4 H BaSO4 HInd After end point n( 1 BaCl ) n( 1 SO H 4 )

43 HEXACYANOFERATOMETRY titrant K 4 [Fe(CN) 6 ]

44 Zinc sulfate solution is titrated directly with potassium ferrocyanide, using mixture of diphenylamine and K [Fe(CN) 6 ] as an indicator, equivalent point can entirely be found owing to the appearance of blue colour. 4 Zn [ Fe( CN ) 6] K ZnK [ Fe( CN ) 6] green Redox Indicator K [Fe(CN) 6 ] + (C 6 H 5 ) NH) diphenylamine n( 8 Zn ) n( 1 4 K 4[ Fe( CN ) 6 ]) This method can be attributed to the precipitation as well as redox titration