1 | ANALYTICAL CHEMISTR CHEMISTRY Y deals ls wit with h the Gravimetric Gravimet ric Meth Method od of Anal Analysis ysis – dea measurement of th the e ma mass ss of a su subs bsta tanc nce e th tha at is chemically related to the analyte. Basic Steps on Precipitation Method 1. Sampl Sample e is dissolved dissolved in an an appropri appropriate ate solvent. solvent. 2. A precipita precipitant nt is used used to convert convert the analyt analyte e into a sparingly soluble precipitate. 3. Th The e pr prec ecip ipit itate ate is co conv nver erte ted d in into to a pr prod oduc uctt of known composition by a suitable heat treatment. . Th The e pe perc rcen enta tage ge of th the e an anal alyte yte in the sa samp mple le is calculated using the gravimetric factor !"#
%$%analyte ∈the sample=
GF =
wt . of precip precipita itate× te× GF × 100 wt .of sample sample
molarmass of analyte analyte xanalyte × molar massof precipita precipitate te y precipitate
found to weigh 2(.*' mg. alculate the 4a7r in the sample. a. ' b. '' c. ( d. '1 ).)&9g 9g sample containing containing chlor chloride ide and iodi iodide de Ex. A ).)& ions gave a silver halide precipitate weighing ).3) g. This precipitate was then strongly heated in a stream of l2 gas to convert the Ag: to Agl; on completion of this treatment< the precipitate weighed ).31*1 g. alculate the percentage of chloride and iodide in the sample. Ans. 4.72% Cl- and 27.05%% I -. meas asur ures es th the e Volm olmetri etric c Meth Methods ods of Anal Analysis ysis – me volume of solution necessary to react completely with the analyte Standard Solution – solution of known concentration Standardization – process of determining concentration of an unknown solution Primary standard – a substance of high purity
the
Ex. The aluminum in a &'(.)* mg of impure aluminum sulfate sample was precipitated as Al!+,$ 3 and ignited at 11))- to yield a precipitate of Al 2+3 weighing 3*&.('3 mg. /0press the result of analysis in terms of Al. a. 2&.)' b. 13.'3 c. 1*.& d. 23.2(
Characteristics Characteristic s of a Good Priar! "tandard
hatt wei weight ght of an impure impure 4a 4al l sample sample mus mustt be Ex. ha taken for analysis so that the weight of Agl precipitate obtained in mg will be e5ual to the l in the sample6 a. 1(.& mg b. 12. mg c. 2.&3 mg d. .) mg
Conditions for #ol$et #ol$etric ric Anal!sis 1. The reaction reaction must be rapid and can be represented represented by a simple balanced e5uation. 2. Th The e re reac acti tion on is co comp mple lete te an and d no si side de re reac acti tion on occurs. 3. An appropriate appropriate indicator indicator must be available available in orde orderr to detect the end point of the reaction
sample ple containi containing ng 4a7 4a7rr and 87r onl only y wei weighs ghs Ex. A sam 2'3.)2 2'3 .)2 mg. The sample sample was dissolve dissolved d in wat water er and treated with e0cess Ag4+3. The precipitate formed was
7y% >A?@2))'1)2*
1. 2. 3. .
,igh purity purity and high e5uivalent e5uivalent weight Stable towards air< air< high temperature and humidity Soluble Solub le in water =eadily available and fairly ine0pensive
Ty!es of Titration
2 | ANALYTICAL CHEMISTRY 1. Direct Titration – the analyte reacts with the standard solution directly 2. Back Titration – an e0cess standard solution is added and the e0cess is determined by the addition of another standard solution 3. Replacement Titration – the analyte is converted to a product chemically related to it and the product of such reaction is titrated with a standard solution
CA""I&ICA'I() (& #(*+E',IC +E'(" There are four general classes of volumetric or titrimetric methods.
Acid"#ase. any compounds< both inorganic and organic< are either acids or bases and can be titrated with a standard solution of a strong base or a strong acid. The end points of these titrations are easy to detect< either by means of an indicator of by following the change in p, with a p, meter. The acidity and basicity of many inorganic acids and bases can be enhanced by titrating in a nonaqueous solvent. The result is a sharper end point< and weaker acids and bases can be titrated in this manner.
Acid-/ase 'itration oncepts of Acids and 7ases B/:S A:C /lectron9p air acceptor 7AS/
/lectron9pair donor
→ , +F F l9 3 !a5$ !a5$
4a+, F , 2+
→ 4aF F ,+9 !a5$ !a5$
'eak %cids&Bases – partially ioniEed in solution ,# F ,2+ 4,3 F ,2+
↔ , +F F #9 3 !a5$ !a5$
↔ 4, F F ,+9 !a5$ !a5$
%utoprotolysis o( 'ater ,2+ F ,2+
↔ , +F F ,+9 3 !a5$ !a5$
) ' +"+,-+ at
/012 Stron$ %cids3 ,l< ,7r< ,:< ,l+ < ,4+3< ,2S+ !only on the Grst ioniEation$ Stron$ Bases3 7ases of "roup :A and 2A 'eak %cids3 ,#< ,4< , 2S+3< ,3D+ and organic acids 'eak Bases3 Ammonia and derivatives Calclation of !H Strong Acids% p, H 9logI acidJ Strong 7ases% p, H 1 F logIn ,+9baseJ eak Acids% p, H 9
1 2 logI8 aacidJ when acid@8 a KK
1))) 7=+4ST/C9B+=? Droton donor Droton acceptor
*hydronium ion protonated !ater or solvated proton **hydro"ide ion Stren$th of Acids and #ases #onization Reaction – reaction involving formation of ions Stron$ %cids&Bases – completely ioniEed in solution
7y% >A?@2))'1)2*
,l F ,2+
eak 7ases% p, H 1 F
1 2 logI8 bbaseJ when base@8 b
KK 1))) At 2'1 . . . p, F p+, H 1
Ex. alculate the p, of 1 0 1) 93 ,l Ex. alculate the p, of 1 0 1) 9* ,l Ex. A ).)3 ,l+ 2 solution is 1).) ioniEed. alculate 8 a. Ex. 8 b for 4, 3 is 1.* 0 1)9' .alculate the ioniEation and the I+,9J in a ).)' solution.
| ANALYTICAL CHEMISTRY Ex. hat is the p, of a ).))3 ,4+ 2 solution that is 1' ioniEed6 Ex. 8a for , 2,3+2 is 1.&' 0 1) 9'. hat is the p, of a ).))3 solution6 Ex. alculate the p, of a ).)* 4, 3 solution. 8 b H 1.* 0 1)9' Ex. alculate the molarity of 4a+, solution if 12.2' mB was used to t itrate ).21' gram of primary standard 8,D. a. ).1)' b. ).13' c. ).2')( d. ).1(& Ex. :n standardiEing a solution of 4a+, against 1.31 grams of 8,D< the analyst uses 3'.') mB of the alkali and has to run back with *.2' mB of acid ! 1mBH1).&' mg 4a+,$. hat is the molarity of the 4a+, solution6 a. ).211* b. ).2) c. ).&*31 d. ).2'(* Common Ion E%ect Lis the shift of e5uilibrium caused by the addition of a compound having an ion in common with the dissolved substance Lreduction in the ioniEation of the weak electrolyte /0. alculate the I, 3+FJ in a ).))' , 2,3+2 solution. 8 a H 1.* 0 1)9' /0. hat is the I,3+FJ in a ).))' , 2,3+2 which contains ).))1 4a 2,3+26 Hydrolysis of Salts Acid and base reacts to form salt and water. As a general rule< salts coming from weak acids or weak bases hydrolyEe in water< that is< only the strong conMugate hydrolyEes in water. An acidic salt is formed from the reaction of a strong acid and weak base. A 4asic salt results from the reaction of a strong base and weak acid. Thus< a neutral salt is a product of the reaction between a strong acid and a strong base.
Hydrolysis Reaction of Salts
7y% >A?@2))'1)2*
Acidic Salt% )4Cl
↔ , +F F 4, 3 3
4,F F ,2+ H 8 @
8 ,
K NH
3
7asic Salt% )aC) 49 F ,2+
↔ ,+9 F ,4
8 , H 8 @
K HCN !H of Salts Acidic Salt% p, H & 9
1 2 log
[ ] C salt K b
when salt@8 , KKK
1)))
7asic Salt% H & F
1 2 log
[ ] C salt K a
when salt @8 , KKK
1)))
Ex. hat is the p, of the resulting solution made by mi0ing 2' mB of ).1 ,l and 1' mB of ).1 4a+,6 a. 1.) b. 1.) c. 1.)) d. ). Ex. hat is the p, of ).2' 4,l6 8 b of 4,3 H 1.* 0 1)9'6 a. (.( b. 11.) c. 11.3 d. .(2 #%er Soltions Solutions that contains weak acid or weak base and its conMugate salt. These solutions tend to resist changes in p,.
4 | ANALYTICAL CHEMISTRY
p8a H &.1)
!H of a #%er Soltion
pH pKa log =
−
[ acidiccomponent ] , Henderson Hasslebalch ! [ basic component ] −
3. ethyl =ed p, Transition =ange% .2 – .3 changes from red to yellow p8a H '.))
:f 8b is given . . . . ethyl +range p, Transition =ange% 3.1 – . changes from orange to yellow p8a H 3.
[ acidiccomponent ] [ basic component ]
pH =14 − p K b− log
&rimary Standards for #ases 1. 7enEoic Acid< ,'++, 2. +0alic Acid< , 22+N2,2+ 3. Dotassium 7iiodate< 8,!:+3$2 . Dotassium ,ydrogen Dhthalate !8,D$< ,!++,$ !++8$ '. Sulfamic Acid !,S+34,2$ &rimary Standards for Acids 1. alcium arbonate< a+ 3 2. ercuric o0ide< ,g+ 3. Sodium arbonate< 4a 2+3 . Tris9hydro0ymethylaminomethane !,2+,$34,2 Indicators for Acid"#ase Titration 1. 7romocresol "reen p, Transition =ange% 3.*9'. changes from yellow to blue p8a H . 2. 7romothymol 7lue p, Transition =ange% .29&. changes from yellow to blue
7y% >A?@2))'1)2*
!T,A$<
'. Dhenolphthalein p, Transition =ange% *.3 – 1).) changes from colorless to pink p8a H (.))
Ex. hat mass in grams of 4a 2,3+2 must be dissolved with ')) mB of ).1)) acetic acid to make 2B of buOer solution of p, H '6 8 a H 1.* 0 1)9' a. 2.2* g b. &.1( g c. &.3* g d. 2.12 Ex. hat is the p, of the resulting solution made by mi0ing ' mB of ).21&* ,l and 1' mB of ).11' 4,36 8 b H 1.* 0 1)9'6 a. (.( b. 11.)) c. (.)2 d. 12.& Ex. hat volume of ).2)) ,l must be added to *) mB of ).1') 4, 3 to produce a 2B buOer solution with a p, of *.))6 8 b of 4,3 H 1.* 0 1)9' a. 3.2 mB b. (. mB c. 2*.* mB d. '.* mB
5 | ANALYTICAL CHEMISTRY Ex. alculate the molarity of 4a+, solution if 12.2' mB was used to t itrate ).21' gram of primary standard 8,D. a. ).1)' b. ).13' c. ).2')( d. ).1(& Ex. :n standardiEing a solution of 4a+, against 1.31 gram of 8,D< the analyst uses 3'.') mB of the alkali and has to run back with *.2' ml of acid !1mB H 1).&' mg 4a+,$. hat is the molarity of the 4a+, solution6 a. ).211* b. ).2) c. ).&*31 d. ).2'(* Alications of Acid-/ase 'itration 1. '(eldahl Method )*etermination of +r$anic Nitro$en, Step 1. Cigestion The sample is o0idiEed in hot< concentrated sulfuric acid< ,2S+ and turns black. . . Step 2. Cistillation The o0idiEed solution is cooled and then treated with 4a+, to liberate ammonia gas% 4,F F ,+9
→ 4, F , + 3!g$ 2
Step 3. Titration 1. Psing an e0cess amount of ,l. . . 4,3 F ,l
→ 4, l
The e0cess ,l is determined using a standard 4a+, solution ,l F 4a+,
→ 4al F , + 2
2. Ammonia distilled is collected in a boric acid solution. . . 4,3 F ,37+3
7y% >A?@2))'1)2*
→ 4, F F , 7+ 92 2 3
Titrate the ,37+394,3 solution with standard acid. . . ,27+392 F ,3+F
→ , 7+ F , 5 3 3 /
Percenta$e Protein in the sample protein H4 L f H '.&) !cereals$ H .2' !meat products$ H .3* !dairy products$ /0. A &'*9mg sample of full cream milk was analyEed by the 8Meldahl method; 3*.1 mB of ).1)&* ,l were re5uired to titrate the liberated ammonia. alculate the 4 in the sample. a. 12.) b. &.( c. 1'.'( d. 1).(3 /0. A '.*&39gram sample beef was analyEed for its 4 content and the liberated 4, 3 was collected in a ').)) mB of ).(1 ,l and a 12.'' mB back titration with ).)2' 4a+, was re5uired. alculate the percentage protein in the beef sample. a. 1&.32 b. '.' c. 3. d. 11.)* /0. A 2)) mg sample of Qour was taken through a 8Meldahl procedure and the ammonium produced was distilled into 1)) mB of ).1)) , 37+3 solution. :f this solution re5uired 3.& mB of ).1& ,l for titration to methyl red end point< what is the percentage of protein in Qour6 Pse '.&) for Qour. Ans. 2.3% 2. *o-le Indicator Method )Mi.tre of #ases,
Ex. A sample that may contain 4a+,< 4a 2+3< 4a,+3< and inert matter alone or in compatible combination is titrated with ).1))) 4 ,l with phenolphthalein as the indicator and the solution became colorless after the
| ANALYTICAL CHEMISTRY addition of *.* mB. ethyl orange is then added and 1.'' mB more of the acid are needed for the color change. :f the sample weighs 2.3' grams< it contains a. '.*2 4a,+3 and .'&& 4a 2+3 b. .'&& 4a2+3 and '.*2 4a+, c. '.&& 4a2+3 and '.*2 4a,+ 3 d. '.&& 4a2+3 and '.*2 4a+,
Ex. A sample consisting of 4a2+3< 4a,+3 and inert matter weighs 1.1&( grams. :t is titrated with ).1)) 4 ,l with phenolphthalein as the indicator< and the solution became colorless after the addition of 2.)) mB. Another duplicate sample was titrated with ,l using methyl orange as indicator. :t re5uired ').2' mB of the acid for the color change. hat is the percentage of 4a,+ 3 in the sample6 a. 1.) b. '.1& c. 12.' d. 21.'* &reci!itation/ :n the case of precipitation< the titrant forms an insoluble product with the analyte. An e0ample is the titration of chloride ion with silver nitrate solution to form silver chloride precipitate. Again< indicators can be used to detect the end point< or potential of the solution can be monitored electrically. +ne of the oldest analytical techni5ues that started in the mid91*))Rs. Silver nitrate !Ag4+3$ is commonly employed in such techni5ue. Titration with Ag4+ 3 is often termed as ar$entometric titration.
Indicators in &reci!itimetry The e5uivalence point can be observed by the following%
Cirect ethod for halides and cyanides Titrant% Silver 4itrate< Ag4+ 3 Titration =eaction% Ag F F l9
→ Agl !hite !s$
:ndicator% sodium chromate< 4a 2r+ :ndicator =eaction% 2Ag F F r+29
→ Ag r+ 2 !s$ red
Drimary Standard for Ag4+ 3% 4al Titration is carried out between p, of &91). Psually< a low concentration of chromate is desired to detect the endpoint clearly since a chromate ion imparts an intense yellow color.
Ex. hat is the molar concentration of Ag4+ 3 solution standardiEed against &12 mg primary standard 4al !'*.' g@mol$ re5uiring 23.* mB of the solution for titration6 a.).')2& b. ).'11* c. ).'23 d. ).'32( Ex. A 1.'))9gram sample of impure All 3 was dissolved in water and treated with '.32 mB of ).1))) Ag4+3 using ohr method. Cetermine its purity as All 3 !133.33$ a. ).2* b. 13.3 c. .* d. 2&.3 . #ormation of colored comple0ion Volhard Method !>acob olhard< "ermany< 1*&$ Cirect method for silver 9 :ndirect method for halides Titrant% Dotassium thiocyanate< 8S4 → Cirect Titration =eaction% Ag F F S491 AgS4!s$ !hite
a. #ormation of a colored secondary precipitate Mohr Method !8.#. ohr< "ermany< 1*'$
7y% >A?@2))'1)2*
:ndirect Titration =eaction% Ag F6e"cess7 F l91 Agl!s$ !hite
→
7 | ANALYTICAL CHEMISTRY
AgF F S491
→
AgS4!s$ !hite :ndicator% ferric alum :ndicator =eaction% #e F3 F S491
→ #e!S4$F2 red
Titration is carried out in acidic condition to hasten precipitation of ferric ion to its hydrated o0ide form.
Ex. hloride in a brine solution is determined by the volhard method. A 1).))9mB ali5uot of the solution is treated with 1'.)) mB of standard ).11*2 Ag4+3 solution. The e0cess silver is titrated with standard ).1)1 8S4 solution< re5uiring 2.3* mB to reach the red #e!S4$2F end point. alculate the concentration of chloride in the brine solution< in g@B. Ans. 5.44 6 Ex. A mi0ture of Bi7r and 7a7r 2 weighing *)) mg is treated ').)) mB of ).1*&( Ag4+ 3 and the e0cess is found to re5uire *.& mB of ).3&1( 8S4 for back titration< using ferric alum as indicator. hat is the percentage of 7a7r 2 in the sample6 a. &.(' b. 32.)' c. 3'.2 d. .3* c. #ormation of a colored adsorption comple0 0a(ans Method !8. #aMans< Doland< 1*&$ Titrant% Silver nitrate< Ag4+3 Titration =eaction% Ag F F l91 → Agl
-/ Titration 1ith Ethylenediaminetetraacetic Acid )E*TA, The structure suggests si0 potential sites !he0adentate$ for metal bonding% the four carbo0yl groups and two amino groups. ommercially< the free acid and the dehydrate are available. Solutions of /CTA combines with any metal ions in a 1%1 ratio. The indicator used for titration is the /riochrome 7lack T. #or metal ion in detections< it is necessary to adMust the p, to & or above so that the blue form predominates in the absence of a metal cation. "enerally< metal comple0es with /CTA are red as , 2:n91. hen an e0cess /CTA is added< the solution turns blue according to the reaction% :n91 F ,?93
→ ,:n92 F ?92
!ine red !s$
!hite
:ndicator% dichloroQuorescein< best determination of halides and cyanides /nd point% color change from yellow t o pink
for
Titration is carried out between p, of 9&. Ce0trin is added to prevent e0cessive coagulation of the Agl precipitate.
7y% >A?@2))'1)2*
Com!le.ometric/ :n comple0ometric titrations< the titrant is a reagent that forms a water9 soluble comple0 with the analyte< a metal ion. The titrant is often a chelatin$ a$ent !a type of comple0ing agent that contains two or more groups capable of comple0ing with a metal ion$. The reverse titration may be carried out also. /thylenediaminetetraacetic acid !/CTA$ is one of the most useful chelating agents used for titration. :t will react with a large number elements < and the reactions can be controlled by adMustment of p,. :ndicators can be used to form a highly colored comple0 with the metal ion.
royal 4lue
Ex. hat volume of ).)3)' /CTA is needed to titrate the a in 1&*.' mg of a+ 36 a.'*.' mB b. 2(.2& mB c. 3.(1 mB d. 1. mB Ex. An /CTA solution prepared from its disodium salt was standardiEed using ').3 mg of primary standard a+ 3 and consumed 2*.') mB of the solution. The standard
| ANALYTICAL CHEMISTRY solution was used to determine the hardness of a 29B sample mineral water< which re5uired 3'.'& mB /CTA solution. /0press the analysis in terms of ppm a+ 3. a.*( ppm b. 31 ppm c. 1'* ppm d. 2( ppm
a. *etermination of Cyanide -y the Lie-i$ Method The titration is carried by the drop wise addition of Ag4+3 in a solution of a cyanide forming a soluble cyanide comple0 of silver% 24 9 F AgF → Ag!4$ 91. 2
Ex. Aluminum is determined by titrating with /CTA% Al3F F ,2 ?29 → Al?9 F 2,F A 1.)) g sample re5uires 2).' mB /CTA for titration. The /CTA was standardiEed by titrating 2'.) mB of a ).1)) al2 solution< re5uiring 3).) mB /CTA. alculate the percent Al2+3 in the sample. Ans. .71%
Ex. hromium!:::$ is slow to react with /CTA !, ?$ and is therefore determined by back9titration. A pharmaceutical preparation containing chromium !:::$ is analyEed by treating a 2.39g sample with '.)) mB of ).)1)3 /CTA. #ollowing reaction< the unreacted /CTA is back9 titrated with 1.32 mB of ).)122 Einc solution. hat is the percent chromium chloride in the pharmaceutical preparation. Ans. 0.221%
The endpoint of the titration is the formation of a permanent
faint
turbidity%
F
AgF →
AgIAg!42$J!s$
Ex. A '))9mg sample containing 4a4 re5uired 23.') mB of ).12'' Ag4+3 to obtain a permanent turbidity. /0press the result of this analysis as 4 9. a. 1'.3 b. 23.)1 c. 1&.2' d. 3).& -/ *etermination of Nic2el An ammoniacal solution of nickel is treated with a measured e0cess of standard cyanide solution and the e0cess of standard Ag4+3 solution according to the reactions% Addition of /0cess yanide% 4i!4, 3$F3 F 491 F,2+
A as8in6 a6ent is a comple0ing agent that reacts selectively with a component in a solution to prevent that component from interfering in a determination.
→ 4i!4$ 91 F4, +,
Ex. A 1.')(9g sample of a Db@d alloy was dissolved in acid and diluted to e0actly 2').) mB in a volumetric Qask. A ').))9mB ali5uot of the diluted solution was brought to a p, of 1).) with an 4, F@4,3 buOer; the subse5uent titration involved both cations and re5uired 2*.*( mB of ).)(') /CTA. A second ').)) mB ali5uot was brought to a p, of 1).) with an ,4@4a4 buOer< which also served to mask the d 2F; 11.' mB of the /CTA solution were needed to titrate the Db 2. alculate the percent Db and d in the sample. Ans. 55.1% P and 44.% Cd
/ndpoint% Ag!4$291 F AgF
7y% >A?@2))'1)2*
Ag!4$291
7ack Titration with Ag F% 2491 F AgF
→ Ag!4$ 91 2
→ AgIAg!4$ J 2 !s$
Ex. A &').2'9mg of alloy nickel was dissolved and treated to remove the impurities. The ammoniacal solution was treated with ') mB of ).1)&' 84 and the e0cess cyanide re5uired 2.2' mB of ).))(2' Ag4+3. Cetermine 4i in the alloy. a. 2).* b. 3&.( c.1).3 d. 1.2&
3 | ANALYTICAL CHEMISTRY
Redction"+.idation/ These Uredo0V titrations involve the titration of an o0idiEing agent with a reducing agent< or vice versa. An o0idiEing agent gains electrons and a reducing agent loses electrons in a reaction between them. There must be a suWciently large diOerence between the o0idiEing and reducing capabilities of these agents for the reaction to go to completion and give a sharp end point; that is< one should be a fairly strong o0idiEing agent !strong tendency to gain electrons$ and the other a fairly strong reducing agent !strong tendency to lose electrons$. ?ou can use appropriate indicators for these titrations< or you may employ various electrometric means to detect the end point.
58#D%9T 8n+ !a$ 8n+ !b
b. ).)3
c. 1.))*
/0. The percentage of n+2 in a ')) mg sample which after the addition of *).)) mB of ).1)' #eS+ solution re5uired *.') mB of ).)*& 8 2r2+& is a$33.'2 b. 3'.)) c. 1&.') d. &.)
/0. A 2)9mg sample of pyrolusite was treated with e0cess 8:. The iodine liberated re5uired .2 mB of ).11)' 4a2S2+3 solution. alculate the n+ 2 in the sample. a$.2& b. 3).*' c. (2.' d. &.12 ombining =atio< also !f$ /0. A sample of iron ore weighing 3*'. mg was dissolved in acid and passed through a >ones redactor. :f ' the resulting solution re5uired '2.3 mB of ).)13 3 8 2r2+& for titration< calculate #e 3+ !231.'' g@mol$ in the ore sample. 2 a$1'.)' b. '.1' c. ().3) 2 d. &.(*
2 + + 1 2 2 1
/0. hat is the molarity of a 8n+ solution standardiEed against 1.3' gram 4a 22+ !13 g@mol$ re5uiring 2'.1 mB of the solution in acidic medium n+ 2. Ans. 7.13% 6
7y% >A?@2))'1)2*
a$).11 d. ).*'
/0. A ).2))9g sample of pyrolusite is analyEed for manganese content as follows. Add ').) mB of a ).1)) solution of ferrous ammonium sulfate to reduce the n+ 2 to nF2. After reduction is complete< the e0cess ferrous ion is titrated in acid solution with ).)2)) 8n+ < re5uiring 1'.) mB. alculate the percent manganese in the sample as n 3+ !only part or none of the manganese may e0ist in this form< but we can make the calculations on the assumption that it does$. Ans. .74% /0. A hydrogen pero0ide solution is analyEed by adding a slight e0cess of standard 8n+ solution and back9 titrating the unreacted 8n+ with standard #e F2 solution. A ).'*&9g sample of the , 2+2 solution is taken< 2'.) mB
10 | ANALYTICAL CHEMISTRY of ).)21' 8n+ is added< and the back9titration re5uires '.1) mB of ).112 #e 2F solution. hat is the percent ,2+2 in the sample6 Ans. .1% /0. A sample of a pyrolusite weighs ).'))) g. To this is added ).& g of As 2+3 and dilute acid. After solvent
7y% >A?@2))'1)2*
action has ceased< the e0cess three9valent arsenic is titrated with '.)) mB of ).1))) 4 8n+ . alculate the o0idiEing power of the pyrolusite in terms of percentage
