J a c k L. Lambert and Clifton E. Meloan Kansas State University Manhattan. 66506
I I
A Simple Qualitative Analysis Scheme for Several Environmentally Important Elements
This is a qualitative analysis scheme for Hg(I), Hg(II), Sh(III), Cr(III), Pb(II), Sr(II), Cu(II), Cd(II), AsUII), Cl-, NO3-, and Sod2-. I t does not require hydrogen sulfide (although sodium sulfide solution is used in two confirmatory tests) and an unknown can be com~letedin one laboratorv period after the student is familiar k i t h the separations and tests. The tests involve oreci~itation.evolution of a eas. complex ion formation, &d a t i m e test. A knowledge ofor: aanic chemistrv is not reauired. - The argumehts u s u a ~ l ~ ~ r e s e nfor t e dteaching qualitative analysis are that i t teaches chemistry and provides the students with some practical experience in chemical reactions. This is true hut in recent years qualitative analysis has declined in popularity because other topics are considered to be more important. There is probably no way that we will ever go back to the old qualitative analysis concept; yet experience in solving practical problems as consultants has indicated that some knowledge of qualitative analysis is immensely beneficial. The following qualitative analysis scheme is an attempt to teach some relevant chemistry, provide some practical knowledge of handling solutions and chemicals, and yet not take up a major portion of an undergraduate laboratory course. The elements involved are all of current environmental concern so student interest is maintained. This scheme is a modification of the scheme of West, Vick and LeRosen ( I ) . Several hundred of our chemical analvsis students have completed an unknown in one three-hourberiod. We have also tried to overcome the tendency of students to do the analyses in "cook book" fashion bv includina the chemical reasoning for each step as a series 0-f notes. In addition to the material presented here, thestudent receives ten pages, single spaced, on the general chemistry of the elemenu, involved with regard to their biological importance, how they react in the body, and what chemists can do to help the body in thedetoxification processes. This material can he obtained from the authors upon request. The Chemistry Behind Thls Qualltatlve Analysls Scheme Step. I . Flcmr Iml: Asillll, Hg(ll1, Cl(lllJ, Cdilll, Pb(ll1. Shill11 produce no vi~ihlceolsr with a flame of this temperature. Species that produce a colored flame are C u C l r b l u e togreen SrCI1-red K = purple-seldom interferes because the Sr red is quite different. N a = yellow-almostalwaysprpaentsoacloen platinum wire is necessary. D i p i n hydnlchloric acid and heal: repeat u n t i l no yellowcol", i s s e n . step 2. The solubilities i n #,oms
Ion Compatibility Table
'West, P. W., Vick, M. M., and LeRosen, A. L., "Qualitative Analysis and Analytical Chemical Separations," The Macmillan Co., New York, 1953 (analysis scheme reproduced in part with permission of the copyright holders).
-
NO.-
SO.'-
i = inlolublso
r
soluble
0 1 means t n a t t h e t w o i o n s c a n n o t b o t h b e in s o l u t i o n , ro d o n ' t report tnat combination. b S O l u b i l i t ~d e p e n d s on pH and c o n c e n t r a t i o n of C I - p r e s e n t .
Step 2 The centrifugeshouldbedone quickly (30-c a t m a r ) t o avoid preeipitstiom uf PbCb upon moling. Y e p 4. HglCla, sometimes called calomel, and once used as e laxative (0.5 g per dose: warning: HAZARDOUS!) is inwlublc i n all m m m m resgqfrr. I@as e very chqracteristie reaction with ammonia i n whieh linely divided H g ihlackland H g i l i l amiduchloride (whitel' are produmd i n a disproportionation reaction. The black overwhelms the whits, so the preeipitste will appear dark gray to black Hg2C12
+ 2NH3
-
Hgill
+ HgNH2Clisl+ NHlt + CI-
W i t h Pb-if the precipitate was not washed mmplctely or the solution was cold when ~enLrifu~d.
-
4PbCIa 3PbO. PbCIa.
+ 6NHa + 4HzO
Ha0 (whitel + 6NHlt
+ 6CI-
Slep5. It isabB~lutelyneeessarythatthepHbebetwpen3.5and4.Oorthechromiumwill
nut preeipitalewhen benzoate iradded. Ifswhitepreeipitalefurms h c r c i l isan indicatim that S h i l l l ) ispraent. hydroxides or hydrated oridesof all melalionsaith theexception uf the alkali S f ~ p 6The . metals. N H ~ + , A s i l l l J ,and i n p a n theslkallneearthsarersth~rdiflievltly soluble i n H20. The hydroxidecontaining the most OH- groups per molwule passes thesmallost K,&.
The effect of valence on solubility where the same dement is involved may b illustrated with the following psirli F ~ ~ o H ) ~6.3 x 10-38 FeiOHh 7.9 x 10-lS
S~IOH), SnIOHJ*
Ix LO-" 2 0 X 10".
Thus by proper manipvlstionoftheOH~mnantratian,hydroridesofthe metalsof high*? valence may he separated from those requiring large.OH- mncentration. The hydwlyds of hut bcnroafe solution is u w d to produce the OH-
Compound (HgnOl HKO SrlOHln CdlOHh PbiOHh CuIOHh AoIOHh CrlOHh SbIOHla
per liter ore:
Lead ehlaride,PbCIzi~ontheborderline betweensolubleand insoluble, hut keeping the solution h o t w i l l keep it from precipifafing.Altho"gh thisamounlofHC1 ~h""ld heample. s cheek for mmplete precipitation o f H g l l l should be made.
CI-
CationrlAnionr
We can see that C r i I l l l and S h i l l l l e m he separated. Actually the mmpounds rearrange and form CrOHBzz and SbOCl which arc equally insoluble. Step 7. Thesodium benwste is w e i t o f a m ebuffer toaid i n k-pingthepH i n t b r o y i m 013.5. Useaboilingwater bath O heat thiasalutionor itrill swcly mmeout of them, t u b . l f v o u had C d l l l i vou had aereenishwlution heforethisateo isa l i r h t blue . Ifvoursoluliun . after you have properly performed thinstep you may a180 haveCuill1. SLm 8. A orwinitate at this m i n t that dms not produce a yellov or ursn~eprecipitate later a false pabitiveand is a b c n m t e . S t e p 9 The HNOs willdi-lve thechromium butnottheSbOC1. &sure wmmhina the washings, which may mntain C.iIII1.
is
~~
~
Volume 54, Number 4. Apil 1977 1 249
Cations onrult the incompatibility table prior t o analysis Flame tert on clean Pt wire (indications. subject t o confirmatory tests: red = Sr, blue t o green = Cu Heat 2 m l unknown t o boiling A d d 6 d r o o l 3 M HCI ficentrifuge
while hot1
. A d d 1M NHAOHt o p H 3.5-4.0 ~ d 2 ddrops 0.5 M NH. benzoate. warm A d d 5 drops 0.5 M Na benzoate. boil [centrifuge, warhl Solution may contain: Pb(ll1 sr(ll1 Hgllil Cu(li1 Cd(ll1 Ar(ll1)
c Ppt = H g p * 4. A d d 5 drops 6 M NHIOH Black ppt = Hg; confirms Hg(11
-Ppt
= Cr(OH)benroate, andlor SbOCi
8. A d d 10 drops 6 M H N 0 , .
warm 9. Warn twice w i t h 6 M HNO,
I-
[centrifuge1
12. s o l n = Cr(NO,l, ~ a k alkaline e with 6 M N a O H p d a 3 drops excerr 6 M NaOH t [ c e n t r i f u g e l -(discard
.. AReadjust d d 5 drops rat'd NaF t h e p H t o 3.4 Let stand 5 min (centrifuge1
1
solution may contain: H9I111 CU(llI cd(l1) Ar(ill1
.~
residue)
c Ppt = SbOCI (white PDt) 10. A d d 5 dropr 6 M NH,OH A d d 5 drop9 1M KNaC,H,O,,
!-
[centrifuge]
-
warm
(di%Cardresidue)
s o l n = K(SbOlC,H,O, Make acidic w i t h 15 M HNO, 11. A d d 2 d r o o l 0.1 M Na*S orange pdt = sb;~,; confirms Sb(lll1
Sol" = NaCrO, 13. ~ a 4adrops 3% H,o,. boil 14. Make barely acidic w i t h 6 M H A c 15. A d d 1 drop 0.1 M Pb(NO,),, and 1t o 2 drops o f 3 M NH,OH Yellow opt = PbCrO,; confirms C r ( l l l 1
-
Ppt = PbF,and/or SrF, 18. A d d 1 0 drops 3MNHIAc. repeat 3 timer
stir. centrifuge
t
[centrifuge1 -Add 20. Combined r o l n = PbAc, Add 1 drop 0.1 M Na S ; P~(III ~ ~ a opt c k = P ~ S confirms
Pot = SrF 3 drp ;s of 12 M HCI 19. crimson flame test = Sr; confirms s r ( i l 1
a k alkaline e w i t h 6 M NaOH (iitmur) Boil free of NH, A d d 1 m16 M NaOH Centrifuge, wash With 1 m l H,O
t
[centrifuge1
c ~ p =t Cd(OH1,. Cu(OH1,. andlor HgO 23. DiSIOiYe in 3 M HCi 24. A d d excess 15 M NH,OH u n t i l basic and 1-2 dropr more-stir centrifuge, wash w i t h 1 0 drops 3 M NH,OH
I
[centritugel
Solution may contain: Cu(II1 Cd(li)
solution may contain: Ar(li1)
7 Combine centrifugate and washing solution A d d 1 2 M HCI t o acidify Boil down t o 2 m l total volume A d d 1 m l 15 M NH,OH
1
4
26.
on-add 5 dropr 0.1 M s n c l , w h i t e p o t which may turn gray.
TO
H ~ ~ c I ~ confirmr - ~ H ~ H~g (, l l 1 2 7 . ~ 0two-aaa 5 dropr of KI ~ e d - o r a n g eopt confirmr H g l l l l
-
sol" + warn [c~(NH,),~'+. [ C U I N H ),I2+ 28. ~ d 1d2 M H C I u n t l ~acidic. heat t o boding 1 7 blue, add solid Na,SO, t o disappearance o f color Ad4 1 m l of 1M NH,SCN
I
[centrifuge1
Solution may contain: Cd(li1
[centrifuge] +(discard residue)
solution may contain: Ar(1ll)
c ~ p =t H~NH,CI 25. A d d 6 M HNO, t o dissolve A d d 10 drop5 H,O, divide into 2 portions
c P p t = CUSCN (white) 29. ~ d 1 d 2 M HCI t o just dissolve A d d 3 drops 15 M HNo,. boil. cool A d d 1 5 M N H a O H u n t i l alkaline Blue roln o t [Cu(NH,),I2+; confirmr Cu(l1)
30. A d d 3 M H C I u n t i l barely acidic Ada 3 drops o f 0.1 M Na,S Yellow p o t = CaS; confirma Cd(il1 ( I f not definmtely yellow-it is not Cd)
A d d 1 0 drops 6 M NaOH A d d pinch A1 granules. glarr wool plug, warm ~ e s evolved t gas with moist AgNO, paper (refer t o explanation i n Step 32 following1 Black opt on paper-Ago; confirmr A r ( l I I 1
Anions u s e 1 m l of unknown solution 1. A d d 5 m l o f 1.5 M Na,CO,. boil in a beaker for 1 0 min. w i t h stirring (keep volume constant b y addition o f water)
[centritugel
- ~ p t= Carbonates, hydroxide5 or oxides o f cations
Divide i n t o t w o portions A. t o one add HCi dropwire u n t i l the CO, evolution ceases; use for NO,-tertr B. t o the other add H N O t o destroy the carbonate; urea for Cl-and 50,'-tertr use 1 m l to_r each o f the fofiowing tertr:
,-'....*
TO ) r m n p r r 8 ,n i ,rrna! 3oUi' aiam iins ur. HL'I i r prcrrnt 'I h l i 8 % x h r 11I YHIUH m,lro s*.,p., lZsl*.hr Ns~,pl(e,r,I~rdd,n:,,,< 8 X n . u " .;.,n.,mr. I . ~ - 1 4 h I ' " l 3 w . w I 8 . ,,.Z", ,.~I.~,~"~OC.'
He%+ 2 ~ - H f
-01:l V
E' = 0.34 V
Cuo
S t e p 27. The unstable ]pale yellow S H II
PhCr04
+ 3OH-
-
? C P O ~ ?+- 2Hf
first h r m e d rapidly changes tu the rtehle redK I will pnduce the d u h l e K z H d r . m,,dilicatbn. inwluhle i n cold water. EXWe have three tort? for Hz(ll1 boause i f Ph(l1) war pre.enLand war not removed it pnduces many similar reactions. The K,. of Hala ia :12 x 10FzB. Step 28. The H C I d e r t n ! y ~the aminemmplexesn~ that C u l l l l can bereduced U ~ C u l l l hy SOa2- which ir the basis for the separaticm of C d l l l i trim C u l l l l . Pb(OHlr-
-
+ Cr012-
Cuz+
Cr2072-+ H z 0
Cd2+
Step 1.5. PhCrOl isinsuluhle,K.,, = 1.8 Y 10-14. BBCrO4.alro.vellmv c#
.!,.-I
, t h v v r , m w e 81.~8 , . o t
n,!r,nt
n.m.:r..\.llll.t \ - -111 n #.8m,cer. , . I s. .m #W.th-n H. I W 3tld ~
i.,..,
iwo1ved in the ,!"era11 reacliiln. 2AI As03:l-
.,,;.,:.,,,u.l
+ 6H20 --2AIiOH)4- + AsHa + OH8Ae + HaAsOs + 6HNO:i GAgNOs + ASH>+ 3HzO +
~
~ is
d
-
st the gas~for the arsine ~ same time , and acts ~ as s carrier ~
2AI C 6 H 2 0
+ 2OH--3H2
and with water AKA-. 3AgN01+ 3Hs0
-
8Ag
t 21AllOHirl-
+ H?AsOa + :IHNO:$
Caution:run a test ons knom.AgN01 paper will diecnlor slightly. Even a lhlank will give orra gar. Holding tho paper with your finper8 will give a pnnitive tort.
Comments on Anion Analysis 11 Tho addition of NaxC03i n this step is to removeas many of the caticm as poerihle. to prevent their interference i n the suhsequontaniunsnalysir,It isdosirahle ,t,clh99P"eth0 OF the s o ~ ~ hefore t i ~and ~ d t e r the d d i t i o n "i the dium carhl,nate. and the c ~ l h r o f Lhc renuking precipitate, hecause this can givevaluable clues as t o what cations may be present'Pha hydrated hn [CUIHSO)~]~* is hlue;consequently,roluliunncantainingCulll) are blue, except i n the presence of strong mmp1exine agents. Solutions containiny C r l l l l l are usually dark green-violet, and will hidethe presence of copper. A l l other cstianr in the scheme are mlorloer i n solution. I n the pr&itaus which form "pun adding NasC01.vm may see color: i f you du, takecareful noto CI(IIII isnormally a green precipitate i n hasic snlulinn, whileCu(ll) forms alight hlue precipitated CU(OH)I. I n theab~encedCrlllli you mayalm beahle t o s e a brown or yellowprecipitate. H g i l l l formaa yellowprecipitate
