A Semimicro Scheme of Qualitative Analysis for the Anions" J. T. DOBBINS and MARY EUGENIA KAPPa University of North Carolina, Chapel Hill, North Carolina
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OMPARATIVELY few systematic schemes of analyms for the anions have been proposed, principally for the reason that satisfactory conditions have not been found for the coexistence in solution of many of these ions. Chief among the difficulties experienced is the fact that certain pairs of the anions undergo oxidation-reduction, therein changing from the original state and completely losing their identity. The anions, for the most part, are complex radicals made from several atoms and are capable of undergoing complicated electron changes. These changes usually produce a rather profound alteration in the nature of the ion. Very few of the cations undergo this change of valence, since they are simple ions consisting only of the residue of the original atom after it has lost one or more electrons. To illustrate, ferrous iron may be oxidized to the ferric state, hut by using appropriate methods a test for iron may be ultimately obtained. In contrast, if the sulfite ion be oxidized, it is changed into an entirely different ion and its identity is permanently lost. For reasons based on the concept of electrode potential these oxidation-reduction reactions take place most readily in a solution of appreciable hydrogenion concentration; hence the question of fundamental importance is how this action may be brought to a minimum. In the proposed scheme of analysis the decrease in hydrogen-ion concentration is produced by dissolving the calcium nitrate, the reagent for the first group, in a saturated solution of calcium hydroxide instead of water. However, there are two pairs of ions (fem'cyanide and sulfide, femcyanide and iodide) which do not involve the hydrogen ion in their reaction; in these cases the oxidation involves only a transfer of electrons from one ion to the other; consequently they never coexist in solution. The anions are divided as follows into five groups according to their bebavior with certain metallic ions: Group I contains these ions which are precipitated as the calcium salts from alkaline solution: carbonate, fluoride, oxalate, tartrate, sulfite, arsenate, arsenite, and phosphate.
Group I1 contains those ions which are precipitated as the barium salts from alkaline solution: sulfate and chromate. Group 111contains those ions which are precipitated as the cadmium salts from alkaline solution: sulfide, femcyanide, borate, and ferrocyanide. Group IV contains those ions which are precipitated as the silver salts from a solution slightly acid with nitric acid: cyanide, thiocyanate, thiosulfate, chloride, bromide, and iodide. Group V contains those ions which cannot be precipitated from solution: acetate, chlorate, nitrate, and, nitrite. In most cases only a limited amount of separation of the ions in any one group is necessary since specific reagents have been found which are reliable in the presence of f i e other members of the group. Organic reagents in some cases give very specific and sensitive tests, but they axe not used entirely. It is desirable to have the substance to be analyzed in the form of its sodium, potassium, or ammonium salt. If any other metal has been found in the cation analysis, a 0.1-gram portion of the material is boiled with 0.4 gram of sodium carbonate in 25 drops of water to obtain the "prepared" solution for use in the analysis of the anions. SCHEMATIC ANALYSIS
Procedure I . Precipitation of Group I. To 20 drops of the "prepared" solution add two drops of Ca(NO&Ca(OH)r solution and centrifuge. Test for complete precipitation: Centrifugate containing Groups II-\7 is saved far Procedure 3. The precipilete may contain calcium salts of COi-, GO4-. CH406-, SOr-, AsOsm,As04-, PO4=,and F-, and is analyzed according to Procedure 2. Procedure 2 ( 0 ) . Analysis of Group I. Wash the precipitate from Prmedure 1 with water until the washings no longer give a test for nitrates; discard the washings. Use two small portions of the ~ r e c i ~ i t a tfor e Carbonate. oxalate. and tartrate tests in procedire 2 ( b ) and reserve the remainder for Procedure 2 ( c ) . Procrdurc 2 (bl.
Test for COa-. C1O.-, and
1st portion
C.HdOa-.
I
2nd Portion
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Presented before the Division of Chemical Education, s t the 104th meeting of the A. C. S., Buffalo, New York, September 9.
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water
Taken from a thesis submitted by Mary Eugenia Kapp to the faculty of the University of North Carolina in partial fulfilment of the requirements for the degree of Doctor of Philosophy. Present address: E. I. du Pont de Nemaurs and Company, Richmond. Virginia.
st low temperature shows oralate and red color appearins
LDT*.
a
Blue color appearing
when crucible ir heated over a flame shown presence 01 lorIrnk
P r o u d w e 2 (c). first.
(
T o t h e remainder of the precipitate from Procedure 2
ccrrrifvgarc may contain
I
Test for SO8- by adding one drop of dilute HCI, five droop BaCie and eentrifugeir ncce;sary. Add five droos HnO,. A white precipitate *hmvs of % -
~~~.~
/ 1 1
Dresenie
sor.
(0)
add ten drops of 3 N metie acid, stir, ecntrifuge, and a n d y ~ ecentrifugatc
Divide in four portions and make t u t s in order listed.
1
2nd portion
3rd Portion
Test for APO? by adding one drop of dilvte HCI and two drops Ha0 end heat t o boiling. Pass in H B . An immediate yellow precipitate shows p r e r ence of arrcnire.
Test for ASOF by adding a few crystals of KBr, 20 droos of -c. HCI. A disappearing on dilution shows praenee of orlendc.
Procedure 3. Precipitation of Group 11. To two drops of the centrifugate from Procedure 1 containing Groups II-V add one drop of Ba(NOs)*. A precipitate indicates the presence of Group 11, but it may be BaCO8. If a precipitate forms, add ten drops of Ba(NO& t o the remainder of the centrifugate and test for complete precipitation. Catrifu@e contains Groups III-V and is saved for Procedure 5. Precipitate contains Bas04 and BaCrO, and is analyzed according to Procedure 4.
1 Rcn'due may contain r-. c10.-.
4th Portion Test for PO,' by adding one drop molybdafetartaric acid solution and allowing t o stand for one minute. Add two drop. of dilute benzidine and one t o eight drops of NHaAe. Blue ring or m1or of shows oresenrr
I
or OH.06-. Test for P- on spot plate by adding a small portion of thereniduc ton mirr u e of one drop of ZrOCIz, one drop alizarin and two drops of 1:s dilvte HCI. The fading of the pink color of the reagent to a straw color showr prenen- of $"wide.
color from white t o yellow to orange t o red and finally t o black If a precipitate forms, add shows the presence of thiosdfale. two drops of dilute NH,OH t o the entire centrifugate. (If tbiosulfate is absent, the treatment with NHdOH may be omitted.) Now add ten drops of A ~ N O I acidify , with HNOI, mix well, and centrifuge. Cenlrifugate contains Group V and is saved for Procedure 9. Precipitate may contain C1-, CN-, CNS-, Br-, I-, and SnOi (as AgzS)and is analyzed according to P rocedure 8.
~Pracrdurr 4.
Analypis of Group 11. Wash the ~precipitateconsisting of Bas04 and BaCr01 with ten drops of water, discarding the warhinga. Treat i t with a mixture of five drops of water and five drops of 3 N HCI. Centrifuge.
Ccntrifugarr may contain C r O P
To one drop of this mlution add one drop of diphenyicarbazide. A reddish-violet color shows the presence of chronoh. Discard the remainder of solution.
Rrlidue may &=in SOI. Treat reidme with ten drops of 3 N HCi. A white precipitate show. the prerence oisui-
Procedure 8. Analysis of Group IV. Wash the precipitate from Procedure 7 until t h e washings no longer give the t e s t for t h e silver ion, discarding the washin& Add fen dropr of ammonineal silver nitrate t o t h e ~recipitafe. Mix well and centrifuge. crnrrifvrorr
may contain CI-. Acidify with HNO.. A w h i t e preeipitate shows presence of chloride.
Procedure 5. Precipitation of Group 111. To two droos of the centrifugate from-~rocedure3 a d d four drops of ~ d ( N 0 ~ ) ~ . A precipitate and not a mere cloudiness indicates the presence of Group 111. If aprecipitate farms, treat the entire centrifugate with ten drops of Cd(NO&, centrifuge and test for complete precipitation. Catrifugele containing Groups IV-V is saved for Procedure 7. Precipilale may contain S-, Boo-, Fe(CN)P, and Fe(CN)a- and is analyzed according to Procedure 6.
Rcriduc may contain CN-, Br-. I-, and A&. R c move small portions t o spot plate and add a f e u crystale of KBr t o each and test each a. directed below. 1st Portion
2nd Portion
T o this add one drop of dilute benzidine and one drop of CuAez. Blue eoior shows presence of cwnidr.
T o this add one drop of 3 N HCI and one drop of PeCIs. Red color shows presence of lhiocymolr.
C.nlrif"golr containing Br- and I a3
zn salts is divided in two por-
Procedure 6. Analysis of Group I l l . Wash the precipitate obtained in Procedure 5 until the washings are oeutrai. discarding the washings. Divide into four portions. 1st portion
s-
is indicated by t h e presence of a ,.ellow p r s i p i tatc. T o confirm. add two drop3 &O and one drop CuAcz to this portion of the prceipitate. The formation of a black nolid shows presence of rul-
2nd Portion
3rd Portion
4th Portion
To a portion on a spot plate, add onc drop of dilute HCI and one drop FeCls. The formation of a deep blue mlor shows presence of
To this portion
To this portion add one drop of saturated benlidine solutioo. A blue or violet
.fc*,oryonidr.
in B "assmle add one drop of con". &SO4 and evapmte t o dryness. Add three drops of carmine - HDO. solution. A red eoior changing t o violet rhows nrescnce of
iid6.
Procedure 7. Precioitation of Grotto IV. To two drops of the centrifugate from Procedure 5 add three drops of AgN08. A precipitate may be Group IV. A white precipitate changing
Rcriduc of silver and un-
used z n 13
&no. l e t portion
2nd portion
Test for Br- by adding two drops of centrifugate to four drops of chromic acid on a watch giar. over a b e a k e r of boilingwater. w e t a oieee of fiiter ;*per with Sehiff'~ reagent. Place it in a s m a l l e r watch g1a.s a n d invert over o t h e r watch glass. Red - violet color with a metallic 1"sler developing on paper ~howa presence of bra-
Test for 1- by adding four drops of CCI. t o rest 01 centtifuglte. acidifying with HAc and addina a small crystal of NeNO,. Dilute to two ml. aodshake well. Violet mlor in CClr shows me"ence of indids.
Procedure 9. Analysis of Group V. Cmirifumlc from Pmeedvre 7 containing NO,-. NOS-,CIOa-, and C~HaOs-irtreated as follows:
rme drop of ecn(rifugnte on spot plate add one drop of Grius diam reagent. The formation 01 a red color immediately and disappcsring after a short time leaving n ydlon ~oivtian shows
Tert lor CIOI': T o one drop of e m *ifusate on spot plate add two drops of milineh~drahioridereblu; color shows presence of chlovelaping m u n d the edge of the droo of NHaOH
Procedure 10. Test for nitrate ion. To two drons of "orepared" solution add a slight excess of AgAc, heat and centriiuge until clear. T o a drop of centrifugate on a spot plate add, in order mentioned, two drops of concentrated H&04, and one drop of alpha-naphthylamine. A purple ring developing around the drop of reagent shows the presence of nitmlc.
SPECIAL REAGENTS Alizarin. Saturated alcoholic solution. Ammoniacal silver nitrate. Dissolve 1.69 g. AgNOs and 25.25 g. KNOs in a mixture of 17 g. concentrated N&OH and dilute to 1 liter with water. (F3?Os is added as a coagulating agent.) Bnsidine, dilute. 0.05 g. henzidine dissolved in 10 ml. glacial HAc and diluted with water to 100 ml. Bneidinc, saturated. 2 N HAc saturated, in the cold, with benzidine. Calcium ndrale-hydrodc solution. 2 N Ca(NOa)r4Hs0 saturated with freshly precipitated calcium hydroxide.
Carmine solution. 0.2 g. in 100 ml. concentrated H1SO4. Diphenyl carbesidc. 1per cent alcoholic solution. 4 Griess diazo reagent. Dissolve 0.5 g. sulfanilic acid in 150 ml. 2 N HAc; bail 0.2 g. alpha-naphthylamine with 20 ml. water. decant the colorless solution and add to it 150 ml. of 2 N HAc. Mix well the two solutions and keep stoppered in cool, dark place. Lanthanum nitrate. 5 per cent aqueous solution. Molybdute-lartoric acid solution. 5 g. (NH&MO,Ow4HzO dissolved in 25 ml. cold water and added t o 17.5 ml. HNOs (sp. gr. 1.2). Add 4 g. tartaric acid t o each 25 ml. portion of this mixture. Solution must he made fresh every few days. dpha-Naphthylomine. Saturated aqueous solution at boiling temperature. Use only clear solution. Schiffs reagent. Dilute with dilute HCI in proportion 1: 1. Prepared by dissolving 1g. rosaniline-HU in 75 ml. water and decolorizing with sulfur dioxide. Zirconium orychloride. 1.7 g. per liter of solution.
SUMMARY
1. A new semimicro scheme of analysis for the identification of the anions, based on the principle of oxidation-reduction through the control of the hydm. gen-ion concentration, has been developed. 2. The anions have been saentifically grouped in such a manner as to minimize the number of operations required to carry out the procedure, thereby reducing the time required for the analysis. 3. Organic reagents have been used wherever advantageous.
