QualitativeAnalysis, with Periodicity, for "Real" Ronald L. Rich Bluffton College, Bluffton. OH 45817
The outline of group separations for a non-HzS analytical scheme applicable to all metallic elements is presented here, along with-an outline of an tnblmviutrd and Aherwlse modified version designed f~~rsomeemphasison nutrition all\^ important metals, with special attention to 10 cations. he more complete scheme will he referred to as the Bromide Scheme and the shorter one as the Iodide Scheme. A survey of the relevant literature from about the last 50 years is included. Major Features
One orohlem in basine teachine on svstems of aualitative analysis is the incomplete correlkon between membership of an element in an analytical group and its position in the periodic table. As stated by Sorum and Lagowski (I)," . . . since the periodic table provides the best available organized picture ofihe propertiesbf the elements and their com&xmds, we should expect the periodic table to serve as a useful guide in many phases of qualitative analysis." The Bromide Scheme is partly the result of a long effort to accentuate that usefulness. Another problem with qualitative analysis has been the interference of various organic and inorganic substances. Even phosphate causes the calcium (carbonate) group, in standard systems, to precipitate too early. Citrate, sugar, EDTA, and polyphosphates are a few of the common ligands that can prevent precipitation of the aluminum (hydroxide) and other groups. Bromide and iodide can prevent precipitation of some sulfides. even that of mercurv. As a result we either disnense artificial unknowns without these important substances, or include tedious wavs ot' dealine with them. Su~dent!.can itill learn a great deal, perhaps even because of this, but one value of the Bromide and Iodide Schemes is that each can overcome most, though not quite all, of these difficulties. Resented at the 6th lnternatioml Conference on Chemical Education, College Park, MD. August 9-14, 1981. and at the 7th Biennial Conference on Chemical Education, Stillwater, OK, August 8-12, 1982.
Many basic ligands, such as carhonate, coordinate to metallic cations, iust as water does, through oxygen atoms. lt is thwefore not &rprising that nnnchelating ox).-1igand.i that ran displace water from some cations will probably react with most of them. This leads to Door selectivitv for initial seoarations. We do well to start, cdusequent~y,wkh reagents acting through (nonmetallic) atoms far from oxygen in the periodic chart and correspondingly different chemically. (Even nearby chloride, in the standard procedures, removes several of the few metals in the first group quite incompletely.) A good general strategy, then, is to use oxy-ligands, and perhaps fluoride, only after many metals have been removed from unknowns by means of more polarizable and selective reagents, such as bromide and iodide. Figure 1 outlines the main Bromide Scheme separations. ~igu;r2 shows the resulting organization of the periodic chart. The numhersareanalytical group numbers. Mn. Fe, Co, and Ni. for examnle. are in Croun 2. The numbers u,ith asterisks designate the'nokmetals that'cwrdinate directly to the cations in the corres~ondineerouos. The 2* over N. for examole. shows that t6e N a&s in cyanocobaltate a;e directly'at: tached to the precipitated cations in Group 2. The Iodide Scheme outline is in Figure 3. We note that the simplified procedures of Firmre 3. without solvent extraction, put Zn into analytical ~ r o u 2p rather than Group 1. Anv specific set of directions raises the question of using a cookboo-k approach. This author also preaches and claimsto practice a strong emphasis on thinking. We should certainly iecognize, though, that students need the most help a t thk beginning of any systematic analysis, partly because they then h u h to work with the largest number of potentially p;esent metals. Readers will see some of our waysof dealing with this. We may add that the proper and improper use i f fixed instructions is not confined to qualitative analysis. Additional comments and procedural details are available. Expensive reagents have been avoided, but information on costs per unknown is also available. (See summary.) ~
~
-
Add HBr red-agt.
MtpBr
pH-3 Add NH,Cfr (or K.HP0,)
CHICI, KI
Heat.
1
~
-
Gr. 5
Met'
---------I Gr.
MetF,
solvent Figure 1. Group separations in the Bromide Scheme
After each group precipitation (or solvent extraction) the remaining aqueous salution is treated by the nen procedure to the rigM The resulting precipitates and
extracts are shown below the correspondingprocedures. Notes: The reducing agent, red-agt, can be ascorbic acid. Mtp is methyltriphenylphosphonium. Met is a metal in the group. Cfr is cupferrate. Bromides of Ge, As, and Se can be distilled out (2)as Subgroup l a before extraction of the bromo-complexes in Group 1 (or lb), for which the [HBr] is made 3
M.
Volume 61 Number 1 January 1984
53
The Role of Organic Reagents
One notable feature of Figure 1is its inclusion of some organic reagents. Does this mean that the student is overwhelmed with rote nrocedures and "incomnrehensible" reactions? By no meahs! Since that may be a'misleading first imnression. however. let us brieflv discuss the role of oreanic reagents inthis work before we further. Dimethylglyoxime and aluminon are accepted almost universally in confirmatory tests. More recently thioacetamide and benzoate (3) also have been accepted as group reagents. One criterion for adopting each group reagent in the present case has been that it can be interpreted to students. A reducing agent is needed in procedure 1, especially for Fe(II1) and Cu(I1). Ascorbic acid works well and is widely known. Its structure is a bit complicated, but the active part is quite comprehensible. Its formula is easily remembered if written as H~C6H606.Mercaptoacetic acid is also effective, very simple structurally, and not too malodorous. The inorganic reductants sulfite and hypophosphite could be used, in fact, but the oxidized products could then precipitate some metallic ions from later groups. (The original presence of sulfate and phosphate in unknown solutions is perfectly acceptahle, however, since incompatible cations obviously cannot also he present.) To help extract the bromo-complexes into methylene chloride we can use a large cation (Cat+ in Table 5) in a moderately priced salt. Methyltripbenylphosphonium (Mtp+, CHdC6H&P+) can be viewed as an analog of ammonium ion. Unlike some snecific organic reagents, but like the reductants just mentioned, its effectivenrss is largely independt.nt ofany mvsterinus details ofstructure.l The students should already hdw encountered the benzene ring. Cyanocobaltutr is similar t u the well-known cvanoferrates andis scarcely organic. I t also lacks the toxicity of cyanide (4). Cupferrate, Cd-Nz02- (not quite a nitrosamine), should he considered as simply a phenyl derivative of the hyponitrite ion, N2022-. Moreover, students can benefit here from learning about a reagent important in practical analysis. Phosphate, however, can be substituted for cupferrate for most, hut not all, members of this group (not vanadium). I t also is used a t a pH of 3 (hot) but is somewhat less effective in competing against interfering chelating agents. For separations within the groups we have used a great variety of procedures, hut always with few organic reagents.
6
' Methyltrioctylammonium ion, i.e. CH3(C8H,,),N+, appears at this writina to be at least as effective if used as an extractant in CHeCCI., " or d~~sobut).l ketone However, II has not yet oeen tested thoroughly Hexadecyltr~melhylammonlumappears I kew~seto oe an exce lent precwant The author would welcome collaborat8on on further testing. >
Both thioacetamide, widely used in sulfide systems, and cupferrate are suspected as possible mild carcinogens. We might note that zinc salts (required nutrients) also have been so classified (5).Whether methylene chloride poses any similar significant health hazard at all is disputed, but it certainly does so less than chloroform (6).In any case, we can probably use other cationic extractants with other solvents.' Our remaining reagents appear to call for no unusual concern. Periodicity Figure 4 shows the distribution of elements in a common form of the hydrogen-sulfide analytical scheme. This reveals clearly the prohlems of explilininh the relarionships among the traditional analytical groups. f e must also recall that even the inter-group boundaries shown are further drastically modified if unknowns contain various common interfering substances. Several points in partial defense of the sulfide scheme, however, are needed for a full and fair appraisal. The HCI precipitation of Ag, W, Hg, T1, and P b could he omitted, esneciallv since the last three are not comnletelv removed anyway, and these metals would then be weli-behaved metnbers of the acid sulfide gmup (7-9). The Iodide Scheme produces a somewhat similar first group, hut it does remove TI and Pb essentiallv n,mpletely from solution, and it ~ u t Cu, s Ag, and Au together. This scheme is not ready a t iresent; however, to handle all the metals in the periodic table. We can illustrate the pedagogical value of periodicity with one condensed interpretation: The heavy-metal cations on the right side of the periodic chart, compared to the cations on the left, have large nuclear charges, and large effective (poorly shielded) nuclear charges. These act on the loosely held, easily polarizable electron clouds of large negatively charged atoms such as Br- and I-, and bind them relatively strongly (10). In our analytical Group 1we thus have such reactions as:
+
--
Cd2+ 4 Br- CdBr4z(or Cd[Hz0In2+ 4 B f CdBraZ- n HzO) CdBrd2- 4 I- CdIa2- 4 Br-
+
+
+
+
Such qualitative considerations, taken alone, are sometimes fallible and they may at times have to serve only as rationalizations. Yet thev connect us with fundamental causes in a way that is possible neither with the more precise language of thermodvnamics, which demands the nrior measurement of quantities similar to those to be pre&&ed, nor with the upper reaches of quantum mechanics, which would elude most beginning students. A discussion of thisgort, deepened, qualified, and hroadened, can give the student an idea of what t o expect for any element, without a detailed study of each one. When, on the
Figure 2. Groups in the Bromide Scheme. Ld = lanthamids or rare earths. Groups separated: 1. Mainly bromo-complexes, MetBr,v-; Metl,Y-: Met. 2. Cyanocobaltates, approx. KM~I[CO(CN)~].
54
Journal of Chemical Education
3. 4. 5.
Cupferrates:MetCfr,; MetO,Cfr,. Fluorides, MelF,. (Soluble).
(Or phosphates.)Clr = C & I - N ~ O ~ .
using both bromide and iodide. (Cyanocobaltate is also nonbasic, but does not interfere in the sulfide schemes.) Various interfering, basic, organic and inorganic ligands, on the other hand, can he inactivated with high concentrations of hydrogen ion. Unfortunately, however, some desirable precipitants are also inactive in acidic solution. Still, we can exploit some basic reagents, such as cupferrate and fluoride (31, for which the metal ions remaining in solution can nevertheless compete against high or moderate acidities. The net result is that we can use acidic conditions throughout our schemes to eliminate most sources of interference. (Oxalate can still precipitate calcium prematurely in Group 3. Bromate can he added, however, to oxidize some HBr to bromine, which, when hot, destroys oxalate.) We tested the effectiveness of this approach. Half of aclass of 36 students (at International Christian University in Tokyo) received the usual variety of unknowns. The other half received the same unknowns, but with half the water consisting of filtered orange juice. There was no difference a t all in the accuracy of results. This was the only test of this sort, however, partly because the possible variety of such unknowns is limitless. I t seems preferable ro continue the exploratury checking of many potential improvements than to concentrare too much on any one version, which we expect to supersede anyway. Students benefit by knowing that this is not a finished science. We have nevertheless done great numbers of tests on synthetic mixtures, checking each of a large variety of "interfering" substances with metals that might be problematic. Difficulties that arose in earlier versions appear to be largely solved.. How does the students' accuracy, using "realistic" un-
other hand. the membershin of several analvtical nrouns aDpears to be distributed somewhat randoml;aroun;d the peiiodic table. even in the absence of interfering- aaents, - . useful correlations may seem less convincing to students. Soon enough. of course. we must introduce the other part of the real worid, the exceptions. But this may be more effective if we can show first that some generalizations do work fairly well in practice. In the present schemes Mn, of the "common" elements, is slightly exceptional. Small amounts of it, less than millimolar, remain after separating Group 2. These do not interfere subsequently, and they are much less than the amounts of T1, Ph, and Hg(I1) left after the usual chloride group, hut they might justify assigning Mn partly to our Group 5. At that point they could be removed with Fe(CN)&. In the Iodide Scheme, Zn behaves similarly. No tests have been done with Po, Fr, Ra, Ac, Pa, the trans-uranium elements, or some rare earths. The other rare metals including Tc, however, have been checked experimentally a t least to a limited extent. The general literature contributes to confidence about the unchecked group classifications. Interference
Interfering species can he classified as basic and non-hasic (with respect to H+). Bromide and iodide ions, which can prevent the precipitation of various sulfides, are non-basic. High levels of acidity, such as required for separation of the first sulfide group from the others, therefore do not inhibit this interference. One thing we can do, however, is t o follow the rule, "If you can't lick 'em, join 'em," as we have done here by
.
as in Table 1
as in Table 1
Add HCI
KI redagt.
POI, etc.
I
AlCfr, CrCfr, etc. (or phosphates).
separately.)
Gr. 4
Gr. 3
Gr. 2
KFeCo(CN). KZ~CO . ( C. N ) ~
CUI
1
1
1
I Gr. 1
GI.5 Na* etc. (Check NH,'
as in Table 1
MgFa
I
CaF, etc.
I
Figure 3. Group separations in the Iodide Scheme.
------
3
4
K
Ca
Sc
Rb
Sr
Y
Zr
Cs
Ba Ra
Ld
Hf
Ac Th Pa ................................... Fr
Mo
Tc
2
Ru. Bi
Po
At
U
Figure 4. Groups in the H,S Scheme. Ld = lanthanoids or rare earths. Groups separated: 1. Chbrides; tungstic acid. 2. Sulfides. 3. Sulfides; hydroxides. 4. Carbonates. 5. (Soluble).
Volume 61 Number 1
January 1984
55
CIP
I
I
HBr distill ete. BrCI-
Ib~Sc I H,S
dNHs
I
I
CH,CO,-
H,Sc
H+' H S O l
~ g " CI- HIS
SO,'-
C0,'-
C!-
NH,IHCO,-h
CI-
SO,'-"
I- Ago
CI-/HNO.-evaporate.
I
NH,
I
I
HS-
I (2)
HCO;'
II H S
CH&O,H C.0,'-' So,'-"
(ll,J2)
NH3 H P O P
(13)f C0,'-g
CI-
CI-
C,O,'-
Classical Scheme, Modern Forms
HCO,?
HSF
NH, HPO,'.
H S
C1-
I
I
OH-
I
HSO.-O
S'
HCI
(14)
C O S 1 HCI CH,CO;
HS-IS,"
SO,'-loxy-anis
S,O.'-
NH,
HC0.-
HzS
NH, etc. HS-
HSO4P'
OH- CO,'-a
HC0.-etc.
(16-IS), (19); OH-
(2621) (22)
i
I
I
I
H* ; NH,
/
oxy-anilHS-"
(33-39). (40-42)f
H* CH3CO2-
NHI
HCO:
CI-
CH,CHOHCO,H H,PO,-
NH,
SCN- C,H,N (pyridine)
(45)
CI -
CH,CO,-
SO,"
OH-
(46)
NaNO, NaOH Na.CO,
OH-
fusd
O
(15)
1 HB~-C~~+P
