Rationale of the rhodamine b method for antimony

VOL.

13 (1955)

RATIONALE

CHX MICA

ASALYTICA

OF

THE

RHODAMINE

B

ACTA

455

METHOD

FOR

ANTIMONY

At tlic present time rhodamine I3 is generally considcrcd to be tile best reagent for the calorimetric dctcrmination of traces of antimony. ‘I’110proccdurc in common use is based on the bcnzenc extraction of rllodnminc 13 chloroantimonate from aclucous solution. WC arc largely indchtcd to W*IXETEI~ AND Fl\rlecausc tlicy Ciln combine with cl~loroantinionatc ion to form the insolul~lc. bcnzcnc-cxtracand I~H,ClSl~Cl,. In (;Ar llydrochloric acid tal>lc, products RHSbCl~, III-i,(SbClJ,, the chief product is the violet salt RHSKI,,, which CilI1 1X isolated in fairly pure with KU%SISTSOV~, who rcportcd the form by precipitation ; this is in apccrncnt formation of this salt. Howcvcr, small amounts of the orange salts IIH,(SbCls), and ICH,ClSbCI, are also formed, cspccially in hydrochloric acid solutions stronger tllan (i:l[. When conccntrnted l~ydrocliloric acid solutions containing rcspcctivcly 0.2 13 in IO ml are miscd, an orange n~illimolc SbCl,- and U.T millimolc rhodaminc precipitate is obtained which is a misture of the two salts mentioned. Accordingly WC have tllc following species in the hydrochloric acid-hcnzenc system containing antimony (V) and rhodamine 13:

Complcxcs

in

bcnzcnc

Co mplcxcs

i XI water

13y.2 forms

in water

RI-ICI 22

RI-I’+

I$-+2 2 -+ SIX&‘i

-2

&I,.‘_A Sb(OH)C&-, (formed by References

p.

458

4

-44 17J-I&l-+ + SbCl,’

-> <-

z-

_* ud

I-ISb(OH)CI,. etc. slow hydrolysis of

SKI,-)

RJ-12(“,

;-

(I< I I&q,

12. W.

4.56

HAMETTE,

E.

E(. SANDELL

VOL.

13

(rgjfj)

The crucial step in the rhodamine 13 method is the conversion of antimony (III) of hydrochloric acid, not less than GM, is required to SbCl,-. A hi& concentration for substantially complete transformation of trivalent antimony into this ion in tho osidation step, Ccrium (IV) is one of the few suitable oxidizing agents. P~rr~an~anate, dichromatc, bromine and hypochlorite all oxidize antimony less completely than ccric sulfate to ShCl,-. Oxidation by cerium (IV) is very rapid at room temperature and only a slight cxccss is nccdcd. As long as no other oxidiz:Me ubstanccs are present, less than the 0.5 millimolc excess specified by WEBSTER AND FAIRHXL~ suffices. WARD AND Li\rCr~~ have pointed out the importance of keeping the solution cool (z 25” C). I-iydroxylaminc hydrochloride is cntircly suitable for destroying cxccss ccrium (LV), which othcrwisc would oxidize rhodsminc I!: to a procluct coloring the benzene I~xu~. It dots not reduce antimony (V) at room tempzraturc in GM llydrocl~loric acid. If only a slight excess of cerium (IV) is added, it is not necessary to blow air into the scparatory funnel to remove any chlorine above the solution, As soon as SbCl; has been formed it @ins to hydrolyze to Sb(OI-I)CI,- (which then hydrolyses to other forms) 2~G. In GM hydrochloric acid at zg” the rate of hydrolysis is about rO/‘oper minute. Since the hydrolysis product dots not react with rhodaminc I-3, it is impcrativc to add the rcngent very soon after oxidation and, moreover, to time the interval if it is more than a fraction of a minute. l”hc hydrolysis of chloroantitnonnte proceeds more slowly at lower hydrochloric ii&I concentrations, possibly l~~ausc the spccics roncting with wntcr is ?fSbCl, and not SbCI,,-. In 2.0 and 3.8M l~yclrochloric ncid the rate constants for the hydrolysis arc rcspcctivcly 2.7-rw3 :ulcl .+/,~-To-~~ min-1, as compared to g.G*rr~-~ min-1 in G.rM hydrochloric ncid (z.5”). Tile extraction cocfficicnt for antimony

is a function of the acidity and the cxccss of rlioclaminc 13. If the simplifying assumptions arc made that antimony is prcscnt as SbCl,,’ (this is at least the predominant species) in the aqueous pl~nsc and as RI-IShCI, in the bcnzcnc phase, with RI-I-‘- as the l~r~c~onlin~l~lt dye cation, WC have the relations:

from which it

follows

that 8 I-

pS-I-~ j/K.

l3ccausc of the coruples composition of rhodnminc 13 solutions (diagram above) it is difficult to write ~lCI-l-t1 as a function of hydrogen ion concentration, chloride ion and total rhodamine 33 concentration, However, for a specified concentration, Ikydrogen ion and chloride ion concentration, we may write

where [dye] refers to the total rhodamine Izcfrlwrces p. 458

I3 concentration

in the acid aqueous phase

VOL.

13 (1955)

RIIODABIINE

I3 METHOD

FOR

ANTIMONY

457

after extraction. Fig. I: shows that E increases linearly with [dye] in the range tested in GM hydrochloric acid. As the hydrochloric acid concentration is increased, the extractability of antimony falls off, chiefly because of the dccreasc in the concentration of RN+. Fig. 2 shows the fraction of antimony (V) extracted into an equal volume of benzene from hydrochloric acid solutions of various strengths at a rhodamine 13conccntratioti of 4.ro-“M. El 16-

Frg.

Ii&g. 2

I

The antimony (V) solutions were obtained by diluting a stock solution in conccntratcd hydrochloric acid, in which hydrolyzed species are not prcscnt. In biI1 hydrochloric acid, roughly ro’% of the antimony is extracted as the orange complex RH,(SbCl,),, based on the observation that the molar ratio Sb/rhodamine 13in the benzene is r.rtfi. A small, but unknown, quantity of RH,ClShCl, may also bc prcscnt. The amounts of cstracted IIH,(SbCl& and RH,ClSbCl, incrcnsc in hydrochloric acid solutions stron(;cr than Ml, but they do not compensate for the decreased extraction of ICHSbCI,, so that the total fraction of antimony extracted decreases with increasing acidity. It is nd\*antageous to carry out the extraction at an acidity lower than Gill, say at 3.11, both because the cxtrnction is then more complctc and the rate of hydrolysis is less. The greater extractability below GM acid has already been pointed out by WEI~STER AXII f;~r~~r~r.r. without explanation. Since a hydrochloric acid concentration of G.qX is required for satisfactory oxidation of antimony, reduction of the acidity must bc carried out by dilution with water immediately after this step. The rcproducibility of such a procedure remains to be tested. More complete cstraction can also be obtained by increasing the rhodamine 13conccntration above the 2.rcr411~ used by WEBSTER AND I~A~I~ALI,. ‘I’hc blank extract rcrnains

virtually

colorless,

even

if the reagent

concentration

is increased

provided the acidity is greater than 31If. Below approximately acid, the bcnzcne extract of a blank begins to become colored Refeucwes

9.

458

to ~.x:o-~J~,

3M hydrochloric violet because of

Ii.

45s

W.

RAMETTE,

I3. B.

VOL.

SANDELL

13

(1955)

increasing extraction of the colorless lactonc form of the dye, which on standing is slowly convcrtcd to the colored form (lIMi-Cl-) as a consequence of the presence extraction coefficient of hydrochloric acid in the benzene. The more favorable resulting from dccrcasing the acidity or increasing the rhoclaminc B concentration would hc of significance if the volume of benzene is relatively small compared to that of water. It would not be of importance if the two phases have equal volumes. ITinally, it should be pointed out that other metals reacting with rhodamine B may intcrfcrc more, or less, or about as much in 3M as in Ghf hydrochloric acid. Qualitative olscrvations on the strength of color imparted to IO ml of benzene by I mg of nictal arc as follows: 311c I-EC1 I-Ig Au

(II)

(III) CL1 (II) Fe (III) (2~ (III) ‘1‘1 (III)

strong strong none strong weak strong

G:ll I-ICI none WdC

none strong strong strong

1t may IX said in conclusion that the original directions of WEJ~STJZH ANJ) I;AIRIIAJ,J. scrvc very well in most work, but at times the slight modifications suggcstcd above may he aclvantagcous.

of

‘I’lw clicmistry Ortlic colorirwtric rlioclnminc 1%bcnzcnc extraction i\~ltilllOl~y is cxziminctl xntl soiirccs of error arc consitlcrctl.

mctlwcl

for the tlrtcrmirurtion

XII moycn clc la rllotlaminc 121 1n~tli:xlc ilc tlflSilKC c:)lorirnbtricluc clc l’anLlm2inc. tlon tlans lc bcndw~* cst cxamink ainsi clue se3 soufccs tl’crrcurs.

11ic hlctliotlc rlcr lcc,lorimcLrisclicn Ihxitinimiing von hntimon mit Rllxlnmin I
lb2ccivccl

13, par cxtrac-

13 mitt&

Nay

htli,

cincr

1955