Complexometric titration of gallium with fluorescent indicator

ANALYTICA

540

CIIIMICA

ACTA

COMPLEXOMETRIC TITRATION OF GALLIUM FLUORESCENT INDICATOR

VOL.

19 (1958)

WITH

bY R. I-1. A. CRRWLEY

l’hc formation of gallium 8-hydroxyquinolatc and its fluorimetric determination has been reported by several autllors1-4. In all of thcsc the authors dctcrmine gallium after extracting the complcs into chloroform, and measuring the fluorcsccncc. It is well known that EDTA (cthylcncdiaminc tctracctic acid) forms a stable complex with gallium, and several papers have been published describing a complcxomctric titration. MII.NI:I<~ titrated gallium with EDTA at p11 2.8 using gallocyaninc as indicator. PATROVSKY~ used morin as a fluorcsccncc indicator in acetic acid medium, while FLASIIKA AND ABDINI;’ added cxccss ISDTA and back titrated with standard zinc or lead solutions, using Eriochromc T3lack T or other indicators. None of these m&hods arc entirely specific, so that a preliminary separation of gallium is normally desirable. The simplest method is that dcscribcd by Mrr.~r:n, WORD AND Woourm~u8 where gallium is estractcd by diethylcthcr (methyl ethyl ketone can also be used) from amisccl sulphuric acid-ammonium chloride solution. After cstraction, gallium is rcturnccl to the aqueous phase by back-&traction with water, and the solution can bc cvaporatcd to low bulk, but not to dryness, without losing any gallium. On examination of the above methods of determination, various disadvantages became cvidcnt. The gallium hydrosyquinolatc fluorescence method wasl-4 found to be very sensitive but not very accurate for quantities of gallium greater than about IO r&g. ‘The EDTA titration method, with gallocyaninc indicate+ gave a good cndpoint when 0.0211~ EDTA was used for > 2 mg gallium, but a rather indistinct endpoint when the 0.002~lf ED’I’A solution was used. The method using E:L)TA and morin as indicator6 also gave a rather indcfinitc end-point, as in our cxpcricncc there was considerable background fluoresccncc after the end-point had been reached. The back titration method’ also gave a rather indistinct end-point when o.oorJi EDTA solutions were used. The possibility of titrating gallium with EDTA was invcstigatcd, using 8-hydroxyquinolinc as a fluorescent indicator. The titration was carried out in near darkness, with a Hanovia fluorescence lamp as U.V. source. It was found that the end-point was very sharp (A 0.1: ml using o.oozJ1 EDTA solution in a total volume of about 20 ml). The decomposition of the gallium hydroxyquinolatc by the EDTA was rapid, in absence of alcohol and chloroform. (Alcohol quenched the fluorescence while chldroform made it impossible to reach an end-point.) When csccss EDTA was added, there -_Nr. Lcuxutcr. * Now employed at Atomic Power Projects, Englh Elcctrlc Co., Wlxtutonc, References p. 54z

VOL.

19 (xgg3)

COSlPLEXOMETRIC

TITRATIOS

OF GALLIUM

5-P

was a very faint residual fluorescence. A solution pre\*iously titrated was therefore retained for comparison purposes. It is necessary to control the pry between about 2.5 and X.5, and tlw pllthalatc buffer clcscribcd by Sns~~sr.~~ was found to be admirable. Using this method. it was possible to dctcrmine gdlium in tlie rangeso-1000 /lg with reasonable accuracy, as Table I illustrates. 13elow so/g high titrations are found. using the conditions dcscribcd. Mow accurxtc titrations can bc obtained if a lower concentration of Y-liydrosyquinolinc is used, but tlw end-point is rntlicr unccrtuin. ‘l’.\l31,13 1

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r11l

50 100 zoo 300 500 20-1 a

Only

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4 .

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two drops

--

--_.

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l1ytlroxycli11noliii~

-

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‘:”

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-

-t 85 +2 -t 3 -t- 0.5 -I- 1.3 I.4 -22 + 0.5 -1-15 _---_

37 5’ 103 :01

3’0 49H 7y7 1010 r8

8900

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56 108 ZOO

1000 __

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-_

orcl1crr1or

blmth

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20

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intl1c;~tor.

15v;rporrrtc il sc~lutlon Cc~lltillllln~ fqlllitttil (for csaniplc tllilt ol~t;Llllctl ;Lftcr uxtraction of @Irum n and I~xlc extraction with water), to abo11t 5 ml*. r\tltl by ctlwr from ~.uIl,ll;Ltc-cl~lorirlc holutlon 5 ml .)y,, pot;i~s1111n hyclro~:cn I~l~thaliltc sol11t1on. and atltl 1o’!i, l wtnssium liydrosiclc (If ncccssary) hyl~ruCh~lJrlth2 solutton to lxirig the pii witlilri tlic mngc 2 5-3 5 r\tlcl 1 ml 20(,?(, Iiytlrosplanilnc In o ~~j,,wxt~c ;LCKI. Titrate, u.slng ;L IJ.1’. lamp In near tlarkncss. nncl I n1lo.o_=J ‘.‘,, I~vtlrox~cltllnolii~c** with 0 oo*Af 151>‘1‘.\, ;dcl1ng this slo\vly ns tllc fluorc?iccncc f;Lclcs, until tllc clltl-lm1nt 1s XWLChCd. This 1s best ohscrvctl by cornparmg with ;L lxcvioub titratccl gnll1um solution. I

h mcthotl is clcscribctl,

inl 0.001 :‘Lf 13lYr.A =

for titrating giilliuni with El)‘1’r\, nuitablc for 50- 1000 pg gallium.

The authclr wislics to cxprcss for thc1r 1ntcrcst 1n this work.

npprccrat~oii

Go.7 pg gallium.

using 8-liyclroxycl111nolinc

to

111s cullcngucs

iii tlio

I3

~1% fluorcsccnt

‘r-11 Rcscarcl1

intl1cator

I.nborntory

1~1~i~ISlI3~NCI~S 1 I,. Ii. * J, \\‘.

~~H,\l>A~:S,

COLLAT

1:. AND

I;l:IGL 1,.

13

ASD ~
1;.

1 IhCllT, .-I #Id

df Ch’~rl

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27

rlCff&,

(19.55)

(1954)

2Gv.

961.

E. 13. SASlJlHI~AD. .~l~~tclysl, 79 (~gj.+) 172. i~ccc~vcrl April zgth, 1958 3

* Avoid boiling strong Ii?-clrochlor1c acid solutions, as gallium may bc lost to subtract the indrcator blank, *+ It ib ncccdsary, with very low gallturn contents, solution. about 0.1 ml l3elow 50 fig gallium, use only 2 drops of I~ydroxyquinol1ilc

usunlly