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dc.contributor.authorSsekaalo, Henry
dc.description.abstractThe pH dependence of formation of tris(2,4-pentanediono)chromium(III)J, Cr(acac), in the absence -end presence" of carboxy1ate ions" ha s been investigated to determine"f'actors;other than the pH" affecting the extent of reaction completion. The reactions were followed by spectroscopic techniques. The Cr(acao)3 formation in the absence of carboxylate ions was used as< a -\asis t'or comparison in all the other studies. The " study was carried out over the pH range ot' 1.0 to 12.0 in view of continuing literature statements about the quantitative formation and extraction of Cr(acac)3 species. These of sodium hydroxide and ammonia t'or the pH adjustment show~ that the formation of Cr(acac)3 was only 93% complete in the case of sodium hydroxide and 95% complete in the case of ammonia at the optimum pH values when the reaction w~s carried out at the boiling temperature (900 ) of the mixture for 30 minutes. However. the reaction as greatly inhibited in the pH range 9.5 to 10.5 then ammonia used. There appears to be little inhibition by sodium h.,vclroxide over this pITz-ange, 'ilio z-eaotLon inhibition by ammonia has been attributed to the formation of stable polymeric ~ecbromium(III) speoies wh.i ch are mer-e inert to 3ubstitut~ on by 2,4-pentanedione (acetylacetone), Hacae. '!he extent of r-eact Lon completion in the r>:resence of e ions at the boiling podrrt of the mixture has been ~tudied over the pTIrange 1.0 to 12.0 for othm10ate, trichloroothanoate and trifluoroethanoate iOM. A pH of 1.0 to 8.0 tras used for monocbloroethanoatc and dichloroethanoate ions. Sodiun bydronde ,.•as used for _the :::>Hadjustment for all the e ions studied. A 100% reaction com~letion for ethanoate and monochloroethanoate ions in -1;hepITrange of 4.0 to 6.0 ahd for triehlorocthanoate ion in the pH r~ of 1.25 to 5.00 traa found. Di.cb.Lor-oc thanoat e and trifluor- oe'bhanoate gave about 95 and 96%reaction respectively at the optimum initiGl pH value of 6.0 '!he )00% reaction completion in the :;:resence of . / . othanoate m1dmon~chloroeth~!oate ions has been attributed to the _fOI'Illation of,'the carbo:qlio/carbaxylate buffer system. · . (vii) In the case of the trichloroethanoate ion the reaction proceeds to 100% completion by virtue of its decomposition to yield the bicarbonate/carbonate buffer system. This system has the widest initial pH range over which the reaction is 100% complete. However, above pH 7..0 the reaction in the presence of the trichloroethanoe.te ion system, was found to be very slow. The slowness of the reaction in this pH region was attributed to the formation of basic trichloroethanoatochromium(III) species. The use of trichloroethanoate ion as a labilizer in the formation of Cr(acac)3 has resulted in the reduction of the time required to get 100% reaction completion f'rointhe previously claimed 30 - 60 to.10 minu- :teitw.hen. it! concentration i.e·;heldbetween 0.08 and 0 •?.8 M". A violet-green extractable species analysed as bis (2,4-pentanedl one )trl.chlo;o~tl'''.an·oatoaquochroummi(III) , ~ has be,en"isolated and characterized when the reaction was conducted in presence of the trichloroethanoate ion. This compound has some i.r. and uv-vis. absorption peaks similar to those o£ Cr(acac)3 but with reduced molar extinction ccef'f'Lcf.enst, Therefore, it was concluded r.: that the fqrmation of Cr(acac)3 in presence of the trich- I ", "' loroethanoate ion goe~'through an intermediate species where the ~cid anion is coordinated to chromium(III). A further proof of the '.carbo~late-chrom~um(III) coordination Was obtained from an ether oxtr~ct of chromium(III)/ trichloroethanoate system. The isolated green species (viii) had. some i.r. peaks similar to those of the violet-green oompound. The rat~B of Qr(acac)3 f()rmation. at temperature and, 00· . pHranges of 60 to 15 and 1.50 to 3.50 respeotively, llere determined to elucidate the labilizing or deactivating effects of car-boxyf.ato ions. The rate constants obtained for the Qr(acac)~ formation in ~he absence of the acid anions uere used for corrpar-Lson, The follmTing kinetic model 1!a5 employed to describe the rate constants. (a.) trhcr-e k2 is the observed ra"te constant Irith respect to Qr3+ in the absence of' carboxy-late ions I 1;;:3 is the rate constant rrhen tho reaction is conducted in the presence of much excess of' £H+], k4 is the rate constant due to the ~~se of fH+] (or baae ) catalyzation of.the reacti~nt Ka is the ionization constant of' Hacao; In tho presence of carboxylato ions Equ~tion (a) tcl(es the form of Equation (b) trher-ek:is the observed rate oonst arrt dl.n£'cr;+J/dt=f:~=k3-ilC4KaL"ffaeacJILff+J+k~RCO;J (b) Hith r-espect tg' er->+ in the prosenoe of car-boxy'lat e ions, ~, k5 is the la;{iliz~t-aon.,o:r: deact~vQ; oonstant 't'Tithre~poct * to the ~'ci.rb?xylatet ~C02t ion. The rolD.tionship in ill:l~ation (b) nas, hosovor-, modified as shotrn in TIiLuation(c) trhor-ekt is th_e labiliz2.t.ion or deactivation dll'1C(:r~+Jlat=k~=k3-tklJ.K;-~cacJj £ir+J+k5' (c) rato oonst-ant trhon fRc0'2J has a"constant naximumeffect. (ix) It was f'ound that cthanoatcr monochloroethanoate, dichloroothcmol1to, trichloroothanoatc, trifluoroothnnol1to and succinate ions labilizcd tho roaction at e pH of 2.50 o and temperatures of 70 and 75. The trichloroethanoate ion had the geea-ce::;-l; 1Tith a re>.teabout tuice that in the absence oi' any carboxylate ion. On the other hand, the oxalate and malonate ions deaotivated the reaction. The oxalate ion decreased the reaction rate by aboirt five. times and the malonate by tHico' the standard rate. The labilizing effect of the monocarboxylate ions ~nd succinate ion Bay be attributed to formation of easily reDlaceable carboxyl~te ligands in the coordination sphere of chromium (III) • 'Ihe slmmess of the reaction in the presence of oxalate and malonate ions, on the other hand, may be due to formation. of stable five- and sixmembE7~ rings 1Tith the metal. r.en
dc.publisherUniversity of Nairobi,en
dc.titleThe Effect Of Trichloroerhanoate And Other Carboxylate Ions On The Formation Of Tris(2,4-pentanediono)chromium(iii)-1 In Aqueous Solutionen

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