Indian Journal of Chemistry Vol. 33A, December 1994, pp. 112~-112:; Estimation of trace quantities of chromium using coprecipitation properties of lanthanum hydroxide ~ S Shrivastav, s'f' Mathur, A Taneja, M M Srivastava, S Das .. S Prakash & R Shrivastav" Department of Chemistry, Dayalbagh Educational Institute. Dayalbagh, Agra 282 005 Received 3 May 1994; revised 10 August 1994; accepted 30 August 1994 Coprecipitation properties of lanthanum hydroxide precipitate has been used for preconcentration of water samples, containing trace quantities (ug dm - 3) of Cr VI/CrIll. The precipitate is able to carry, quantitatively both forms ot Cr at jl.a'"] > lOOmgdm- 3 atpH > 7 and can be effectively used for trace analysis of Cr. The acute toxicity of chromium in hexavalent state has led to the development of various analytical techniques for its estimation in trace quantities'«. Inspite of very low detection limit (0.05-1.5 ug dm - 3) of some methods, preconcentration of samples is often required. The tendency of Cr to quantitatively coprecipitate in situ, with series of precipitates can be used for preconcentration of water samples- - 5. Iron(III) hydroxide and aluminium hydroxide have been widely used in this regard. However, the concentration range and other conditions, most favourable for an effective preconcentration are still ' obscure for many precipitates. In this study in situ coprecipitation ofCr, with La(OHh precipitate, was studied under different conditions, from water samples containing 0.01-1.0 mg dm -3 of chromium. The extent of binding ofCr with La(OH)3 precipitate, followed by its recovery in residue, was determined by Flame Atomic Absorption Spectrometry and by tracer technique using 51Cr isotope. The same preconcentration method was used to determine total Cr in some ground water samples. Experimental Potassium dichromate and lanthanum chloride used for preparing stock solutions ofCr vl and LallI were of A R grade and 51Crwas received from BRIT, BARC, India. The preparation of samples and preconcentration was achieved by adopting the following procedure. In the water samples 250 ern", [CrVI] = 0.1 mg dm - 3), prepared by diluting stock solution of Cr VI using preacidified (PH = 3.5)water, different doses of La"I (Table IA) were added. Sodium sulphite solution was used to reduce Cr VI in order to generate samples ofCr III . The precipitation of La III , alongwith coprecipitation of Crill Cr VI was achieved by adding calculated (necessary to generate the required pH) volumes of NaOH (6N). After centrifuging at 3200 rpm, the residue, thus obtained, was dissolved in 25 em! (final volume) of HCl (PH = 2). The chromium concentration was determined in this solution by employing Perkin Elmer 2380 Flame Atomic Absorption Spectrometer (F AAS). While using tracer technique, I cm ' of 51Cr (equivalent to 3~ Ci) solution, in respective oxidation state, was added before precipitation and 5 cm ' of the final solution was counted in a standard geometry over NaI(TL) gamma ray detector, coupled with 4k multichannel analyser. Double distilled deionized water was used through out the experiments carried out in triplicate. The results were reproducible within ± 4% deviation. Results and discussion The mean values of results, obtained for each set of experiments, are presented in Table 1. The excess of Lalli do not interfere in FAAS measurements, but at [La III ] < 100 mg dm - 3, preconcentration of samples is not quantitative with respect to chromium. Hence, further experiments were conducted only at [Lalli] = 100 mg dm - 3, which is the minimum concentration of La III able to quantitatively carry Cr from water samples, with initial concentrations ofCr ranging from 0.01-1.00 mg dm "? (Table IB). The variation inpH has alsoa marked effect and effective preconcentration is possible only at pH > 7 (Table IC). In case of Cr VI , the effect of pH might be associated with the equilibrium between CrOl- and HCrO;;. AtpH > 7, CrO~- is the major species which is largely coprecipitated, Regarding CrIll, it is known" that it exists at acidic pH as the hexa-aqua ion [Cr(H 2 0)6P +, which has pK of 4. At slightly higher pH the hydroxide ion [Cr(H 2 0)5(OH)F + is'formed which can give soluble dimers and polymers. At still higher pH, dark green gels are formed, which are coprecipitated with La(OHh. Coprecipitation of an ion by a precipitate is generally, accompanied by its adsorption followed by incorporation into precipi- tate matrix. Therefore, net carrying over of the trace