Measuring Water Solubility of Platinum Group Metal Containing Substances

Transcription

1 , JOHNSON MATTHEY TECHNOLOGY REVIEW Mesuring Wter Solubility of Pltinum Group Metl Contining Substnces Solubility dt vilble in the literture for the first time By Mtthew Gregory Johnson Mtthey Technology Centre, Blounts Court, Sonning Common, Reding, RG4 9NH, UK *Emil: The wter solubility of 22 pltinum group metl (pgm) contining substnces ws evluted to provide useful dt for regultory complince nd to id ssessment of their environmentl impct. The fl sk method from OECD Guideline 105 (1) for the testing of chemicls (wter solubility) ws used to test ech mteril. For substnces tht could not be isolted s pure solids, simplifi ed wter solubility test ws crried out. The results provide relible dt on solubility previously unvilble in the literture. Introduction There is pucity of robust dt on the wter solubility of mny pgm compounds. Besides being fundmentl property for ech substnce, this dt is essentil for number of purposes: for exmple to comply with regultory requirements (see further discussion below) nd s bsis for modelling the environmentl effects or environmentl distribution of these compounds. Wter solubility levels cn lso serve s n indiction for biovilbility of substnce nd re thus n importnt considertion when plnning nd interpreting ecotoxicology or mmmlin toxicology studies. It hs become necessry, s prt of the Europen Union regultion Registrtion, Evlution, Authoristion nd Restriction of Chemicls (REACH) (2, 3) to determine the wter solubility of every substnce plced on the mrket in quntities greter thn one tonne. In order to chieve this, the industry hs orgnised itself into consorti representing sectors of common interest to shre costs nd minimise testing. The work presented in this pper covers pgm compounds for which no relible dt on wter solubility ws vilble in the existing literture nd which re being considered by the Precious Metl Consortium (PMC) under the uspices of The Europen Precious Metl Federtion (EPMF). To our knowledge, this is the fi rst comprehensive publiction of wter solubility dt of pgm compounds. Wter Solubility Testing The wter solubility of substnce is defi ned s the sturtion mss concentrtion of the substnce in pure wter t given temperture. Wter solubility is expressed s the mss of solute per volume of wter with the SI unit of kg m 3, but g l 1 is lso commonly used. OECD Guideline 105 (1) describes two methods for determining wter solubility. These re the column elution method nd the fl sk method. The column elution method involves the use of micro column chrged with n inert crrier mteril nd n excess of test substnce. The test mteril is then eluted with wter. Wter solubility is determined when the mss concentrtion (the mss of mteril dissolved in given volume) is constnt s function of time. The fl sk method involves dissolving the test mteril in wter t higher temperture thn the test temperture. The sturted mixture is then cooled nd llowed to equilibrte t the test temperture. The mss concentrtion of the test substnce is then determined by inductively coupled plsm-opticl emission spectrometry (ICP-OES). According to the Guideline, Johnson Mtthey

2 solubility below 10 2 g l 1 should be evluted by the column elution method, wheres the fl sk method is preferred for substnces with wter solubility bove 10 2 g l 1. In this investigtion only the fl sk method ws used. The column elution method ws not considered to be pproprite for tests on inorgnic compounds due to the diffi culty of coting the support mteril with the test substnce. A preliminry test ws crried out on ech substnce to determine pproximte wter solubility. As well s determining which method ws to be used it lso estblished how much smple ws needed to crry out the test. A diffi culty in following this test method is tht severl of the substnces cnnot be isolted s pure solids (some re described s dmp solids ), nd whilst known mss of the mteril cn be tested this does not relte to known quntity of the pure substnce. In such cses Simplifi ed Wter Solubility Test ws crried out. This test involved prepring sturted solution, which ws then fi ltered, nd the mss concentrtion of the test substnce determined by ICP-OES. In the simplifi ed test the experiment ws conducted over 24 hours, which ws deemed suitble for fulfi lling the wter solubility end-point, wheres the fl sk method is repeted t 48 hours nd 72 hours (1). All of the experimentl work for ech substnce ws crried out in duplicte to give extr confi dence in the dt nd llow for clculted errors of the wter solubility of ech substnce. Some dditionl wter solubility studies were crried out by n externl lbortory. This ws done for confi dentility resons, with some compnies involved in the study feeling uncomfortble providing their mterils to competitor for nlysis. For this reson there is no comprison dt between lbortories. Experimentl Instruments A Grnt OLS200 thermostticlly controlled shking wter bth ws used to gitte the solutions t controlled temperture. The Precis 2625MA-FR nlyticl blnce ws used to ccurtely weigh out the required mount of ech substnce. A Denley BS400 centrifuge ws used to seprte the solid nd liquid phses of the test mteril fter specifi c time periods. The concentrtion of pgm in the resulting solution ws determined using Perkin Elmer Optim 3300 RL ICP-OES. The ph of the test solutions were determined with Mettler Toledo MP225 ph meter. Chemicls De-ionised wter with resistivity of 18.2 MΩ cm, produced by Milli-Q Advntge A10 system from Millipore, ws used to dissolve the test substnces. For ICP-OES mesurements, pgm stock stndrds, prepred in-house nd stndrdised grvimetriclly, were used to provide clibrtion stndrds. These were used to quntify the pgm content of the sturted solutions obtined from the test method. The pgm mterils tested were sourced from EPMF member compnies registering these substnces under REACH. The tested mterils were considered representtive smples. For commercil resons, the producers re not specificlly mentioned for ech individul mteril but re listed in the Acknowledgement in this pper. Preliminry Test In order to clculte how much smple ws required for the nlysis, preliminry test ws crried out to determine the pproximte solubility. Incresing volumes of wter were dded to 0.1 g of test mteril with the mixture being shken between ech ddition for 10 minutes. The ddition of wter ws continued until either the substnce hd completely dissolved or 100 ml of wter hd been dded. In the ltter cse n pproximte solubility of <1 g l 1 ws recorded. OECD 105 Flsk Method From the preliminry test, the quntity of test mteril necessry to sturte 10 ml of wter ws clculted. Five times this mount ws weighed into conicl fl sk. This ws repeted in fi ve further fl sks to llow the study to be crried out in duplicte, for three different nlysis times. To ech fl sk, 10 ml of deionised wter ws dded. The fl sks were stoppered tightly with rubber bungs nd gitted t 200 rpm in shking wter bth t 30ºC. After 24 hours two fl sks were removed from the wter bth nd llowed to equilibrte t room temperture for further 24 hours. Smples from these fl sks were then trnsferred to centrifuge tubes nd centrifuged for 30 minutes to seprte the solid from the liquid. The solution ws nlysed by ICP-OES for the pgm content nd the ph ws mesured. The other four fl sks were treted similrly, Johnson Mtthey

3 nd fter initil equilibrtion t 30ºC two fl sks were removed t 48 hours nd the fi nl two fter 72 hours. If the lst two vessels vried by less thn 15%, the test ws deemed stisfctory. If the lst two vessels vried by more thn 15% then the nlysis ws repeted with longer equilibrtion time until the vrition ws less thn 15%, stisfying the criteri specifi ed in the Guideline (1). Simplified Wter Solubility Test 0.2 g of test mteril ws dded to 20 ml of deionised wter in conicl fl sk. In cses where the substnce ws only vilble s solution, the equivlent of 0.2 g of the initil solution (for exmple 2 g of 10% solution) ws used. This ws done for rhodium 2-ethylhexnote solution in 2-ethyl hexnol, dihydrogen tetrchloroplldte, pltinum(0) divinyltetrmethylsiloxne complex, plldium(ii) nitrte nd pltinum(ii) nitrte. The fl sks were stoppered tightly with rubber bungs nd gitted t 200 rpm in shking wter bth t 30ºC. After 24 hours the solutions were fi ltered using 0.1 μm cellulose nitrte membrne fi lters nd the pgm content ws determined by ICP-OES. The ph of the solution ws lso mesured. The nlysis ws repeted for ech mteril to provide duplicte dt. Additionl Tests Some dditionl wter solubility studies were crried out by seprte lbortory (AQur GmbH, Mrl nd Hnu, Germny) on further fi ve pgm contining substnces. The sme fl sk method from OECD Guideline 105 ws used for ech substnce. The following pprtus were used in this dditionl work: Heidolph Promx 2020 shking mchine; Mettler Toledo XS205 nlyticl blnce; Lud Thermostt RMS 6; Hereus TM Vrifuge TM GL centrifuge; Horib Ultim 2 ICP-OES; Metrohm 736 GP Titrino burette; Hmilton Mikroelektrode ph electrode. The regents used in the tests were: purifi ed wter produced from Milli-Q system; 1.0 g l 1 stndrds of pltinum, plldium, iridium nd ruthenium trceble to stndrd reference mterils (SRM) from the Ntionl Institute of Stndrds nd Technology (NIST). Results The solubility of ech of the substnces ws clculted using the pgm concentrtions obtined from the ICP-OES nlysis nd the pgm ssy from the certifi cte of nlysis nd conformnce provided with ech substnce. Tble I shows summry of the wter solubility of ech of the test mterils nlysed by the OECD 105 Flsk Method. The results for dicrbonyl(2,4-pentnedionto)rhodium(i) showed tendency of incresing vlues t ech equilibrtion time. The experiment ws repeted t dditionl 24 hour intervls until the method criteri were met. In this prticulr cse the smples were repeted with equilibrtion times up to 192 hours nd 216 hours. Tble II shows summry of the wter solubility of ech of the test mterils nlysed by the simplifi ed wter solubility test. The mount of test mteril used ws suffi cient to produce sturted solution for substnces with wter solubility below 10 g l 1. For substnces tht were soluble t this level, >10 g l 1 ws reported s the limit for the wter solubility. The ph of ech of the test solutions ws mesured nd the results of this cn be seen within Tble I nd Tble II. A summry of the wter solubility dt obtined from AQur cn be seen in Tble III, long with the ph of ech test solution. Tble I Wter Solubility of Substnces Tested Using OECD 105 Flsk Method (1) hexchloropltinte(iv) ± Plldium(II) chloride ± Dimminedichloroplldium(II) ± Rhodium(III) iodide < (Continued) Johnson Mtthey

4 Tble I (Continued) Rhodium(III) nitrte hydrte ± Ruthenium(III) chloride hydrte ± Dicrbonyl(2,4- pentnedionto)rhodium(i) hexchlororhodte(iii) Dimmonium sodium hexkis(nitrito-n)rhodte Tetrmmonium decchloromu-oxodiruthente(iv) ± ± ± ± Iridium(IV) oxide ± hexchlororuthente(iv) ± This method ws pplied to pure substnces Tble II Wter Solubility of Substnces Tested Using Using the Simplified Test Rhodium trihydroxide ± Rhodium 2-ethylhexnote solution in 2-ethyl hexnol ± Ruthenium trihydroxide < Dihydrogen tetrchloroplldte Pltinum(0) divinyltetrmethylsiloxne complex Tetrkis(triphenylphosphine) plldium(0) >10 b ± < Plldium(II) nitrte >10 b 1.2 Pltinum(II) nitrte ± hexchloroplldte(iv) hexchloroplldte(iv) ± >10 b 1.3 See text for description. This method ws used for substnces in which the quntity of pure mteril could not be determined, including those supplied s dmp solids b The wter solubility of these substnces ws t or bove the level specifi ed in the test Johnson Mtthey

5 Tble III Wter Solubility of Substnces Investigted by AQur tetrchloropltinte(ii) Tetrmmineplldium(II) chloride Dihydrogen hexchloroiridte(iv) hydrte ± ± ± 5 <0.1 Ruthenium cette ± Plldium(II) 2,4-pentnedionte ± Tests were crried out using the OECD fl sk method (1) Conclusion A rnge of pgm contining substnces were nlysed for wter solubility. Ech substnce ws mesured in duplicte nd n ssocited error hs been quoted in ech cse. Two methods were used to determine the wter solubility. The shke fl sk method ccording to OECD 105 guideline ws used for pure solids nd simplifi ed wter solubility test ws used for substnces tht could not be isolted s pure solids. A column elution method is lso described in the guideline for substnces with solubility below 10 2 g l 1 but this ws rejected due to diffi culties in coting the support mteril with the test substnces. This investigtion hs provided relible wter solubility dt for rnge of pgm contining substnces which were previously unvilble in the literture. Acknowledgements The study ws supported by the Precious Metls Consortium (PMC), c/o Europen Precious Metls Federtion (EPMF), with specil cknowledgement to Dr Klus Rothenbcher, scientifi c mnger of EPMF, for providing dditionl informtion into the necessity of the study. Smples were provided by C. Hfner GmbH & Co KG, Hereus Precious Metls GmbH & Co KG, Johnson Mtthey Plc, Umicore NV nd Vle Europe Ltd. References 1. Test No. 105: Wter Solubility, in OECD Guidelines for the Testing of Chemicls, Section 1, OECD Publishing, Adopted 25th July, Regultion on Registrtion, Evlution, Authoristion nd Restriction of Chemicls (REACH), Regultion (EC) No 1907/2006, 18th December, 2006; Officil Journl of the Europen Union, L 396/1, 30th December, Corrigendum to Regultion (EC) No 1907/2006; Officil Journl of the Europen Union, L 136/3, 29th My, 2007 The Author Mtt Gregory grduted from the University of Surrey, UK, with n MChem in Chemistry. In 2007 he joined the Anlyticl Deprtment t Johnson Mtthey Technology Centre, Sonning Common, UK, where he currently works s Reserch Scientist in the Assy tem specilising in nlysis using inductively coupled plsm-mss spectrometry (ICP-MS) nd inductively coupled plsmopticl emission spectrometry (ICP-OES) Johnson Mtthey