Pertanika 12(1), 79·82 (1989)
The Differences in Thermal Breakdown of H 2PtC16
and cisPt(NH3)2C12 in Carbon Furnace Atomic Absorption.
MD. JElAS HARON and WAN MD. ZIN WAN YUNUSDepartment of Chemistry,
Faculty of Science and Environmental Studies,Universiti Pertanian Malaysia,
43400 UPM Serdang, Selangor Darul Ehsan, Malaysia
Key words: Thennal decomposition; platinum compound; carbon furnace atomic absorptionspectrophotometry.
ABSTRAKKajian menggunakan analisis termogravimetri dan penyerakan x-ray menunjukkan larutan asid heksakloroplatinik (Hj'tCIJ, yang selalu digunakan sebagai larutan piawai di dalam teknik spektrofotometri serapanatom (AAS), diubah kepada PtClzyang mudah menvap sebelum menjadi logam platinum. Sebaliknya larutancis-diaminodikloroplatinum (II) (Cis-Pt(NH)zClz; cis-DDP) diubah terus kepada logam platinum. Perbezaanmekanisma penguraian oleh haba ini mungkin menjadi salah satu daripada sebab mengapa isyarat spektrofotometri serapan atom relau karbon (CFAAS) bagi HzPtClr, lebih rendah daripada cis-DDP.
ABSTRACTA study using a combination of thermogravimetric and x-ray diffraction analyses has shown that a solutionofhexachloroplatinic acid (Hj'tCI(), usually used as a standard in atomic absorption spectrophotometric (AAS)measurements, was broken down to yield volatile PtClz before being converted to metallic platinum. On theother hand, a solution ofcis-diaminodichloroplatinum(II) (cis-Pt(NH) zClz ; cis-DDP) was converted to metallicplatinum in a single step. The differences in their thermal decomposition mechanisms could be one of the l-easonsas to why carbon furnace atomic absorption spectrophotometric (CFAAS) signal from Hj'tCI~ was found tobe lower than that of cis-DDP in aqueous solution.
INTRODUCTIONStudies by earlier workers have shown differences in absorbance signals of platinum fromdifferent platinum complexes when analysedby flame AAS technique. For example, theplatinum absorbance of ( H4)2PtC14 (50 ppm)is about twofold greater than that of ~PtCI6
(Macquet and Theophanides 1974) and thecis-DDP signal is about 12% higher than that ofH2PtCI6 (Macquet et al. 1974). These differences are said to be due to the differences inthermal stability of the complexes. The abilityof the more stable complex to survive as volatilespecies in the flame is greater than the lessstable complexes; a volatile species is more easilydecomposed to free atoms hence a higher AASsignal is obtained. (Macquet and Theophanides
1975). The same explanation was also given forthe depression of platinum signals from HltCI6in the flame by several anions and cations.
HltCI6 is a compound which is formedwhen Pt metal is dissolved in aqua regia anddiluted in hydrochloric acid after the evaporation of nitric acid. The solution is used asstandard for platinum analysis using AAS techniques because it is available commercially orcan be easily prepared.
In our preliminary attempt to analyse Ptin serum and urine from patients who werebeing treated with cis-DDP, using CFAAS, it wasfound that the recovery of analysis was alwayshigh (129-165%) (Haron, MJ. 1982). Theanalysis and the recovery was done usingH 2PtCI6 as standard. It was also found that at a
MD.JELAS HARON AND WAN MD. ZINWAN YUNUS
concentration of 500 ppb Pt in 0.05M HCI, theabsorbance peak height signal of cis-DDP washigher by 38% compared to HltC16 ·
This article describes the differences inmechanism of the thermal breakdown betweenHltC'6 and ciss-DDP solutions that possiblyoccur in a carbon furnace atomic absorptionduring the atomization process. This work wascarried out as part of the effort to optimize theCFMS technique for the analysis of platinumin biological fluid i.e. serum, urine and cerebral fluid from patients after treatment with thedrug cis-DDP for various types of cancer(Rozencweig et al. 1977) .
MATERIALS AND METHODSStock standard containing 1000 ppm Pt asHltCI
6in 1M HCI was obtained from BDH
Chemicals Ltd. Cis-DDP salt was obtained fromMead:Johnson Ltd and an appropriate amountwas dissolved in 1M HCI to give the 1000 ppmPt solution used as stock standard.
The curve for thermal breakdown of thecompounds from the solutions were recordedusing a Stanton Redcroft TG 750 thermobalance which consists of a microfurnace and electronic balance, and a two-pen Speedomax stripchart recorder.
A platinum crucible (40 ml), which wassupplied by the manufacturers, was tared onthe microbalance at an argon flow rate of 20 mlmin,l. A sample solution (30 Ill) containingabout 3 mg of platinum as H 2PtCI6 or cis-DDPwas transferred into the crucible using an Eppendorf micropipette. The solutions were preparedby evaporation of 100 ml solution of 1000 ppmplatinum in 1M HCI to I ml; the evaporationwas accomplished at a temperature between 90100°e. The thermogravimetric curve was recorded as the sample was heated at a heatingrate of 30°C min,l from ambient temperatureto the maximum available temprearure of1000°C.
A Philips x-ray generator equipped with aDebye-Scherrer camera was used to record thex-ray diffraction patterns of the residues formedby heating the H
2PtCI6solutions to about 350°C
in the thermobalance or in carbon furnace usingHGA 500 Perkin Elmer graphite atomiser.
The preparation of samples in the carbonfurnace for the x-ray analysis was done as fol-
lows: A stock solution of HltCI6 (lOIlI) wasinjected into the furnace and the s9lution wasdried at lOO°C for 20 seconds with 20 secondsramp time. The temperature was increased to
350°C for 40 seconds with 40 seconds ramptime, After cooling, the procedure was repeatedthree times to obtain enough sample. Theresidue was scraped from the furnace using astainless steel spatula and analysed by x-raymethod.
RESULTS AND DISCUSSIONFigure 1 shows a trace of the thermal breakdowncurve of HltCl6 as recorded by the thermobalance. The pattern was similar to previouslyreported data (Rowston and Ottaway 1979).Schweizer and Kerr (1978) found that thethermal breakdown ofH
2PtCl
li. H
20 salt to form
platinum metal occurred through several stepsas follows;
150°-180°CH
2PtCI
6) PtC'4 + 2HCI
300°C-320°CPtCI
4) PtClt + Cit
375°C-510°CPtClt ) Pt + CI
2
Plateaus A, B, C and D (Figure 1) COlTespond to HltC1
6, PtCI
4, PtCI
2and platinum metal
respectively. In order to confirm the existenceof PtCl
t, the residue from a thermogram on
heating the solution to the plateau C and fromcarbon furnace on heating the solution to 350°was analysed by x-ray powder diffraction, following the method published by Rowston andOttaway (1979). The d spacings of the x-raypattern were calculated and compared with thestandard values for PtCI
2(The Powder Diffrac
tion File 1972). The results showed that theresidue from the thermobalance at plateau Cand the carbon furnace contained only PtCI
2,
A typical example of the x-ray pattern of theresidue from carbon furnace is shown in Table1.
PtCI" is known to have different crystalline form~, ex and ~ (Wiese et al. 1970), Thelatter, which is formed in this process (Schweizerand Kerr ]978), contains discrete Pt1iCl
l2units
and is volatile (Schafer et at. 1967); Landsbergand Schaller, 1971). From the thermogravimetric traces, the number of moles of platinum
80 PERTA! lKA VOL. 12 NO. 1,1989
metal and PtCI2
were calculated. It was foundthat 12% (mean) of the platinum is lost duringthe breakdown of PtCI 2 to Pt metal(Haron 1982). However the amount of platinum lost during the conversion varies withsample size, heating rate and flow rate of theinert gas used (Schweizer and Kerr 1978).
TABLE 1X-ray powder diffraction analysis of residue
obtained after heating 30 ul aqueoussolution of H
2PtCl
6(0.3 mg Pt) in carbon
furnace AAS to 350°C. X-ray conditions :source Cu K (1.54 nm) radiation
Lattice parameters (nm)
7 .00
6 .00
A
5 .00
4.00
u 3.00
~.'"
7 .00
Temperature uC
Themwgravimetric l'U1Ve (~ for a solu.tion of
H ,PtCl" in hydmchloric acid. 111e residue on thepleteau were H,PtCl" (A), PtCl, (B) , PtCllC) and Ptmetal (D). '[fIe line (0) is a temperature calil7ration.
Calculated
0.6730.4000.3740.3230.2920.2650.2270.1990.1870.1760.164
Literature* (PtCl)
0.6690.4030.3780.3250.2930.2680.2280.2000.1870.1770.163
Fig 1.
2 .00
700 600 500 400 300 200 100
* The Powder Diffraction File, (1972).
On the other hand, the trace of thermal breakdowll curve of cis-DDP solutionshows that the compound is converted tometallic platinum in the temperature range290-500°C in a single step (Figum 2). This issimilar to the thermogravimetric data onPt(NH
3) 4Cl2 reported earlier by Kerr and
Chester (1971). They have shown that DDP,which was formed as an intermediate duringthe thermal breakdown of Pt(NH3)4C12' wasconverted to platinum metal in a single step,as shown below:
6 .00
5.00 ciS-PdNHJ) 2C1 2
'"-S.0
4.00'0
~~
a
~3.00
~
2.00
Temperature °C
nennogravimetric CU1Ve ~ for cis-Pt(NH,),Cl, in ahydrochlOJic acid solution. The line (0) is a temperaturecali/nation.
290°-500°C5Pt( H3)2C12 ) 5Pt + 4HCl + 2N2
+ H2 + 6 H 4CI
From the thermogravimetric traces (Figure2)it can be seen that cis-DDP was converted tometallic platinum without any significant loss.
Fig 2.
700 600 500 400 300 200 100
PERTANlKA VOL. 12 NO.1, 1989 81
MD.JELAS HARON AND WAN MD. ZIN WA J YUNUS
CONCLUSIONIn a carbon furnace, cis-DDP could also be converted to metallic platinum in a single step,whereas H
2PtCl
6was shown to be converted to
PtC!., before being converted to metallic platinUln-. The PtCl
2formed, which is volatile, could
be swept out by the flow of the argon gas (usedto protect the carbon furnace) in the samemanner as in the thermobalance. This could bethe explanation for the lower platinum signalsgiven by H 2PtCl6 compared to those by cis-DDPin CFAAS. It is therefore suggested here thatcis-DDP be used as a standard for analysis ofbiological samples containing cis-DDP.
ACKNOWLEDGEMENTThe authors wish to thank Dr G.S. Fell, RoyalInfirmary, Glasgow and the late Dr. W.B.Rowston,Glasgow College of Technology fortheir assistance and for the provision of equipment for the study.
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LANDSBERG, A. and J.L. SCH/\LLER. 1971. The Kinetics and Equilibria of the Platinum-chlorineSystem. j. Less Common Metals 23: 195-202.
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(Received 31 May, 1988)
82 PERTANIKA VOL. 12 NO. I, 1989