Utilization of hydralazine hydrochloride in the Potentiometric Determination of Osmium(VIII): Analysis of Binary and Ternary Mixtures

-A simple, rapid and accurate potentiolnetric method is described for tlie determination of Os(VII1) in tlie co~icentration range 0.4-4.0 mg/ml. Tlie method is based on tlie addition of hydralazine hydrochloride to Os(VII1) to reduce i t to 0s (IV). Tlie excess of hydralazine l~ydrocliloride was oxidized by iodine dissolved ill acetic acid. The liberated iodide was then potentio~netrically titrated against mercury(I1) using silver amalgam as the indicator electrode. The potential of this inetliod for siine-microdeterininatioil is important in practical applications where tlie above reaction should proceed quantitatively towards completion. The relative standard deviation for six replicate deterinination of osmiuin(VII1) in binary and ternary mixtures without the need for extractioil or heating.

The present study describes a new potentiometric method for the simetnicrodetermination of osiniuin by reduction with excess of liydralazine hydrocl~loride and then oxidation the unreacted hydralazine hydrochloride irsi~ig iodine, the liberated iodide was then titrated potentioinetrically with Hg(1I).7'lie proposed tnethod has the advantage of being simple, rapid, accurate, precise, highly reproducible with reasonable selectivity, over conventional method (3.'2."' , wide range of determination over other methods ''-I", in addition to siinplicity of reagetits and apparatus.Further, osmium in synthetic binary and ternary mixtures of colnlnon lnetal ions that usually accoinpany os~niurn in nature and manufactured sa~nples is determined by this method.

Solulions
All solutions were prepared from analaR grade chemicals and deionized water, and were standardized by the recommended method (I4).0.0385M inercilric nitrate (Aldrich product) were prepared by dissolving the appropriate weight in water and standardized according to previously method 'I4).
Osn~ium solution; Osiniu~n tetroxide stock solution was prepared by dissolving 1 .OO g of osmium tetroxide (Aldrich) from a sealed ampoule in about 100 ml of 0.2 M sodium hydroxide solution in a glass-stoppered 1.00 L. Calibrated flask (taking into account the significant vapour pressure of Os04 at room temperature and slow reduction of Os(VII1) to Os(IV) when osrniu~n tetroxide is dissolved in NaOH solution.The solution was then diluted with water and standardized iodolnetrically 'I2'.
Working solution of 0.0105 M was prepared by appropriates dilution of the above stock solution and standardized iodometrically (I2).
Hydralazine hydrochloride solution; 0.0205M7 Dissolve the solid in water and ,standardized iodometricall y.
Vanadiun~(IV) solution; 0.0405 M , The appropriate weight of atn~noeiunl metavanadate(N&V03) dissolved in water.The resulting solution was reduced with Na2S03 ' 1 5 ) , boiled for 20 min to ensure co~nplete transformation of V(V) to V(1V) and to expel the excess of SO2, and diluted with water to the required volu~ne.Tlle solution was standardized potentiometrically with EDTA as complexone tising platinum electrode as indicator electrode.
Urotropine solution; 8 %.Solution was prepared in distilled water.This solution was used for pH adjustment.

Apparatus
Tlie titration assembly consisted of a 200-m1 beaker fitted witli a silver amalgam indicator electrode and a saturated calomel electrode, a 5 .O-inl(O.O1 scale microburette, a magnetic stirrer and a potentiometer (an Orion Research Model 601 NDigital ionalyzer).

-lletern~ination of osinium (VIIr)
Aliquot of 2.0 1111 of 0.0105 M Os(VII1) solution was transferred from tlie microburette into a 100-in1 Pyrex cell and 1.0 ml of2.0%HC1 was added and followed by 2.0-8.0 in1 Of 0.0205 M hydralazine hydrochloride (pH 3.2), tlie solution was left for about 2.0 inin to ensure complete reduction of Os(VII1) to Os(IV), wliich is sliown by transformation of the yellow to a brownish red colour, an excess of iodine solution was added and dissolved in acetic acid (43) of oxidise unreacted liydralazine hydrochloride and the liberated iodide was titrated witli 0.0385 M Hg(I1) [Fig.11 as optimum titrant for highly change in potential at the end point.Tlie ainount of hydralazine hydrochloride equivalent to osiniuin(VII1) as calculated.

2-Analysis of synthetic mixtures
Synthetic mixture were prepared by mixing different concentration of pure standard materials prepared above witli different volume of 0.0105M osiniu~n solution as recorded in Table (3 and 4) and completed to tlie mark with water in a 10-in1 calibrated flask.The mixtures were analyzed using following procedures.

Results and Discussion
The proposed method depends essentially on adding to osinium(VII1) a known excess amount of liydralazine HCl, then oxidizing unreacted liydralazine HCl with iodine dissolved in acetic acid and titrated the liberated iodide witli Hg(I1) using silver amalgam as the indicator electrode Careful investigations were carried out to demonstrate the most favourable conditions to achieve inaxiintuin and constant mV reading in tlie quantitative determination of osmiuin [Fig.11.The effect of tlie following variables on tlie reaction was examined.

b.ecl o f reagent concentration
The results obtained indicated that at least 2.0 m10.0205M hydralazine HCl should be present for complete reduction process in low concentration level of osmic~m whereas 6.0 ml of the reagent should be present to achieve maximu~n reduction in high concentration level.However, 2.0-8.0 ml 0.0205 M hydralazine HCI was used in the present study to insure quantitative reaction at the upper limit of determination.

lj'fsect of acidity
The effect of acidity was investigated using nitric, phosphoric, sulphoric, acetic and hydrochloric acid.Complete reduction process with highly accuracy and precision was obtained on using l~ydrochloric acid rather than acids under examination due to the formation of O S C ~~ with the Os(1V) results from the reduction process.The effect of varying amount of HCl was investigated and found to be 1.0 ml of 2.0 % HCl which adjusted the pH of solution mixture to 3.2, the CIion present in the solution introduce in the formation of 0~~1:'.Excess amount of HCl causing interference in the determination of iodide with Hg(I1) and so decrease the accuracy of the determination.

Effect of tin~e and ten~peralure
The results obtained indicated that colnplete reduction of Os(VII1) to Os(1V) was occurred instantaneously at room temperature and the brownish red colour of Os(IV) remains on raising the temperature upto 100 OC.In aqueous solution with a pH greater than 7.0, hydralazine decotnposes, forming phthalazine; the rate of In order to establish whether the proposed inetllod exhibits any fixed or proportional bias, a simple linear regression '20) for Os(VI I I) concentrat ion was calculated (dependent variable).A student's t-and Ftest '2J'(at 95% confidence level) was applied to the slope and the intercept of the regression line [Table 21.Statistical analysis of the data shows that the calculated slope and intercept do not differ significantly froin the ideal value of unity and zero, respectively.Hence it can be concluded that there are no systematic difference between the determined and true concentrations over the examined range.The measured standard deviation (S.D.), relative standard deviation (R.S.D.), the standard analytical error and co~lfidence limits [Table 11 can be considered satisfactory at least for the level of concentrations examined.

Parameter Os(VII1) Parameter Os(VII1)
Optimum corlcentration 0.4-4.0Calculated t-value 1.93 range (mglin I) (2.57)' Regression coefficient 0.9978 Calculated F-2.07 or slope of regression value (5.05)' Shift or ifltercept of the 0.0 19 Range of error % 1 00 + I . 1 5 regression line * 95 % confidence limit.The present method for binary and ternary mixtures is lligllly reliable, because they involve potentio~netric endpoint determination [Fig. 21,wbich is known to be highly accurate [Table 3 and 41.  decompositions depends on pH, temperature and tlie kind and the concentration of anion present.At higher temperature and in acidic medium, hydralazine decomposed mainly to hydrazine (I9).The quantitativeness of the redox reactions involving Os(VII1) as oxidant and liydralazine as tbe reductant may be predicted by considering the standard reduction potentials of the half-reactions involved.
C8H8N4 + 2 P + N2H4 + C8H6N2 0 ~0 4 By using the relation of EO =(0.0592/n) log K, where E0 is the standard potential, n is the number of electrons evolved to reduce one mole of Os(V11I) to Os(VI), and K is tlle equilibrium formation constant of the reaction.The log K value was calculated to be 82.57.Such value indicates that tlie above reaction should proceed quantitatively towards completion.The degree of completion of the reaction (a) is calculated according to a = log K/nl + 112 = 82.5716= 13.76.

Analytical data
The data obtained froin potentioinetric titration show that accurate results are obtained with a good recovery over the concentration range 0.4-4.0mglml, above which low accuracy with more error than 2.0% are obtained as summarized in Table (1).The equivalence points were accurately determined with potential breaks ranging from 202 to 21 8 mV/O. 1 ml of Hg(II), sharp enough for tlie accurate determination of the endpoint.' Average of five deter~nil~ations.

Conclusion
The proposed method compares will wit11 the other("10), in regard to simplicity, rapidity, llighly reproducibility with reasonable selectivity, highly accurate and precise, in addition to siinplicity of reagents and apparatus.With the proposed method it is possible to determine oslniu~n at wide range from 0.4-4.0mglml.The neth hod was applied to a variety of syntl~etic binary and ternary mixtures to evaluate its effectiveness [Table 3 and 41.Mixtures of coininon metal ions that usually accompany osrniilln in natural and manufactured samples were prepared and analyzed.

Table ( 1
): Evaluation of accuracy and precision of the proposed potentiolnetric titration method.

Table ( 3
):Representative results for analysis of binary complexes.

Table ( 4
): Representative results for analysis of ternary mixtures