EVALUATION THE PRECISION OF " DOCTORE TEST" FOR MERCAPTANS DETECTION IN PETROLEUM PRODUCTS BY SULPHIDE ION-SELECTIVE ELECTRODE

: Mercaptan sulphur in petroleum products ( Motor fuel, kerosene, aviation fuel ,….etc) were detected with standard test method of British Institute of Petroleum ( IP) that marked by the number IP-30 . The detection of this method was qualitatively and the precision has not been established. In this research a précised results were tabulated for the IP-30 test method, using sulphide ion selective electrode. A comparable results of quantitative value for mercaptans of IP-30 test method now are available.

A method designated as British institute of petroleum (IP-30)(13), which corresponded to that of American Standard Test Method (ASTM-D484-52)(14) was intended for the detection of mercaptans sulphur in motor fuel, kerosene and similar petroleum products.
The results of testing in this method are reported as negative , slightly positive and positive depending on the discoloration of sulphur floating at oil-water interface or by discoloration of either of the phase as a result of shaking of the mixture of the sample with sodium plumbite solution and flower sulphur(Table .1) The precision of this method has not been established, so in this research, an evaluation has been done using Sulphide ionselective electrode.The British institute of petroleum methods (IP-30) will be then suitable for setting specifications on petroleum products as a quality control tool.
These standard solutions were checked for mercaptane sulphure using the potentiometric methods.The sample was dissolved in 100 mls of titration solvent and then titrated against alcoholic 0.1N silver nitrate; the end point was checked by Sulphide ionselective electrode.
Each standard sample was checked for mercaptan sulphur presence by (IP-30) test methods.Fresh untreated petroleum products were examined by the potentiometric mode and the (IP-30) test methods for comparison.-------(2) Because of the appreciable hydrolysis of the sulphid ion according to equations (3,4)
[ -----------(5) Thus, the sulphid measurements are difficult, since it is subjected to protonation by hydrogen ions and also it is subjected to loss by volatilization,

At pH { = < 4} , the sulphid is almost exclusively present as dissolved hydrogen sulphid gas [ H2S ] , at pH { => 4} the hydrogen sulphid progressively dissociates to form bisulphid ions [ HS -1 ] , at pH { = 7} , the solution contains equal proportions of [ HS-1 ] and [ H 2 S ] . between pH {= 9-10} the dissolved sulphid would be almost exclusively present as hydrogen sulphid ions [ HS-1 ] . If the pH raised again , hydrogen sulphid ions dissociates to form divalent sulphid ions [ S -2 ] which will be predominate at pH { => 13}.
To overcome the protonation phenomena during measurements , sulphid samples and standards were buffered and the determination of sulphid at pH { => 13} is practically useful and was recommended by Orion Research Inc. ( 15 ) In this research , buffer titration solvent was used from Isopropanole and 1% aqueous ammonium hydroxide following the USA method (UOP-163) (12 ) The sulphid samples were stored under purified nitrogen atmosphere to overcome the oxidation phenomena of sulphid ion by dissolved oxygen to sulphite , thiosulphate and sulphate.(16) The prepared standard mercaptan samples that checked by sulphide ionselective electrode showed that The standard mercaptan concentrations of 0.5 to 2.5 ppm gave negative results with (IP-30) test method.

Standard
mercaptan concentrations of 2.75 to 5 ppm showed slightly positive results with (IP-30) test method.
Standard mercaptan concentrations with (6) ppm mercaptan and above (Table 2), showed a positive results of (IP-30) test method.
To evaluate the work, fresh sample of kerosene hydro treating unit and untreated kerosene from storage tanks were checked.The negative result of (IP-30) was of 2.0 ppm mercaptan sulphur with sulphide ion-selective electrode for kerosene from the unit.
A slightly positive results of (IP-30) was of 5 ppm mercaptane sulphur with sulphide ion-selective electrode for kerosene from storage tanks.Now it can be concluded that if a sample was tested for mercaptane sulphur with (IP-30), so, its roughly results can be estimated as a quantitative results giving fast and accurate mercaptan sulphur concentration (Table -3).
AKNOWLEDGEMENT: Special thanks to laboratories department of Baiji-Refinery , the manager , Chemists and workers who helped us in arranging samples for this research.
In the past few years a large numbers of researches have been done in the field of ion-selective electrodes, the development and various applications of new electrochemical sensors continue to be a rapidly growing area of analytical chemistry.Sulphid ion-selective electrode is sensitive and capable of measuring very low levels of Ag+ and S-2 .The electrode responds either directly to silver ions or indirectly to sulphid ions.When it is used as a sensor for silver ions , it's potential follows the Nernst equation .E = EO + 2.303 (RT/ ZF) Log a Ag + -----(1) When it is used as a sensor for sulphid ions , it's potential follows the Nernst equation .E = EO ‫ـ‬ ‫ـ‬ 2.303 (RT/ ZF) Log a S -2 -