Verificacion de la acidez de la gasolina

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Application Bulletin
Of interest to: Petrochemical industry A 1, 5
Determination of the acid and base number in
petroleum products
Summary
The determination of the acid or base number plays a significant part in the analysis
of petroleum products. This is manifested in the numerous standard procedures in
use the world over (internal specifications of multinational companies, national andinternational specifications of ASTM, DIN, IP, ISO, etc.). These procedures differ
mainly in the composition of the used solvents and titrants. Fortunately, the days
seem to be over where one laboriously worked with pH meter, rod burette and colour
indicators. Modern titrators are gradually replacing the time-consuming, manual
titrations as well as the calculation of the results.
Thisbulletin describes the determination of the acid and base number in petroleum
products by means of automatic potentiometric titration.
Instruments and accessories
• 702 SET/MET Titrino, 716 DMS Titrino, 736 GP Titrino, 751 GPD Titrino or 785
DMP Titrino or
• 726 Titroprocessor with 700 Dosino or 685 Dosimat
• 2.728.0040 Magnetic Stirrer
• 6.3014.223 Exchange Units 20 mL (with flat PCTFE/PTFEstopcock)
• 6.0229.100 LL Solvotrode with 6.2104.020 electrode cable or
• 6.0431.100 Pt Titrode and 6.0729.100 Double junction Ag/AgCl reference electrode
with two 6.2104.020 electrode cables (for three electrode technique with
differential amplifier input of the titrator).
Reagents
This bulletin does not include advice concerning the use and handling of the described
chemicals or the safetyprecautions to be taken. For this information, we
recommend that you refer to the relevant literature.
Reagents for the determination of the acid number
• Titrant: c(TBAOH) = 0.1 mol/L in isopropanol (IPA) or
c(KOH) = 0.1 mol/L in IPA
• Solvent: 500 mL toluene + 495 mL IPA + 5 mL H2O
For environmental reasons one should refrain from using chlorinated
solvents.
Application Bulletin No. 80/3 eDetermination of the acid and base number in petroleum products Page 2
• Electrolyte for
the electrodes: Tetraethylammonium bromide c(TEABr) = 0.4 mol/L in ethylene
glycol (Metrohm no. 6.2320.000)
• Standard: c(benzoic acid) = 0.100 mol/L in ethanol (1.220 g benzoic acid
in 100 mL ethanol)
Reagents for the determination of the base number
• Titrant: c(HClO4) = 0.1 mol/L in glacial aceticacid or
c(HCl) = 0.1 mol/L in IPA
• Solvent: Toluene : glacial acetic acid = 1 : 1 (volume ratio)
• Electrolyte for
the electrodes: LiCl sat. in ethanol (Metrohm no. 6.2312.000)
• Standard: c(CH3COONa) = 0.100 mol/L in glacial acetic acid (0.820 g
anhydrous CH3COONa in 100 mL glacial acetic acid)
Treatment of the electrode(s), control of the electrode’s slope
• When not in use, store theSolvotrode and the reference electrode in the respective
electrolyte solutions. The Pt Titrode should be stored in dist. water.
• Upon completion of the titration, rinse the electrodes first with toluene/IPA, then
with ethanol and finally with dist. water. (In sample changer operation, immerse
the electrodes, while stirring, for 10 s each in toluene/IPA, then ethanol, and finally
rinse well ina separate beaker with dist. water.)
• Control of the electrode’s slope:
Rinse the electrodes with dist. water and gently dry with a soft paper towel (do
not rub). Dip them in buffer solution pH = 7.00 (Metrohm no. 6.2307.110), stir
and after 1 min read off the voltage value in mV. After rinsing the electrodes with
dist. water, repeat the same procedure with buffer solution pH = 4.00 (Metrohmno. 6.2307.100).
For good electrodes the resulting voltage difference is > 150 mV (at 20 ... 25
°C). If the difference is less than 150 mV, loosen the ground sleeve of the diaphragm,
so that a little electrolyte can flow out. Then repeat the measurements
using the two buffer solutions.
Example:
Voltage for buffer pH = 7.00: 18 mV
Voltage for buffer pH = 4.00: 195 mV
Resulting voltage...
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