DETERMINATION OF WATER CONTENT IN LIQUID PETROLEUM GAS USING KARL FISCHER TITRATION


Product Group: Hydrocarbons, Petroleum Products


General Information Concerning the Product Group Hydrocarbons


Saturated hydrocarbons can in most cases be titrated according to standard methods. To overcome solubility problems of unpolar or weakly polar substances, the addition of a solubiliser to the solvent is necessary. In the case of long-chain and cyclic hydrocarbons, long-chain alcohols (e.g. propyl alcohol or decyl alcohol) or chloroform are thus recommended. Toluene, xylene or chloroform improve the solubility of aromatic compounds. Unsaturated hydrocarbons can usually be titrated in the same way. Interferences due to double bonds only occur with some very reactive compounds. In the case of interferences (unstable end point or none at all) a methanol-free, alcoholic solvent (e.g. CombiSolvent or CombiSolvent Keto) should be utilised instead of methanol. Recommended methods are both the volumetric titration with one or two component reagents, as well as the coulometric analysis. The latter is predominantly applied for low water concentrations (< 0.1 %).


Petroleum Products


Petroleum products are mixtures of long-chain or aromatic hydrocarbons. They are hardly soluble in methanol. Water determination by Karl Fischer therefore requires the addition of solubilisers. For light oils, long-chain alcohols are suitable. For dissolving of heavier oils toluene, xylene or chloroform are added. For the volumetric titration specifi c KF solvents for oils are available. Due to the very low water concentration titrants with a low factor (2 mg/ml or 1 mg/ml) are recommended. During coulometric determination without diaphragm 20% solubiliser can be added to the working medium, or 40% solubiliser to the anolyte in the case of coulometry with diaphragm. Note that oils are often heterogeneous compounds with uneven distribution of water and should thus be homogenised (e.g. with Ultra-Turrax) prior to KF determination. Additives in oils can cause side reactions during KF determination. Here, the direct coulometric analysis is not possible, the volumetric titration only conditionally. As an alternative, the KF oven technique can be utilised in combination with coulometry, whereby the release of water is best achieved at temperatures between 120 and 140 °C.


Special Information Concerning the Sample and the Methods


To improve the solubility of the sample, long-chain alcohols are added to the methanolic titration medium as solubilisers or, alternatively, special KF solvents (for long-chain substances) are used.


Titration One Component System Reagents: Titrant


188002 Aquastar™ - CombiTitrant 2 - One component reagent for volumetric Karl Fischer titration, 1 mL = approx. 2 mg water


Solvent


50 mL 188008 Aquastar™ - CombiSolvent - Methanol-free solvent for volumetric Karl Fischer titration with one component reagents


or


40 mL / 10 mL 188009 / 803463 Aquastar™ CombiMethanol / Decanol - Solvent mixture for one component titration


Titration Parameters: Default titration settings, e.g.: I(pol) = 20 - 50 µA, U(EP) = 100 - 250 mV Stop criterion: drift < 20 µL/min Sample size: 5 - 10 mL Procedure:


The titration medium is fi rst placed into the cell and titrated dry by means of the titrant. Then the sample is added with a syringe (exact sample weight determination by weighing of syringe before and after injection) or volumetric pipette and the titration is started.


Titration Two Component System Reagents: Titrant


188011 Aquastar™ - Titrant 2 - Titrant for volumetric titration with two component reagents, 1 mL = approx. 2 mg water


Solvent


50 mL 188016 Aquastar™ - Solvent oils & fats - Solvent for volumetric Karl Fischer titration with two component reagents for oils & fats


or


40 mL / 10 mL 188015 / 803463 Aquastar™ Solvent / Decanol - Solvent mixture for two component titration


Titration Parameters: Default titration settings, e.g.: I(pol) = 20 - 50 µA, U(EP) = 100 - 250 mV Stop criterion: drift < 20 µL/min Sample size: 5 - 10 mL Procedure:


The titration medium is fi rst placed into the cell and titrated dry by means of the titrant. Then the sample is added with a syringe (exact sample weight determination by weighing of syringe before and after injection) or volumetric pipette and the titration is started.


Coulometry with Diaphragm Reagents: Catholyte


5 mL 109255 Aquastar™ - CombiCoulomat frit - Coulometric Karl Fischer reagent for cells with diaphragm


Anolyte


80 mL 109255 Aquastar™ - CombiCoulomat frit - Coulometric Karl Fischer reagent for cells with diaphragm


and


20 mL 803463 Decanol - as solubilizer Titration Parameters:


Default coulometer settings for cell with diaphragm: For end point indication, e.g.: I(pol) = 5 - 10 µA, U(EP) = 50 - 100 mV Stop criterion: drift < 10 µg/min Sample size: 1 - 2 mL Procedure:


The Karl-Fischer reagent is placed into the cathode and anode compartment of the titration cell with diaphragm. The coulometer is started and the solvent is titrated dry. After preliminary titration and stabilisation of drift the sample is injected into the titration cell with a syringe (exact sample weight determination by weighing of syringe before and after injection) and the water determination is started.


Coulometry Without Diaphragm Reagents: Working medium


80 ml 109257 Aquastar™ - CombiCoulomat fritless - Coulometric Karl Fischer reagent for cells with or without diaphragm


and


20 mL 803463 Decanol - as solubiliser Titration Parameters:


Default coulometer settings for cell without diaphragm: For end point indication, e.g.: I(pol) = 5 - 10 µA, U(EP) = 50 - 100 mV Stop criterion: drift < 10 µg/min Sample size: 1 - 2 mL Procedure:


The Karl-Fischer reagent is placed into the titration cell without diaphragm. The coulometer is started and the solvent is titrated dry. After preliminary titration and stabilisation of drift the sample is injected into the titration cell with a syringe (exact sample weight determination by weighing of syringe before and after injection) and the water determination is started.


Author Contact Details Merck KGaA • 64271 Darmstadt, Germany • Tel: +49(0)6151 72-2440 • Web: www.merckgroup.com EMD Millipore Corporation • 290 Concord Road, Billerica, MA 01821, USA • Tel: +1-978-715-4321 • Web: www.emdmillipore.com


www.PEFTEC.com


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68  |  Page 69  |  Page 70  |  Page 71  |  Page 72  |  Page 73  |  Page 74  |  Page 75  |  Page 76