Breakdown of ASTM D6304 (ppm water content determination using coulometric Karl Fischer titration)
Oct 19 2021
ASTM D6304 comprises of three procedures used to determine moisture in petroleum samples using coulometric Karl Fischer titration. In November 2020, the new revision of ASTM D6304 was approved, introducing several updates to existing procedures whilst further introducing new procedures to expand the sample administration techniques used in the standard. This article will outline the basic principle of each procedure, provide a comparison between the three and explain how ECH have been at the forefront of these method developments.
Procedure A is the method of direct injection, ideal for light liquids and fuel samples without any known compound interferences. A known volume or mass of sample is to be injected into a pre-conditioned titration cell where any present water will be automatically titrated and results calculated and shown in ppm, % or mg/kg. This technique has been the entry level method for several years and remains a tried and tested procedure. The Aquamax KF Plus from ECH has been conforming to D6304 since its inception.
Procedure B, the headspace technique. Each sample is contained within a sealed vial and administered into an oven module. Each vial is heated to the point the water vaporizes from the sample, the vaporized water is then transported from the sample into the titration cell using a non-reactive carrier gas such as pre-dried ambient air, nitrogen, or argon. Procedure B is currently the only available automated method of coulometric Karl Fischer titration, with various manufacturers offering autosamplers with a variety of samples plates. Using a closed loop system with pre dried ambient air such as the Aqua 40 Vario from ECH Scientific can provide end-users with over 1000 + samples in one charge of coulometric KF reagents, significantly more than the market average. To read more about the Closed Loop system within the Aqua 40 Vario check out the following article - ECH Closed Loop Technique for ppm water in oil determination (new revisions to ASTM D6304).
The oven evaporation technique is ideal for low ppm water content samples as unlike procedure B, there is no blank value required. A know volume or mass of sample is to be injected via a gas tight syringe into the heating chamber and the sample should be heated to the point of water vaporization. The water is transferred by a dry carrier gas (such as pre-dried ambient air) directly into the titration cell where the coulometric reaction occurs. The sample itself remains in the evaporation chamber and is subsequently removed via automatic waste removal pumps. Sample heating must be monitored to avoid the risk of decomposition from the samples itself which could lead to unwanted compounds from the sample entering the titration cell and causing as a side reaction. Using the correct temperatures and a closed loop carrier gas system yields benefits of analysing over 1000 samples in one charge of Karl Fischer solvent. The next generation of Aquamax KF systems, the Aquamax KF Pro Oil was specifically designed for titration according to procedure C.
To learn more about ECH coulometers and the changes made to D6304 sign up our e-leaning “What are the latest trends and developments in coulometric Karl Fischer titration methods used within the petroleum related markets?”.
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