ASTM D4928 for water in crude oils by Coulometric Karl Fischer titration
6. Apparatus
6.1 Karl Fischer Apparatus, using electrometric end-point. Presently there are available on the market a number of commercial coulometric Karl Fischer titration assemblies. Instructions for operation of these devices are provided by the manufacturer and not described herein.
6.2 Mixer, to homogenize the crude sample.
6.2.1 Non-Aerating, High-Speed, Shear Mixer - The mixer shall be capable of meeting the homogenization efficiency test described in Practice D5854 (API MPMS Chapter 8.3). The sample size is limited to that suggested by the manufacturer for the size of the mixing probe.
6.2.2 Circulating sample mixers, such as those used with automatic crude oil sampling receivers, are acceptable providing they comply with the principles of Practice D5854 (API MPMS Chapter 8.3).
6.3 Syringes:
6.3.1 Samples are most easily added to the titration vessel by means of accurate glass syringes with LUER fittings and hypodermic needles of suitable length. The bores of the needles used should be kept as small as possible but large enough to avoid problems arising from back pressure and blocking while sampling. Suggested syringe sizes are as follows:
6.3.1.1 Syringe, 10 µL with a needle long enough to dip below the surface of the anode solution in the cell when inserted through the inlet port septum. This syringe is used in the calibration step (Section 10). It should be of suitable graduations for readings to the nearest 0.1 µL or better.
6.3.1.2 Syringes, 250 µL, 500 µL, and 1000 µL (1 mL), for crude oil samples. For the volumetric determination procedure, the syringes should be accurate to 5 µL, 10 µL, and 20 µL (0.02 mL), respectively.
7. Reagents and Materials
7.1 Purity of Reagents - Reagent grade chemicals shall be used in all tests. Unless otherwise indicated, it is intended that all reagents shall conform to the specifications of the Committee on Analytical Reagents of the American Chemical Society, where such specifications are available. Other grades may be used, provided it is first ascertained that the reagent is of sufficiently high purity to permit its use without lessening the accuracy of the determination.
7.2 Purity of Water - Unless otherwise indicated, references to water shall be understood to mean reagent water as defined by Type IV of Specification D1193.
7.3 Xylene, reagent grade. Less than 0.05 % water. (Warning - Flammable. Vapor harmful.)
7.4 Karl Fischer Reagent - Standard commercially available reagents for coulometric Karl Fischer titrations.
7.4.1 Anode Solution, shall be 6 parts of commercial Karl Fischer anode solution with 4 parts of reagent grade xylene. Fresh Karl Fischer anode solution shall be used. Anode solution shall not be used past its expiration date. Anode solution should be replaced after 7 days in the titration vessel. (Warning - Flammable, toxic by inhalation and if swallowed, avoid contact with skin.)
NOTE 1 - Other proportions of anode solution and xylene can be used and should be determined for a particular reagent and apparatus. The precision and bias were established using the designated anode solution and xylene.
7.4.2 Cathode Solution, use standard commercially available Karl Fischer cathode solution. Cathode solution shall not be used after the expiration date and should be replaced after 7 days in the titration vessel. (Warning - Flammable, can be fatal if inhaled, swallowed, or absorbed through skin. Possible cancer hazard.)
