7. Sampling and Sample Preparation
7.1 Sampling is defined as all the steps required to obtain an aliquot representative of the contents of any pipe, tank, or other system, and to place the sample into the laboratory sample container. The laboratory sample container and sample volume shall be of sufficient dimensions and volume to allow mixing, as described in 7.4. (Warning - The results of the round robin have shown that for reliable results, strict adherence to the sampling and mixing procedure is of the utmost importance.)
7.2 Laboratory Sample - The sample of crude oil presented to the laboratory or test facility for analysis by this test method. Only representative samples obtained as specified in Practices D 4057 and D 4177 shall be used for this test method.
7.3 Test Sample - The sample aliquot obtained from the laboratory sample for analysis by this test method. Once drawn, the entire portion of the test sample will be used in the analysis. Mix the laboratory sample properly, as described in 7.4, prior to drawing the test sample.
7.4 Homogenize the laboratory sample of crude oil immediately (within 15 min) before drawing the test sample to ensure complete homogeneity. Mix the sample at room temperature (15 to 25°C), or less, in the laboratory sample container, and record the temperature of the sample in degrees Celsius immediately before mixing. Heat waxy samples, solid at room temperature, to 3°C above their pour point in order to facilitate test sample withdrawal. Select the type of mixer related to the quantity of crude oil in the laboratory sample container. Before any unknown mixer is used, the specifications for the homogenization test (see Annex A1) shall be met. Reevaluate the mixer for any changes in the type of crude, the quantity of crude, the shape of the sample container, or the mixing conditions (such as mixing speed and time of mixing).
7.5 For small laboratory sample containers and volumes, 50 to 500 mL, a nonaerating, high speed (3000 r/min), shear mixer is required. Use the mixing time, mixing speed, and height above the bottom of the container found to be satisfactory to Annex A1. For larger containers and volumes, appropriate mixing conditions shall be defined by following a set of procedures similar to those outlined in Annex A1 and Practice D 4177 but modified for application to the larger containers and volumes. Clean and dry the mixer between samples.
7.6 Record the temperature of the sample immediately after homogenization. The rise in temperature between this reading and the initial reading prior to mixing (see 7.4) shall not exceed 10°C, otherwise excessive loss of volatile vapors can occur or the dispersion can become unstable.
7.7 In order to ensure that crude oils with rapidly settling impurities are properly sampled, withdraw the test sample container immediately after homogenization by lowering the tip of the sample tube (see 5.3) almost to the bottom of the container, and withdrawing the test sample as quickly as possible. Clean and dry the sample tube before and after sampling.
8. Preparation of Apparatus
8.1 Extraction Apparatus - To reduce the risk of superheating and the resulting hazards, introduce a gentle stream of air into the bottom of the extraction apparatus. This can be done by passing a length of hypodermic tubing through the bore of the tap so that the lower end reaches the bottom of the heating tube, while the upper end of the tubing is passed through a rubber bung in the top of the thistle tube. Place the extraction apparatus behind a safety screen. Shield all electrical resistances and devices; alternatively, remove them from the immediate vicinity of the extraction apparatus.
8.2 Potentiometric Titration Equipment
8.2.1 Glass Electrode - Before each titration (or each series of titrations), rinse the electrode with water and soak it for at least 10 min in 0.1 mol/L hydrochloric acid (see 6.6). Then rinse again with water. After titrations store the electrode immersed in reagent water.
8.2.2 Silver Electrode - Polish the silver electrode before each set of titrations with polishing paper (see 6.13) until a clean, polished metal surface is obtained.
