ASTM D2008 for Ultraviolet Absorbance and Absorptivity of Petroleum Products
ABSORPTIVITY OF SOLIDS AND LIQUIDS
12. Summary of Test Methods
12.1 The range of absorptivities for petroleum products can be very wide. Probably most absorptivities of interest would fall in the range from 10(-4) to 10 L/g•cm.
12.2 In determining absorptivities it is necessary to measure absorbances in the range from 0.1 to 1.0 for optimum results. This is done by preparing solutions and selecting cells of sample path length to give absorbances in the 0.1 to 1.0 range. For an individual petroleum product the absorptivity may change so rapidly with wavelength that it is necessary to prepare several solutions in order to cover the required wavelength interval. Consideration must be given to the selection of solvent, the selection of concentration levels, and the selection of sample paths lengths to obtain optimum results.
13. Selection of Solvent
13.1 Refer to the applicable section of Practices E 169 for a brief discussion of solvents for ultraviolet use. The choice of solvent is dictated by the solubility of the petroleum product and the transparency of the solvent in the region of interest.
13.2 Use isooctane unless restricted by solubility requirements.
13.3 Use decahydronaphthalene as the first alternative solvent to be used if the sample is not sufficiently soluble in isooctane.
13.4 If neither isooctane nor decahydronaphthalene will dissolve a sufficient quantity of sample to prepare the required solution, then one of the solvents listed in the table in Practices E 169. As indicated in the table in Practices E 169, not all of these solvents are usable over the entire spectral range covered by this test method. For the purposes of this test method a solvent shall be considered to have sufficient "spectroscopic purity" when its absorbance in a 1-cm cell, using reagent water in a 1-cm cell as a reference, is less than 0.05 at all wavelengths where a sample absorbance is to be measured in a 1-cm cell. Cyclohexane (Warning - Extremely flammable. Harmful if inhaled.), carbon tetrachloride, chloroform (Warning - Carbon tetrachloride and chloroform can be fatal if swallowed. Harmful if inhaled. Can produce toxic vapors if burned.), and the alcohols listed in the table in Practices E 169 are useful alternative spectroscopic solvents.
14. Selection of Solution Concentration
14.1 Select an initial concentration of the sample high enough to provide measurable absorbance (0.1 to 1.0) at the wavelength of weakest absorption to be measured but not over 40 g/L and necessarily within the solubility limitations of the solvent.
14.2 The lowest concentration that can be prepared conveniently in the initial solution is about 1 g/L. If the sample is not sufficiently soluble at room temperature to prepare such a solution, then proceed at elevated temperature in accordance with Section 16.
14.3 Table 1 lists four recommended concentration levels for the initial solution and the required sample weights and solution volumes. Column 4 gives the range of absorptivities that will give absorbance readings between 0.1 and 1.0 when the solution is measured in a 1-cm cell.
14.4 Select from Table 1 the concentration required to measure the lowest absorptivity of interest in the sample. Note the recommended sample weights and solution volumes. These shall be used in preparing the initial solution of the sample.
14.5 If concentration levels lower than 1 g/L are required because absorptivities at the wavelengths of interest exceed 1, prepare an initial solution at the 4 g/L level (Table 1) and dilute as follows: pipet 1 to 10 mL of the initial solution into a 25 to 100-mL volumetric flask to obtain a dilution factor in the range from 2.5 to 100. Select dilution factors to obtain an absorbance reading in the range from 0.1 to 1.0 at the wavelength of measurement.
NOTE 1 - For example, 1 mL of the initial solution (4 g/L) pipetted into a 25-mL volumetric flask, which is then filled to the mark with solvent and shaken, will give a dilution factor of 25 and a concentration level of 0.160 g/L in the first dilution. Repeating this procedure on the first dilution would produce a second dilution containing 0.0064 g/L. The dilution factor for the second dilution would be 625.
15. Selection of Sample Path Length
15.1 Unless otherwise specified in a particular application of this test method, use a sample path length of 1.0 cm, and the alternative sample path length of 10.0 cm.
15.2 The procedures given in Sections 16 and 17 were written assuming the use of the recommended sample path length, 1 cm, and the use of the alternative sample path length, 10.0 cm. If in a particular application of this test method different sample path lengths are specified, the smaller shall become the recommended sample path length and shall be used where a 1.0-cm cell is specified. The larger shall become the alternative sample path length and shall be used where a 10.0-cm cell is specified.
16. Procedure at Room Temperature
16.1 Weigh by difference the recommended sample weight into the volumetric flask (see Table 1). Add solvent to partially fill the flask and shake to dissolve the sample. Fill to the mark with solvent. Mix well.
16.2 If sample does not go into solution rapidly, warm the solution by heating the flask under warm tap water. When the sample has dissolved, dilute to volume and shake to make homogeneous. Cool to room temperature. Add solvent to the mark.
16.3 Fill a 1.0-cm sample cell with the initial solution or dilution to be measured. Fill a 1.0-cm reference cell with solvent. Make sure the windows are clean. Position the cell in the cell compartment of spectrophotometer and measure the absorbance of the sample at wavelengths of interest within the range from 220 to 400 nm.
16.4 If the absorbance reading in the 1.0-cm cell is less than 0.1 at one or more of the wavelengths of interest, repeat 16.3 using a pair of 10-cm cells to obtain an absorbance reading in the 0.1 to 1.0 range.
16.5 If the absorbance reading in the 1.0-cm cell is greater than 1.0 at one or more of the wavelengths of interest, dilute to obtain absorbances in the readable range (0.1 to 1.0). Dilution factors of 2.5 to 100 can be obtained by pipetting volumes of 1 to 10 mL of the initial solution into 25 to 100-mL volumetric flasks and filling to the mark with solvent.
16.6 Determine the cell correction by measuring the absorbance of the solvent-filled sample cell compared to the solvent-filled reference cell.
17. Procedure at Elevated Temperature
17.1 If it is not possible to obtain a homogeneous solution of the sample at room temperature in the recommended solvents (16.2), it will be permissible to determine the absorbance at a test temperature high enough to ensure solubility. However, do not exceed 66°C (150°F).
17.2 Weigh by difference the recommended sample weight into a clean, calibrated volumetric flask and record the weight to the nearest 0.1 mg (see Table 1).
17.3 Partially fill the flask with solvent, and place the flask in a water bath at the test temperature. When the sample is completely dissolved, dilute to volume using solvent at the test temperature. Shake to make homogeneous and bring to the test temperature in a water bath.
17.4 Prepare spectrophotometer cell holders capable of maintaining the sample cell and reference cell at the desired test temperature.
17.5 Take proper precautions to ensure that the temperature of the solution at the time of absorbance measurement is within more or less 1.1°C (2°F) of the test temperature at which the solution was diluted to volume.
17.6 Transfer a sample of the solution (hypodermic syringe heated to the test temperature is convenient) into a 1.0-cm sample cell previously heated to the test temperature. Fill a 1.0-cm reference cell with solvent at the test temperature in a similar manner. Stopper the cells firmly. Make sure the cell windows are clean. Position the cells in the cell compartment of the spectrophotometer. Allow enough time for temperature equilibrium and obtain the absorbance as required at wavelengths of interest within the range from 220 to 400 nm.
17.7 If the absorbance reading in the 1.0-cm cell is less than 0.1 at one or more of the wavelengths of interest, repeat 17.6 using a pair of 10-cm cells to obtain an absorbance reading in the 0.1 to 1.0 range.
17.8 If the absorbance reading in the 1.0-cm cell is greater than 1.0 at one or more of the wavelengths of interest, prepare a second initial solution at a lower concentration level selected from Table 1. If this is not possible, repeat 17.6 using a pair of 0.1 or 0.5-cm cells to obtain an absorbance reading in the 0.1 to 1.0 range. Record all readings.
17.9 Determine a cell correction by using solvent in the sample cell and compare it to the solvent-filled reference cell.
18. Calculation
18.1 Calculate the absorptivity, a, of a solid or liquid sample at the specified wavelength as follows:
a = Af/cb
where:
A = absorbance of sample solution at specified wavelength minus cell correction,
b = path length of sample cell in centimetres,
c = concentration of sample in grams per litre of the initial solution, and
f = dilution factor, the ratio of the volume of the diluted solution to the volume of the original solution containing the same quantity of solute as the diluted solution. For the initial solution, f = 1.
19. Report
19.1 Report numerical values of absorptivities along with a statement of the wavelength, solvent, concentration, and cell path length used in obtaining the absorbance measurement.
