Measuring Transparent Liquids in Very Small Quantities
The instrument illustrated below is an UltraScan PRO. Measurements on an UltraScan VIS or ColorQuest XE would be carried out in a very similar fashion.
Batch-to-batch color consistency (or colorlessness, for some) is an important indicator of quality for many transparent liquids. In the pharmaceutical industry, for instance, the color of a transparent liquid may be an indication of its freshness or purity. Transparent liquids require special handling when being evaluated either visually or instrumentally. The color of a transparent sample changes when the path length is changed, so the path length must be fixed. Transparent liquids should be measured in a transmittance mode on a benchtop sphere instrument in order to assess the color of the liquids when light shines through them.
A HunterLab UltraScan PRO diffuse/8° spectrophotometer standardized in total transmittance (TTRAN) mode can be used to measure the transmittance of transparent liquids that are contained in optically-clear sample cells held in place over the transmission port using a semi-micro cell holder and optical assembly. This method is recommended by HunterLab for the measurement of very small quantities of transparent liquids.
The UltraScan PRO
The Application
Transparent liquids have several non-uniform characteristics that require compensating preparation and presentation techniques in order to ensure a repeatable sample measurement.
Liquids must be contained in and measured through a clear sample cell in order to be effectively made solid.
They are transparent--not opaque--and will be sensitive to path length changes, so a constant path length should be used.
Liquids may contain bubbles or nonhomogeneous areas that alter the color measurement, requiring the averaging of several readings with replacement.
Transparent liquids in the pharmaceutical industry have the additional characteristic of being very expensive, and so normal liquid sampling techniques are not economically suitable. Sampling techniques that use a very small sample size (approximately 0.4 mL to 8.0 mL) are more cost-effective in this situation.
| Recommended Color Scale | CIE L*a*b* or Hunter L, a, b as a full color descriptor |
| Recommended Single-Number Indices | YI D1925 (2/C), YI E313 when samples are near clear, APHA, ASTM D1500, Gardner, and/or Saybolt when samples are yellowish, Haze |
| Recommended Illuminant/Observer | D65/10° or C/2°. |
Measurement Method
| 1. Configure your software to read using the desired color scale, illuminant, and observer. | |
| 2. Install the semi-micro cell holder and optical assembly (HunterLab Part Number L02-1012-202) in the instrument's transmission compartment as described in your User's Manual, with the large lens of the optical assembly toward the instrument lens and the small lens of the assembly toward the opening in the sphere. |  |
| 3. Standardize the instrument in the total transmittance mode for the large area view, sliding the black card device between the cell holder and the hole in the sphere when prompted for the black card. |  |
| 4. When prompted for the cell blank, fill the desired analytical cell (10 mm or 20 mm standard or semi-micro) with distilled water or another clear liquid you would like to use as your top of scale. Some cells will have a fill line. The liquid should at least reach this line. | |
| 5. Slide the transmission cell into the cell holder as far as it will go. If your cell has an indicator arrow, point the arrow toward the hole in the instrument sphere. |  |
| 6. Complete the standardization by measuring the cell blank as the top of scale. | |
| 7. Remove the cell blank from the cell holder and rinse it with the sample to be measured. Fill the cell with the sample and measure it the same way you did the cell blank. Take a single color reading of the sample. Empty and refill the sample cell and read the sample at least once more. Average the multiple color readings for a single color measurement representing its color. Averaging multiple readings with replacement between readings minimizes measurement variation associated with non-uniformity. |  |
| 8. Record the average color values for the sample. |