Precautions for correct selection and use of cuvettes
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July 14 01:05:20, 2025
It has recently been discovered that errors in measurement—often caused by improper selection or use of cuvettes—are common in many experiments, and these issues are frequently overlooked by the experimenter. Today, we’ll discuss the proper selection, usage, and important precautions when working with cuvettes.
1. **Choosing the Right Cuvette**
The light-transmitting surfaces of a cuvette are made from materials that allow light to pass through within the desired wavelength range. For measurements between 200–350 nm, quartz or fused silica cuvettes are essential. (Note: Fused silica is less commonly seen, and most people still use quartz. If there’s any oil on it, make sure to check carefully.) Quartz cuvettes can be used in both ultraviolet and visible regions, but they tend to be more expensive. Therefore, if you’re not working in the UV range, it’s more practical to use regular glass cuvettes. This saves money and reduces the risk of breakage—no need to worry about losing one. Cuvettes made from ordinary silicate optical glass are only suitable for visible wavelengths between 350–1000 nm. While some may claim they can go up to 2000 nm, we won’t get into that here.
2. **Proper Use and Precautions**
When using a cuvette, ensure that the two transparent sides are parallel and placed vertically in the cuvette holder. This ensures that the incident light is perpendicular to the light-transmitting surface, minimizing reflection loss and ensuring a consistent optical path.
Most cuvettes are rectangular, with frosted sides and two polished optical surfaces. When handling them, keep the following in mind:
- **2.1** Always hold the cuvette by the frosted sides to avoid touching the optical surfaces.
- **2.2** Avoid contact with hard objects or dirt on the optical surfaces. Fill the cuvette to about two-thirds full. If liquid remains on the optical surface, gently blot it with filter paper, then wipe it with lens paper or a soft cloth.
- **2.3** Do not leave corrosive solutions in the cuvette for extended periods.
- **2.4** Rinse the cuvette immediately after use. Soaking in 1:1 hydrochloric acid and rinsing with water can help remove stubborn residues.
- **2.5** Never place the cuvette directly on a flame or electric stove, or dry it in an oven.
- **2.6** If there's any doubt about the consistency of the cuvettes during measurements, perform a self-check. Set the spectrophotometer to the actual wavelength being used, fill a set of cuvettes with distilled water, adjust one to 95% transmittance (or 100% on digital displays), and measure the others. If the difference in transmittance is no more than 0.5%, they can be used together.
- **2.7** Some experienced users suggest:
- **2.7.1** Soak the cuvette in a 2% nitric acid solution for 24 hours before use, then rinse thoroughly with water and distilled water.
- **2.7.2** Before colorimetric analysis, test each cuvette with distilled water at the measurement wavelength. Select those with an absorbance error of ±0.001 to minimize measurement discrepancies.
- **2.7.3** When emptying the cuvette, tilt it along the frosted side and pour out the liquid slowly. Avoid flipping the cuvette or placing the opening directly on filter paper. Instead, blot the remaining liquid and rinse the inside with distilled water, repeating the process to avoid contamination and unnecessary wiping.
3. **Cleaning Methods for Cuvettes**
Cleanliness plays a crucial role in the accuracy of spectrophotometric measurements. Here are some effective cleaning methods:
- The general rule is to clean based on the type of solution measured. If the solution was acidic, use a weak alkaline solution; if it was basic, use a weak acid. For organic samples, use alcohol or other compatible solvents.
- According to literature, the washing solution should effectively clean the cuvette without damaging it or affecting future measurements.
- **3.2.1** Chromic acid solutions are often used for cleaning, but they are not recommended for cuvettes. They can cause thermal stress and damage the bonding areas, and traces of chromium might remain, interfering with UV measurements.
- A better option is to soak the cuvette in a mixture of nitric acid and hydrogen peroxide (5:1) and then rinse with water. For particularly stubborn residues:
- **3.2.2** Soak in a sodium carbonate solution (20 g/L) with a small amount of anionic surfactant, then soak in a 5:1 hydrogen peroxide and nitric acid mixture for half an hour.
- **3.2.3** In a fume hood, wash with a mixture of hydrochloric acid, water, and methanol (1:3:4).
By following these guidelines, you can significantly reduce measurement errors and improve the reliability of your experimental results.