Solubility Curves of Colored Compounds

Most active pharmaceutical ingredients are white or off white, dissolving towards a colorless solution. Other industrial segments like dye, food, personal care and agroindustry, often make use of colored compounds. The possibility to measure the solubility of colored compounds on the Crystal16 was investigated at Radboud University Nijmegen by Dr. Hugo Meekes and Evelien Huijs.

One of the investigated compounds was potassium dichromate. Different  concentrations of potassium dichromate in water (100, 200, 300, 400, 500, 600, 700 and 800 mg/mL) were investigated with the use of the Crystal16 and its turbidity technology. Clear and cloud points were determined and plotted using the CrystalClear software. The solubility curve of this orange compound obtained with the Crystal16 was in good agreement with literature data [1].

One of the advantages of the Crystal16 instrument is the tuning step that a scientist can perform on the investigated samples. The turbidity measurement can be tuned to clear solutions in order to make it more sensitive. In the tune step, the intensity of the turbidity LED is adjusted to a value, such that the transmission signal will be set to 100%. The tune step should therefore be programmed to take place when the solution is clear. The result is that, as soon as particles appear in the solution, the turbidity signal will change, notifying the user of a change in the reactor. If the signal is not tuned, the LED intensity may be too low to reach 100% turbidity, or if the intensity is too high, the change in turbidity only occurs after a lot of particles have appeared in the reactor.

Tuning of the aqueous potassium dichromate did lead to a value of 100% transmissivity. The transmissivity is mainly observed in the region of 540 to 700 nm. Since this spectral range is overlapping with the wavelengths of the light source, detection of turbidity is possible.

Figure 1. Left: UV-vis spectrum of a saturated solution of potassium dichromate in water. Right: Picture of the measured solution.

For concentrations ranging from 200 to 700 mg/mL two extra peaks were observed: one before total dissolution and one after nucleation during cooling. In figure 2, both the extra peak during heating and the peak reaching 100% transmissivity are marked as a clear points.

Figure 2. Turbidity diagram of 600 mg/mL mixture of potassium dichromate and water. Measurements were performed on the Crystal16 under magnetic stirring with a bottom stirrer. Two dissolution peaks were observed which were both marked as clear points. The yellow and green line are the temperature and the transmissivity respectively. Clear and cloud points are indicated in red and blue.

When these clear points were plotted together with the observed cloud points (figure 3), a second solubility curve was found. A possible explanation is the formation of a different crystal form; probably this is a hydrate. However potassium dichromate is always mentioned in literature as an anhydrate. More research is performed at the moment to confirm the existence of the hydrated form of potassium dichromate.

Figure 3. Solubility diagram of potassium dichromate in water. Two dissolution peaks per temperature cycle were observed, both are incorporated in this solubility diagram. The clear and cloud points (mg/mL) are plotted versus temperature (°C) in red and blue respectively, the drawn lines are a guide to the eye.

The Crystal16 can be exploited for colored compounds which have a transmissivity within the range of wavelengths of the light source. This means that solutions which are yellow, orange can generally be measured. For other colored solutions, the possibility to measure turbidity is dependent on the concentration; measurements can be performed for solutions which have enough transmissivity in the range of wavelengths of the light source.


Technobis Crystallization Systems is highly grateful to Dr. Hugo Meekes and Evelien Huijs from Radboud University Nijmegen, The Netherlands for the help with this case study.
[1] Korin, E. and L. Soifer, Solubility of potassium dichromate in dilute aqueous ethanol solutions in the temperature range 283 K to 303 K. Journal of Chemical & Engineering Data, 1997. 42(6): p. 1251-1253.