Certain inorganic salts retain some water molecules when they crystallize. These hydrous salts are capable of changing colors when the bound water is removed. This article discusses the use of UV-Vis diffuse reflectance spectroscopy to examine the color change of powdered hexahydrate nickel sulfate, NiSO4(H2O)6 upon heating.
A Praying Mantis™ diffuse reflectance accessory with its High Temperature Reaction Chamber was used to make measurements (Figure 1). The K-type thermocouple and heater from the reaction chamber were both linked to Harrick’s Temperature Controller (Figure 2).
Figure 1. Praying Mantis™ shown with its High Temperature Reaction Chamber.
Figure 2. Temperature Controller.
The controller was interfaced to a PC through an RS-485 to USB adapter and was monitored with the Watlow EZ-Configurator software. Before being used, the temperature controller was automatically tuned to 600°C.
A commercially available UV-Vis spectrophotometer was installed with the diffuse reflectance accessory. The aperture was decreased, the data interval was set to 0.1 nm, and the spectra were measured over a range of 650 nm - 200 nm.
Initially, the sample was heated in increments of 5° beginning from ambient conditions. To ensure that the sample had reached equilibrium at each temperature, a single spectrum was taken immediately as soon as the preferred temperature was obtained, and thereafter every 4 minutes until no more difference is detected from the earlier spectrum.
In some cases, about four spectra were required. Once the sample cup achieved 70°C, the powder was heated in increments of 10°, and after 120°C, was heated in increments of 20°. The Kubelka-Munk function of the spectrometer was then used to transform the reflectance data.
Results and Discussion
Hydrous salts like NiSO4 occur as bright glowing colors in an air environment. This is because of the presence of water vapor in the air which permits nickel sulfate to develop NiSO4(H2O)6, a coordination compound with water. At room temperature, this complex is teal in appearance.
As the powder’s temperature increases, water evaporation takes place and the complex begins to gradually lose its color. Pure anhydrous NiSO4 is yellow in color, but the color becomes quite pale if it is visible to the naked eye during the heating process.
This change is obvious in UV-Vis spectra, where the sample becomes extremely yellow once heated (Figure 3). This is supported by the peak maximum shifting from 400 nm, which closely matches to blue, towards higher wavelengths. It should be noted that the peak never reaches yellow (570 nm).
Figure 3. Diffuse Reflectance Spectrum of NiSO4 at temperature from 20°C to 180°C, with the arrows showing increased temperature trends.
The Praying Mantis™ diffuse reflectance instrument coupled with the high temperature reaction chamber is suitable for identifying changes in powders with temperature. This attachment can be used to detect even the change induced by a temperature difference of 10°.
This information has been sourced, reviewed and adapted from materials provided by Harrick Scientific Products, Inc.
For more information on this source, please visit Harrick Scientific Products, Inc.