The angle of rotation changes slightly with changes in temperature. The angle of rotation of most substances is expressed at 20.0°C.
In order to measure the precise angle of rotation, measurements must be taken at a constant temperature.
There are two ways to keep the temperature constant: ① circulating water at a constant temperature around the outside of the observation tube and ② maintaining a constant temperature in the observation tube's chamber. Generally, method ① is quicker at achieving a constant temperature throughout all of the liquid within the observation tube and is more stable. ATAGO offers observation tubes for method ①. If great precision in angle of rotation measurement (the converted specific rotation value) is not required, the room containing the polarimeter can be kept at approximately 20℃.
For reference, the specific rotation of sucrose changes 0.0313 degrees with a 1℃ change in temperature. According to the Japanese Pharmacopoeia, the standard specific rotation for sucrose (white sugar) is 65.0–67.0°. Therefore, even if the temperature deviates slightly from 20℃ when using ATAGO's instruments, it should not be a major issue from a control standpoint.
In the sugar refining industry, measurements using the International Sugar Scale (ISS) or of purity are expressed at 20.0℃.
Measurements are often taken while circulating water at a constant temperature of 20℃ around the outside of the observation tube. ATAGO offers observation tubes compatible with this method.
Only samples of sucrose, however, can be measured using the ISS or purity, so the values can be adjusted based on the ISS value changes corresponding to temperature. Simply put, even if measurements are taken at a temperature other than 20.0℃, the ISS at 20.0℃ can be expressed.
ATAGO's AP-300, SAC-i, and SAC-i 589/882 are equipped with two scales: ISS (International Sugar Scale without temperature compensation) and ISS(TC) (International Sugar Scale with temperature compensation). When converting to purity, temperature is already compensated for. The RePo-1's scales for ISS and purity also compensate for temperature. The standards of the International Commission for Uniform Methods of Sugar Analysis (ICUMSA) call for temperature compensation of sucrose for the ISS within 18–30℃.
Making use of automatic temperature compensation eliminates the hassle of water circulation at a constant temperature.