Acid–base indicators: transition colours and pH ranges determined in select aqueous–organic mixed solvents

Abstract

Transition colours and pH ranges are reported for nine acid-base indicators dissolved in selected aqueous-methanol and aqueous-l,4-dioxane solvent mixtures.

Determination of cyclamate in low-calorie foods by high-performance liquid chromatography with indirect visible photometry

“…This dominant anionic form does not change even on addition of methanol to the buffer and is consistent with the results obtained by other researchers. 23 An intense absorption band with a peak maximum of 520 nm is observed if the pH is lowered to 3.8 because of the formation of the red protonated form of Methyl Red. 24 In principle, the red protonated form of Methyl Red can also be used as the visibleabsorbing ion in the mobile phase for indirect photometric detection.…”

“…Different concentrations of phosphate buffers (0-0.2 mol dm 23 ) at pH 7.0 were used to prepare 3+2 buffer-methanol. It was found that a lower concentration of buffer gave a very unstable baseline for detection.…”

“…There is also a risk of precipitating the phosphate salt if a high concentration of buffer is used. From our experience, an optimum concentration of phosphate buffer of 0.02 mol dm 23 was chosen throughout the remainder of the experiments.…”

“…The effect of Methyl Red concentration on the sensitivity for the detection of cyclamate was investigated by varying its concen-tration in the mobile phase from 10 to 50 mmol dm 23 . A series of cyclamate standards were injected into the column.…”

“…It has been suggested that the photometric error would be smaller if the absorbance range can lie between 0.2 and 0.8. 30 Therefore, the eluent ion concentration (10-30 mmol dm 23 ) chosen should have a background absorption that lies within this range. The limit of detection can be determined as three times the standard deviation of the peak area of the blank and from results for the linear regression equations in Table 1.…”