Reexamination of Litmus from an Educational Point of View (II) -The Color Change of Litmus Paper and the Color Absorption Curve by pH Change of Litmus Solution-

Abstract

According to Japanese Industrial Standard (JIS), the litmus paper has been changing color clearly from blue to red at pH 5, and from red to blue at pH 8, using buffer solution. But, when using general solution of acid or base, the color change of litmus paper does not happen at such pH. Therefore, when color of the litmus paper (made in TOYO-Roshi) changed practically by use of typical acid and base, the authors tried to measure and ascertain the pH value. The measured color changing pH value was 3. 9 in the case of using hydrochloric acid (HCl), and 11. 4, 4. 4, and 10._4 were respectively in the case of sodium hydroxide solution (NaOH), acetic acid (CH₃COOH)and ammonia water (NH₄OH). The absorption spectra of litmus solution, impure azolitmin solution and pure azolitmin solution, these many spectra were measured at various pH values. For example, the absorption spectra of impure azolitmin solution that effused by 30% ethyl alcohol on potato starch column are shown in figure 2. As shown in figure 3, when the difference of absorbance happens over 0. 020 on the same wavelength, the difference of the solution color can be confirmed by careful observation. As shown in figure 4, when the wavelength of the peak of absorption spectrum differs over 20nm on the same absorbance, the change of the solution color can be confirmed clearly. According to the figure 5, when the absor bance differs over 0. 020 and the wavelength differs over 20nm at the same time, a little difference of the color and a slight change of the color can be confirmed clearly. Therefore, by investigating many absorption spectra, the correlation between pH change and a little difference of wavelength can be obtained as coordinates. When the::e coordinates are plotted with pH value as ordinate and the wavelength as abscissa, the color change curve for pH is obtained. In figure 7, the two lines are color change curve for pH that based on the absorption spectra of the litmus solution on the market. And the line a and b point out that there are two kind of litmus. Line a represents one kind of litmus that the solution of itis not respond so sensitively, with the pH changes on acid side. But when the pH changes from 8 to 9 on basic side, the color of it changes from red to purple and blue. Line b represents another kind of litmus that the solution of it changes from red to purple and blue, with the pH changes from 3 to 4. In figure 8, line a is a color change curve for pH by impure azolitmin that effused by water on potato starch column, but when using of 30% ethyl alcohol, the curve of impure azolitmin effused by the column is shown line b or c. Line d shows a curve by Merk's azolitmin. Line e shows a curve by almost pure azolitmin that is purified by repeating -0f liquid chromatographic techniqne.