Plant Pigments, (1906)
Irvin Walter Brandel

p. 42

Another characteristic of the quinhydrones is the fact that they are very readily reduced to the colorless hydroquinones which in turn readily absorb oxygen resulting in the reproduction of the colored compound. Many red and blue flowers, especially those which give the quinhydrone test (colorless solution in alcohol), are readily decolorized when held in an atmosphere containing a small percent of sulphur dioxide.

Thus the purple alcoholic residue from the flowers of the purple pansy (Violaceae) becomes colorless when held in an atmosphere of sulphur dioxide but regains the purple color gradually when allowed to stand in the air. In the presence of moisture the restored color is red instead of purple, due to the fact that some of the sulphur dioxide is retained by the moisture and the resulting acid turns the purple residue to red. None of the yellow flowers examined, are at all affected by even strong sulphur dioxide vapors.

p. 52

MALVACEAE Observer Color of flower Color of alc. sol. Color of residue
*Althaea rosea Brandel red slightly brown dark red
 Althaea rosea71 Elsner purple partly decol. purple red
" "72 Hanausek      
" "73 Glan red    
" "74 Vogel rose    
" "74 Vogel light violet    
" "75 Vogel dark violet    
*Anoda hastata Brandel blue colorless blue
 Gossypium herbaceum76 Perkin yellow    
 Hibiscus syriacus Brandel purple colorless purple
" " Brandel white colorless colorless
" " Brandel blue colorless blue
 Hibiscus rosa sinensis Brandel purple pink purple
 Hibiscus syriacus77 Fremy blue colorless blue
 Lavatera trimestris78 Vogel rose    
 Malva rotundifolia Brandel rose colorless colorless
 Malva sylvestris79 Elser red colorless red
" "74 Vogel rose    
 Malva rosea80 Hunefeld      
 Malva rotundifolia81 Vogel rose    

Hibiscus syriacus biblio