following exactly the same procedures for the actual samples. When the effects of extraneous compounds are not known, one can effectively include a so-called internal standard by first fully developing the color for the unknown sample; then, a known amount of sugar is added to this sample. The increase in the absorbance upon the second color development is equivalent to the incremental amount of sugar added.
The concentration of the colored complex can be determined with the spectrophotometer set at 540nm.The sugar concentration of unknown sample can then be read off a calibration curve (standard curve) created using known sugar concentrations. The dilutions of a solution of known concentration are used to determine the concentration of unknown.
The figure illustrating how glucose react with DNS (Dinitrosalicylic acid) to give 3-amino-5-nitrosalicylic acid (red-brown) which is detected in spectrophotometer. The background information about the nature of sugar is very essential. This method detects glucose, fructose and other monosaccharide only. Sucrose cannot be detected by this method directly. Therefore, the sample containing sucrose must be digested so that glucose and fructose products can be detected; (Lanthong et