EXPERIMENT NO. 4A

GLUCOSE ASSAY

BY DINITROSALICYLIC COLORIMETRIC METHOD

Prepared by
Nam Sun Wang
Department of Chemical & Biomolecular Engineering
University of Maryland
College Park, MD 20742-2111
ENCH485

Table of Contents

Method

This method tests for the presence of free carbonyl group (C=O), the so-called reducing sugars. This involves the oxidation of the aldehyde functional group present in, for example, glucose and the ketone functional group in fructose. Simultaneously, 3,5-dinitrosalicylic acid (DNS) is reduced to 3-amino,5-nitrosalicylic acid under alkaline conditions: 
                           oxidation
aldehyde group            ----------> carboxyl group

                           reduction
3,5-dinitrosalicylic acid ----------> 3-amino,5-nitrosalicylic acid
Because dissolved oxygen can interfere with glucose oxidation, sulfite, which itself is not necessary for the color reaction, is added in the reagent to absorb the dissolved oxygen.

The above reaction scheme shows that one mole of sugar will react with one mole of 3,5-dinitrosalicylic acid. However, it is suspected that there are many side reactions, and the actual reaction stoichiometry is more complicated than that previously described. The type of side reaction depends on the exact nature of the reducing sugars. Different reducing sugars generally yield different color intensities; thus, it is necessary to calibrate for each sugar. In addition to the oxidation of the carbonyl groups in the sugar, other side reactions such as the decomposition of sugar also competes for the availability of 3,5-dinitrosalicylic acid. As a consequence, carboxymethyl cellulose can affect the calibration curve by enhancing the intensity of the developed color.

Although this is a convenient and relatively inexpensive method, due to the relatively low specificity, one must run blanks diligently if the colorimetric results are to be interpreted correctly and accurately. One can determine the background absorption on the original cellulose substrate solution by adding cellulase, immediately stopping the reaction, and measuring the absorbance, i.e. 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.

List of Reagents and Instruments

A. Equipment

B. Reagents

Procedures

  1. Add 3 ml of DNS reagent to 3 ml of glucose sample in a lightly capped test tube. (To avoid the loss of liquid due to evaporation, cover the test tube with a piece of paraffin film if a plain test tube is used.)
  2. Heat the mixture at 90º C for 5-15 minutes to develop the red-brown color.
  3. Add 1 ml of a 40% potassium sodium tartrate (Rochelle salt) solution to stabilize the color.
  4. After cooling to room temperature in a cold water bath, record the absorbance with a spectrophotometer at 575 nm. 
        3ml                             1ml                         O.D.
  reagent  --->----+            Rochelle soln --->----++-------> at 575nm
                 | |  |                             | || |
                 +-+--+                             +-++-+
                 |    |      heat                   |    |
                 |    |    -------->                |    |
                 |    |                             |    |
                 |    |                             |    |
                 |____|                             |____|
             3ml sample soln

Notes

  1. Phenol, up to 2g/l, intensifies the color density. It changes the slope of the calibration curve of absorbance versus glucose concentration but does not affect the linearity. The above procedure yields an absorbance of 1 for 1 g/l of glucose in the original sample in the absence of phenol in the reagent, as opposed to an absorbance of 2.5 for 1 g/l of glucose in 2 g/l of phenol. This property can be exploited to achieve the maximum sensitivity for dilute samples.

    Questions

    1. How much time was needed for the complete color development? Justify your answer with a plot of color intensity as a function of time.
    2. Obtain an absorption spectrum over wavelengths in the visible range (i,e. 400-700 nm). Justify the use of 575 nm chosen in the Procedure.
    3. Find the procedures for at least two other methods commonly employed to measure sugar concentrations. List the advantages and disadvantages of these methods.

    References

    1. Miller, G.L., Use of dinitrosalicylic acid reagent for determination of reducing sugar, Anal. Chem., 31, 426, 1959.

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    Glucose Assay
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    Nam Sun Wang
    Department of Chemical & Biomolecular Engineering
    University of Maryland
    College Park, MD 20742-2111
    301-405-1910 (voice)
    301-314-9126 (FAX)
    e-mail: nsw@umd.edu