for the first NMR measurement, then adjust the stock solution pH in steps of 0.5 using NaOH, and after each titration remove an aliquot for the measurement. As an internal pH-independent standard 0.1 M 1,4-dioxane (δ_C = 67.6) is used. In alkaline solution vitamin C is very readily oxidized; thus the samples should be measured immediately or kept under an inert atmosphere.

Load standard ¹³C NMR parameters. Set and control the temperature at 300 K. You have to set:

decoupler attenuation and 90° pulse for CPD
td	64 k
sw	200 ppm
o1	middle of ¹³C NMR spectrum
o2	middle of ¹H NMR spectrum
p1	30° ¹³C transmitter pulse
d1	2 s
ns	32

5. Processing

Use standard 1D processing as described in Experiment 3.2 with exponential multiplication (lb = 2 Hz).

6. Result

The figure shows the ¹³C NMR spectrum of ascorbic acid at pH 1.2 obtained on an AMX-500 spectrometer with a multinuclear probe-head. The signal labeled D arises from the internal standard dioxane. The complete titration diagram is shown on the next page, indicating the two deprotonation steps, which can best be seen in the signals of C-1, C-2 and C-3. From the inflection points of the curves, the pK_a values were calculated as pK_a1 = 4.05 and pK_a2 = 11.7. These values compare well with literature results obtained by electrochemical methods, taking into account the relatively high concentration used in this experiment and the influence of temperature.

Carbon-13 NMR spectrum of ascorbic acid at pH 1.2