| HIRANUMA APPLICATION DATA | Automatic Titrator | Data No. | K8 | Feb. 10,2021 |
| Organic acid | Fractional determination of ascorbic acid and sodium ascorbate |
1. Abstract
Ascorbic acid has properties as an acid and a strong reducing agent. On the other hand, sodium ascorbate doesn’t have a function as an acid but works as a reducing agent as well as ascorbic acid. The quantitative determination method for ascorbic acid is prescribed in JIS K 9502 and Japanese pharmacopeia. There are two determination methods for ascorbic acid; neutralization titration and iodine titration.
The fractional determination method for ascorbic acid and sodium ascorbate is introduced in this report. First, ascorbic acid is determined by the neutralization titration with sodium hydroxide standard solution (formula (1)). After that, the total amount of ascorbic acid (ascorbic acid and sodium ascorbate) is measured by the redox titration with iodine standard solution (formula (2) and (3)). The sodium ascorbate is determined by the subtraction of the ascorbic acid from the total amount of ascorbic acid.
| (I) Reaction formula for neutralization titration | ||
| C₆H₈O₆ ＋ NaOH → C₆H₇NaO₆ ＋ H₂O | ・・・(1) | |
| (II) Reaction formulae for redox titration | ||
| C₆H₈O₆ ＋ I₂ → C₆H₆O₆ ＋ 2HI | ・・・(2) | |
| C₆H₇NaO₆ ＋ I₂ → C₆H₅NaO₆ ＋ 2HI | ・・・(3) | |
2. Configuration of instruments and Reagents
| (1) | Configuration | ||
| Main unit | : | Automatic Titrator COM series | |
| Option | : | One buret and one buret head | |
| Electrode | : | Glass reference electrode GR-501BZ Connect to IE-1. Platinum electrode PT-301 Connect to IE-2. |
|
| (2) | Reagents | ||
| Titrant | : | 0.1 mol/L Sodium hydroxide standard solution 0.05 mol/L Iodine standard solution |
|
| Additive | : | 2 % Metaphosphoric acid solution | |
3. Measurement procedure
| (1) | Measurement for ascorbic acid | |
| (i) Take approx 0.4 g of sample into a 100 mL beaker and weigh it accurately. | ||
| (ii) Add approx 40 mL of DI water. | ||
| (iii) Immerse the electrodes to start titration with 0.1 mol/L sodium hydroxide standard solution. | ||
| (2) | Measurement for sodium ascorbate | |
| (i) Take approx 0.4 g of sample into a 100 mL beaker and weigh it accurately. | ||
| (ii) Add 50 mL of 2 % metaphosphoric acid solution. | ||
| (iii) Immerse the electrodes to start titration with 0.05 mol/L iodine standard solution with using optional buret and buret head. | ||
4. Measurement conditions and results
Examples of titration conditions
(1) Measurement for ascorbic acid
| Cndt No. | 1 | |
| Method | Auto | |
| Buret No. | 1 | |
| Amp No. | 1 | |
| D. Unit | pH | |
| S-Timer | 5 | sec |
| C.P. mL | 0 | mL |
| T Timer | 0 | sec |
| D.P. mL | 0 | mL |
| End Sens | 200 | |
| Over mL | 0.2 | mL |
| Max Vol. | 20 | mL |
| Constant No. | 1 | |
| Size | 0.4036 | g |
| Blank | 0 | mL |
| Molarity | 0.1 | mol/L |
| Factor | 1.004 | |
| K | 176.13 | |
| L | 0 | |
| Unit | mg/g | |
| Formula | ||
| (D-B)*K*F*M/S | ||
| Decimal Places | 4 | |
|
Auto input Param.
|
None | |
| Mode No. | 4 | |
| Pre Int | 0 | sec |
| Del K | 9 | |
| Del Sens | 0 | mV |
| Int Time | 3 | sec |
| Int Sens | 3 | mV |
| Brt Speed | 2 | |
| Pulse | 40 | |
(2) Measurement for sodium ascorbate
| Cndt No. | 2 | |
| Method | Auto | |
| Buret No. | 2 | |
| Amp No. | 2 | |
| D. Unit | mV | |
| S-Timer | 30 | sec |
| C.P. mL | 7 | mL |
| T Timer | 30 | sec |
| D.P. mL | 0.5 | mL |
| End Sens | 500 | |
| Over mL | 0.2 | mL |
| Max Vol. | 20 | mL |
| Constant No. | 2 | |
| Size | 0.4042 | mL |
| Blank | 0 | mL |
| Molarity | 0.05 | mol/L |
| Factor | 1.001 | |
| K | 176.13 | |
| L | 0 | |
| Unit | mg/g | |
| Formula | ||
| (D-B)*K*F*M/S | ||
| Decimal Places | 4 | |
|
Auto input Param.
|
None | |
| Mode No. | 20 | |
| Pre Int | 0 | sec |
| Del K | 2 | |
| Del Sens | 0 | mV |
| Int Time | 3 | sec |
| Int Sens | 3 | mV |
| Brt Speed | 2 | |
| Pulse | 40 | |
| Cndt No. | 3 | |
| Method | Calc | |
| Constant No. | 3 | |
| Size | 0.4042 | g |
| Blank | 0 | mL |
| Molarity | 0 | mol/L |
| Factor | 0 | |
| K | 116.92 | *1 |
| L | 1.125 | *2 |
| Unit | mg/g | |
| Formula | ||
| (CA-K)*L | ||
| Decimal Places | 4 | |
|
Auto input Param.
|
None |
*1 K (coefficient 1): Result of ascorbic acid by neutralization titration
*2 L (coefficient 2): Coefficient to convert ascorbic acid to sodium ascorbate (C₆H₇NaO₆ (198.11) / C₆H₈O₆ (176.13))
Measurement results
Measurement results of ascorbic acid
| Meas. No. |
Size (g) |
Titrant volume(mL) |
Ascorbic acid (mg/g) |
Statistic calculation | |
|---|---|---|---|---|---|
| 1 | 0.4036 | 2.671 | 117.028 | Avg. | 116.92 mg/g |
| 2 | 0.4021 | 2.659 | 116.931 | SD | 0.12 mg/g |
| 3 | 0.4055 | 2.678 | 116.796 | RSD | 0.10 % |
Examples of titration curves
Measurement results
Measurement results of sodium ascorbate
| Meas. No. |
Size (g) |
Titrant volume(mL) |
Total ascorbic acid (mg/g) |
Sodium ascorbate (mg/g) |
Statistic calculation | |
|---|---|---|---|---|---|---|
| 1 | 0.4042 | 9.858 | 215.001 | 110.341 | Avg. | 110.70 mg/g |
| 2 | 0.4056 | 9.922 | 215.671 | 111.095 | SD | 0.38 mg/g |
| 3 | 0.4064 | 9.925 | 215.275 | 110.649 | RSD | 0.34 % |
Examples of titration curves
5. Note
Another method for successive titration
The iodine titration method is used for the determination of ascorbic acid in this report, but there is an indophenol method as another determination method for ascorbic acid; the color change of indophenol from blue to red under metaphosphoric acid is detected as the end point of titration in this method. The relatively-high selectivity on the determination of ascorbic acid in fruit juice etc. compared with the iodine titration method is the character of the indophenol method.
Keywords： Fractional determination of ascorbic acid and sodium ascorbate, Neutralization titration, Redox titration, Iodine titration
*Some measurement would not be possible depending on optional configuration of system.
