GB/T 9725-2007Chemical reagent -- General rule for potentiometric titration (English PDF)

5.1 Potentiometric titration device

Potentiometric titration device shown in Figure 1.

5.2 Potentiometer

Potentiometer accuracy of ± 2mV.

5.3 Acidity meter

The accuracy of pH meter is ± 0.02.

5.4 Electrode

See Appendix A for electrode selection.

This standard specifies.

a) glass electrode for indicating electrode, silver electrode and platinum electrode;

b) saturated calomel electrode for reference electrode, double salt bridge saturated calomel electrode;

c) Composite electrode.

5.5 Electromagnetic stirrer

1 --- Burette;

2 --- indicating electrode;

3 --- reference electrode;

4 --- beaker

5 --- test solution;

6 --- electromagnetic stirrer;

7 --- potentiometer or pH meter;

8 --- display window.

Figure 1 potentiometric titration device

6.1 Determination methods

Sampling according to the provisions of the product standards and preparation of test solution. Insert the specified electrode, start the electromagnetic stirrer, with the provisions of the standard titration solution into Line titration. From the burette, a standard titration solution of about 90% of the desired titration volume is added and the potential or pH of the solution is measured.

After each drop plus 1mL or standard titration solution to measure a potential or pH, before and after the stoichiometry, 0.1mL per drop should be added Quasi-titration solution, measured once. Continue to titrate until the potential or pH changes little. Record each drop after dropping standard titration solution the tube readings and measured potentials or pH values are used as a plot or second-order derivative to determine the titration endpoint.

6.2 Determination of the end point

6.2.1 Mapping method

The titration curve is plotted using the mV or pH of the indicator electrode as the ordinate and the reading of the burette (mL) as the abscissa. Do two and the abscissa 45 ° titration curve tangential line, and in the two tangent line with a tangent distance equal to the parallel line, the line with the titration curve the intersection point is the titration end point (see Figure 2). The abscissa of intersection point is the dosage of standard titration solution when titration end point, and the ordinate of intersection point is drop Set the end of the potential or pH.

1 --- titration curve;

2 --- tangent

3 --- parallel equidistance line;

4 --- titration end.

Figure 2 titration end point curve

6.2.2 Second-level micro-business law

Buret reading (mL) and the corresponding potential E (mV) or pH are tabulated and the following values calculated.

The volume of standard titration solution is applied dropwise (delta ).

Changes in potential or pH (deltaE or deltapH) each time a standard titration solution is dropped.

A level of micro-value. That is, the change of potential or pH value caused by standard titration solution per unit volume is equal to deltaE/delta or deltapH/delta .

Second-level derivative. Numerically equal to the difference between adjacent first derivative.

First derivative, the second derivative is equal to zero when titration end.

The volume of the standard titration solution titration end point to 0, the value expressed in milliliters (mL), according to equation (1) Calculated.

0 = + - × delta () (1) where.

--- second derivative for the volume when the standard titration solution, in milliliters (mL);

--- second derivative before the two micro-value;

--- second derivative after two micro-value;

delta --- from the second derivative to to second derivative for when the volume of the standard titration solution, in milliliters (mL).

An example of a second-level micro-business method for determining the end point is shown in Table 1.

Table 1 two micro-business law to determine the end of the instance

deltaE /

delta /

A micro-business

(deltaE/delta )

Second-level micro-business 33.00 405 33.40 415 33.60 422 33.80 431 34.00 443 34.10 455 34.20 470 34.30 514 34.40 569 34.50 588 34.60 599 34.70 606 10 0.40 25 7 0.20 35 9 0.20 45 12 0.30 60 12 0.10 120 15 0.10 150 44 0.10 440 55 0.10 550 19 0.10 190 11 0.10 110 7 0.10 70-360-80-40 in Table 1, the maximum of the first derivative is 550, and the second derivative is the zero between 110 and -360. Checked from Table 1 = 110, = -360, = 34.30 mL, delta = 0.1 mL.

0 = 34.30 mL + 110110 - (- 360) [] × 0.1 mL = 34.30 mL + 0.02 mL = 34.32 mL Appendix A.

(Informative)

Electrode Selection Reference Table Titration method Electrode system (Indication - reference) Description 1. Aqueous Neutralization Glass - Saturated calomel Composite electrode (1) glass electrode. the new electrode should be soaked in water for more than 24h before use, standing after use that is, cleaned and soaked in water.

(2) saturated calomel electrode. When using the upper electrode pin rubber plug must be pulled out to prevent Only produce diffusion potential, affect the measurement results. Electrode potassium chloride solution can not be There are air bubbles to prevent disconnection. The solution should be kept a little potassium chloride crystals to protect Certificate of potassium chloride solution saturation. Note that the electrode liquid department is not stained or blocked, and Ensure that the liquid part of the appropriate oozing flow rate.

(3) composite electrode. the use of the top of the electrode pin rubber plug must be pulled out to prevent production Health diffusion potential, affect the measurement results. The external reference electrode of the electrode is potassium chloride Solution (3mol/L), replenishment can be added from the top hole. No need to measure when the electrode protection cap should be put on the cap should put a small amount of potassium chloride solution (3mol/L), to keep the electrode bulb wet. Electrode Avoid prolonged immersion in the distillation Water, protein solution and acidic fluoride solution and avoid with silicone grease contact.

2. Redox platinum - saturated calomel platinum electrode. before use should pay attention to the electrode surface can not have oily substances, if necessary, acetone or chromic acid wash solution, then washed with water.

3. Silver volume method silver - saturated calomel (1) silver electrode. Before use, use a fine sandpaper to polish the surface, and then immersed in a small amount of nitric acid Sodium dilute nitric acid (1 + 1) solution, until there is gas evolution until removed and washed with water clean.

(2) double salt bridge saturated calomel electrode. salt bridge casing filled with saturated ammonium nitrate or potassium nitrate Solution. Other considerations are the same as for saturated calomel electrodes.

4. Nonaqueous acidometric glass - saturated calomel (Glacial acetic acid as solvent) (1) glass electrode. usage and aqueous solution of the same.

(2) double salt bridge saturated calomel electrode. salt bridge casing saturated potassium chloride anhydrous B Alcohol solution. Other considerations are the same as for saturated calomel electrodes.

5. Non-aqueous alkali Alkali glass - saturated calomel (Alcohol or acetonitrile as solvent) Glass electrode and double salt bridge saturated calomel electrode. usage and non-aqueous acid method the same.