(1) Principle of use a. Potentiometric titration uses the change in potential to determine the end point. The potential is a relative value, which is the potential difference between the reference solution and the solution. Potentiometric titration is based on the potential change (S-curve) of the electrode during the titration process. The end point is usually determined by the maximum derivative. Of course, there are many ways to determine the end point. b. The permanent stop method uses current or voltage changes to determine the end point. When the current or voltage reaches a constant value and remains constant for a certain period of time, the end point can be considered. The permanent stop titration method is based on the sudden change of the current of the platinum electrode during the titration (maintaining a certain time, such as 10 seconds or more) to determine the end point. The permanent stop titration method is mainly current titration, but it can also be potentiometric titration (determined by titration property).
(2) Use a. Potentiometric titration can be used for titration, complexation, non-aqueous and acid-base titration. b. The permanent stop rule is mainly used for the titration of sulfonamides. Of course, the moisture of Karlsberg is also a kind of permanent stop.
(3) Electrode a. Potentiometric titration uses two different electrodes. One is the indicating electrode, and its electrode potential changes with the change of the ion concentration of the analyzed component in the solution; the other is the reference electrode, and the electrode potential is fixed. When the end point of the titration is reached, the potential of the analyte is sharply changed to cause a sudden drop or sudden increase in the potential of the indicator electrode. This turning point is called a sudden jump point. b. The permanent stop titration method uses two identical platinum electrodes. When a low voltage (for example, 50 mV) is applied between the electrodes, if the electrodes are polarized in the solution, there is only a small or no at the end of the titration. The current passes; but when it reaches the end point, the titrant is slightly excess, the electrode is depolarized, and there is current flowing through the solution. The galvanometer pointer is suddenly deflected and no longer returns. Conversely, if the electrode is changed from depolarized to polarized, the galvanometer pointer will return from zero to zero and will not change.
(4) Method of use a. Potentiometric titration: The beaker containing the test solution is placed on a magnetic stirrer, immersed in the electrode, stirred, and the titration solution is added dropwise from the burette; at the beginning, more can be added at a time. The amount, stir, record the potential; until near the end point, you should add a small amount each time, stir, record the potential; until the jump point has passed, you should continue to add several titrations, and record the potential. Determination of the end point of the titration: using the coordinate paper with the potential (E) as the ordinate and the titration volume (V) as the abscissa, the EV curve is drawn, and the center of the steeply rising or falling portion of the curve is the end point of the titration. Or △ E / △ V (that is, the ratio of the potential difference between two adjacent and the volume difference added to the titration solution) as the ordinate, and the titration volume (V) as the abscissa, draw (ΔE / ΔV) - The V curve, the volume corresponding to the maximum value of ΔE/ΔV, is the end point of the titration. The second derivative can also be used to determine the endpoint. According to the obtained ΔE/ΔV value, the difference between adjacent values, that is, ΔE/ΔV, is calculated, and the (ΔE/ΔV)-V curve is plotted. The volume at which the curve crosses zero is the end point of the titration. If the selection is for the color change domain of the indicator, the indicator is added before the titration, and the color change from the end point to the end point is observed to select the color of the indicator at the end of the variety. b. Permanent stop titration: When used as the end point indication of the diazotization method, the R is adjusted so that the voltage applied to the electrode is about 50 mV. Take the appropriate amount of test sample, accurately weighed, placed in the beaker, unless otherwise specified, can add 40ml of water and hydrochloric acid solution (1 → 2) 15ml, and then placed on a magnetic stirrer, stir to dissolve, then add potassium bromide 2g After inserting the platinum-platinum electrode, insert the tip of the burette into the liquid surface at about 2/3, and titrate rapidly with sodium nitrite titration solution (0.1mol/L or 0.05mol/L). The tip of the burette is raised to the liquid surface, the tip is rinsed with a small amount of water, the washing solution is incorporated into the solution, and the titration is continued slowly until the galvanometer pointer suddenly deflects and no longer returns, which is the end point of the titration. When used as the end point indication for moisture measurement, R can be adjusted so that the initial current of the ammeter is 5 ~ 10μA, and the current is suddenly increased to 50 ~ 150μA, and does not return for several minutes, which is the end point of titration.
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