Titration analysis method-acid-base titration (1)

2.

Acid-base titration is a titration analysis method based on acid-base reaction

Acid-base titration using a strong acid or alkali commonly as titrant, for example, as a standard solution with HC1 acid, a basic substance can be titrated, such as of NaOH, of Na ₂ CO.

1.

During the acid-base titration, the solution itself does not change in appearance.

The acid-base indicator has the following dissociation equilibrium in aqueous solution:

The expression of the dissociation equilibrium constant K _HIn (set K1=K _HIn ) is then

In the discussion formula, the pH value can be seen

At that time, the color was acidic, and the pH of the solution was less than or equal to pK1-1;

At that time, the color was basic, and the pH of the solution was greater than or equal to pK1+1;

When the solution pK _{1 ^-1} ≤pH≤pK ₁ +1, it shows a mixed color

Definition: pK _{1 ^-1} to pK ₁ +1 is the theoretical variation range of the indicator, and pK1 is the theoretical discoloration point

For example, using NaOH to titrate HCl: At the end of the titration, phenolphthalein is colorless → red; methyl orange, orange → yellow

Table 2.

The narrower the color range of the indicator, the better, because the pH value changes slightly, the indicator changes from one color to another immediately, and the indicator changes sharply, which is beneficial to improve the accuracy of the analysis results

The indicators listed in Table 2.

The mixed indicator is artificially formulated, and the complementary effect between the two indicator colors is used to narrow the color range, shorten or disappear the transition color duration, and narrow the color range, making it easy to observe the end point

Table 2.

There are generally two ways to form a mixed indicator:

(1) A dye that does not change with the concentration of H+ is mixed with an indicator, such as a mixed indicator composed of methylene blue and methyl red

(2) Two different indicators are mixed in a certain ratio, such as bromocresol green (pK _HIn =4.
9) and methyl red (pK _HIn =5.
0).
Both indicators change with the pH value.
After being mixed in a certain ratio, when the pH value is 5.
1, the red wine changes to green or vice versa, which is extremely sensitive
.

This indicator is used for the determination of alkalinity in water
.

If methyl red, bromothymol blue , thymol blue, and phenolphthalein are mixed in a certain proportion, dissolved in ethanol to prepare a mixed indicator, the mixed indicator will gradually change color with the pH value as follows
.

The wide pH test paper is made with the above-mentioned mixed indicator to determine the pH value
.

Related links: Basic knowledge of titration analysis