In Chemistry, What Is a Salt Bridge?

The salt bridge is a high-concentration electrolyte solution that is connected between two solutions in order to reduce the liquid junction potential and transfer ions. Salt bridges often appear in galvanic cells and are composed of agar and saturated potassium chloride or saturated potassium nitrate solution.

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Often used

Insert a salt bridge between the two solutions to replace the direct contact between the two solutions, reduce and stabilize the liquid junction

The concentration of saturated KCl solution is as high as 4.2mol · dm-3. When the salt bridge is inserted into the interface between two electrolyte solutions with a small concentration, two interfaces are generated, and K + and Cl- in the salt bridge diffuse outward. The mainstream of ion diffusion at these two interfaces. Due to the similar diffusion rates of K + and Cl-, the junction potentials produced by the salt bridge in contact with the two solutions are very small, and the directions of the two are opposite, so they cancel each other down to 1 ~ 2mV. The principle of selecting the electrolyte in the salt bridge is that the concentration is high, the number of positive and negative ion migration is nearly equal, and it does not occur with the solution in the battery.

1. Agar-saturated KCl salt bridge: 3g agar and 97ml distilled water are added to the beaker, and the agar is heated to complete dissolution using a water bath heating method. Then add 30 grams of KCl and stir well. After the KCl is completely dissolved, use a dropper or siphon while hot to add this solution to the glass tube that has been bent in advance, and leave it to stand until the agar has coagulated and then use it.

Excess agar-saturated KCl was plugged with a ground plug and reheated during use.

If there is no agar, you can also use cotton to plug the ends of a U-shaped tube containing a saturated solution of potassium chloride in place of the salt bridge.

The agar-saturated KCl salt bridge cannot be used in solutions containing ions that react with Cl- such as Ag +, Hg2 +, etc., or containing substances that react with K + such as ClO4-.

2.3% agar -1mol · dm-3 K2SO4 salt bridge: suitable for use with the solution, Hg-Hg2SO4-saturated K2SO4 electrode can be used in this solution.

3.3% agar -1 mol · dm-3 NaCl or LiCl salt bridge: suitable for the solution containing high concentration of ClO4-, in which mercury-calomel-saturated NaCl or LiCl electrode can be used.

4 NH4NO3 salt bridge and KNO3 salt bridge can be used in many solutions, but it has no common ions with various electrodes, so the common electrode will change the concentration of the reference electrode and introduce foreign ions, which may change the potential of the reference electrode. . In addition, agar salt bridges cannot be used in solutions containing high concentrations of acids and ammonia.

5. (1) Simple method

Use a dropper to inject a saturated KNO3 (or NH4NO3) solution into the U-shaped tube. After filling up, plug the two ends of the U-shaped tube with twisted filter paper. There must be no air bubbles in the tube.

(2) Gel method

Weigh 1g of agar and put it into 50mL saturated KNO3 solution, soak it for a while, and then slowly heat it to boiling. After the agar is completely dissolved, cool it a little, insert the washed salt bridge tube into the agar solution, and suck the solution from the top of the tube (There must be no air bubbles in the tube.) Keep this filled state and cool to room temperature, that is, the gel is solidified and fixed in the tube. Remove and wipe for future use.