What is a salt bridge?
A device that is used in an electrochemical cell with the purpose of connecting its oxidation and reduction in half cells is called a salt bridge. Keep in mind that a weak electrolyte is mandatory. To make you understand this device, you should know that a salt bridge works as a junction that works on connecting the anodic and cathodic compartments of a cell or an electrolytic solution.
A strong electrolyte is present in a salt bridge which further consists of ions. For example, AgNO3, KCl, etc. Salt bridges are generally used in a galvanic cell such as a voltaic cell or Daniel cell.
The function of a Salt Bridge
Now let’s take a look at the function of a Salt Bridge. The main purpose of a device such as a salt bridge is to help maintain the electrical neutrality which exists in the internal part of the circuit. It also works in being able to prevent the cell from taking away its reaction to equilibrium. If the device, the salt bridge, is found to be absent or if it is not being put to use, then the reaction in action will mostly proceed to continue and the solution in one-half electrodes will collect from around a charge that is negative in nature. You must also know that in the other half, the electrodes are accumulating a charge that is positive in nature. The result of this is the reaction coming to a halt and the electricity that will be produced is zero.
Thus, the device salt bridge acts in the prevention of the collection or gathering the positive and negative charges around the respective electrodes which further allows a reaction to take place smoothly. It also functions in the electron to flow continuously. But be aware that the function of a salt bridge is not to just work in moving the electrons from the electrolyte; rather it also aids in maintaining the charge balance as the electrons have been proven to be moving from one-half cell to the other
The device Salt bridge also functions to prevent the diffusion of the solution from one-half cell to another. It also works on minimizing the liquid-liquid junction potential. Why? Because Potential appears when they are in contact with each other between the two solutions. The device also performs as an electrical source of contact between two half cells.
Types of Salt Bridges
In electrochemical cells, there are mainly two types of salt bridges used. One is a Glass Tube Bridge and the other is the Filter Paper Bridge.
Let’s take a look at Glass Tube Bridge:
The tube is U – Shaped which is filled with electrolyte. A few generally used electrolytes are – Sodium Chloride (NaCl), Potassium Chloride (KCl), Potassium Nitrate (KNO3). The electrolyte must be unreactive with other chemicals that are present in the cell and have cations and anions that possess a similar migratory speed. Also, remember that the concentration of the salt solution and the diameter of the glass tube acts as a role of importance in conductivity. If the lowering concentration occurs, the diameter of the tube decreases the conductivity.
Now, let’s take look at the filter Paper Bridge:
It is another bridge that is commonly used which consists of filter paper, a porous material soaked in an electrolyte. Some commonly used electrolytes are sodium chloride (NaCl) or potassium chloride (KCl). The concentration, porosity, and roughness of filter paper tend to have an impact on the conductivity. In order to seek higher conductivity, a filter paper of smooth absorbent kind is used as they bring out higher conductivity than a paper which is rough in nature with lower absorbent.
As mentioned earlier, the main function of a salt bridge is to work in maintaining the electrical neutrality between two beakers. In order to do so, the salt present must be inert. It is crucial for the ions to move back and forth between the two half cells. Potassium chloride (KCl) and potassium nitrate (KNO3) are better inert salts, unlike other salts. A salt that is inert is used in order to prevent the reactions from taking place between the salt and solution. The inert salt potassium chloride (KCl) contains the potassium and chloride ions which have a very common diffusion coefficient and junction potential which is minimizing, however, the potassium chloride is not used as much as an electrolyte when the electrode used is lead or silver. This is because they form a precipitate.
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