PHOTOVOLT TECHNICAL NOTE
How To Clean Photovolt Coulometric Generators
Part 1-One-Piece Cartridges

1. REAGENT AND EQUIPMENT

1.1 ALCOHOLS Methanol or ethanol, reagent grade, should contain a low amount of water. Alcohols are used to rinse the frit after nitric acid cleaning and water rinsing.

1.2 OTHER SOLVENTS A wide variety may be used to remove sample build-up on the frit. Oils can best be removed with petroleum solvents - xylenes, toluene, chloroform, methylene chloride, etc. With other samples, the analyst is usually aware of solvents in which their samples are soluble. Use these solvents to remove any build-up on the frit. Rinse the frit with water to remove the solvents before cleaning with nitric acid. Although water will not remove all of the solvents, it is not good practice to add nitric acid to an organic solvent, so therefore, use the water rinse. The frit material, the platinum anode, and cathode are relatively inert in most solvents and acids; however, strong alkalis should be avoided for they may damage the frit.

1.3 NITRIC ACID Reagent grade is suggested. Technical grades can be used if reagent grade is not available. Nitric acid is preferred to other acids. Nitric acid is obtained from any chemical supply house. The cleaning procedure requires about a 75% aqueous solution. Make this by placing 25 cm3 (mL) of water in a container and adding 75 cm3 (mL) of concentrated acid to the container. The quantities do not have to be exact, but try to have a mixture with one part water to three (3) parts nitric acid. Handle and dispose of solutions in an approved manner.

2. CONTAINERS Any acid and solvent resistant glassware or plastic ware may be used.

3. APPARATUS

3.1 Some means of obtaining a slight vacuum or some pressure is necessary in order to provide some means of pushing through or pulling up acid through the generator frit. Some users prefer to fill the generator with the solutions and push the solvent through with pressure while the traditional way has been to pull the solutions up through the frit with slight vacuum. The PHOTOVOLT cleaning kit (P/N 4091004) is useful for this purpose.

3.2 Explosion-proof oven for drying the generator cartridge after rinsing with alcohol. Maintain the oven at 40° - 60° C. Use of a drying oven is optional. See 4.6.

4. PROCEDURE

4.1 Remove the sensing electrode, the white plastic generator cap, and the TEFLONÔ plug (if it is being used) from the vessel cover and set aside.

4.2 Release the three thumb screws on the vessel cover; remove the cover with the generator still in place. The generator can be removed from the cover by gently unscrewing and lifting it out of the vessel cover or it may be cleaned while still attached to the cover. Rinse the generator cartridge with water to remove any remaining Karl Fischer solution.

4.3 Immerse the generator cartridge in a small container of diluted nitric acid and draw about 5 to 20 cm3 (mL) of acid into the cartridge. It will probably come through very dark, therefore, discard the darkened acid and draw more acid through the frit until the acid comes through the frit clear.

4.4 Replace the acid container with one containing water and repeat the process by drawing water through the frit to remove all traces of nitric acid.

4.5 Replace the water container with one containing the driest alcohol available and repeat the process by drawing alcohol through the frit.

4.6 Finally immerse the generator in a small amount of vessel solution and draw a few cm3 of this through the frit. At this point the generator may be placed in the vessel instead of oven drying.

4.7 Add about 5 cm3 (mL) of fresh generator solution and place the re-assembled vessel on the instrument. Set point should be achieved in a short time. Before replacing the white cap on the generator cartridge, observe the hole in the center by holding the cap up to the light. If the hole is plugged, clean it with a fine wire. The hole must be kept open to equalize pressure inside and outside the generator cartridge. The vent hole in the top of the vessel cover must also be kept open. Ensure this by pushing an applicator stick through the hole occasionally.

1994-02-10
Revised 1998-12-16