Electrolyte boiling

Spontaneous low resistance internal short circuits can develop in silver—zinc and nickel—cadmium batteries. In high capacity cells heat generated by such short circuits can result in electrolyte boiling, cell case melting, and cell fires. Therefore cells that exhibit high resistance internal short circuits should not continue to be used. Excessive overcharge that can lead to dry out and short circuits should be avoided. 

In virtually all of the simple immersion and two electrode experiments carried out so far, in-diffused H has been detected at the 1016/cm3 level or less. There has been no demonstration that large densities (> 1018/cm3) of defects can be passivated by these methods, and where plasma and electrochemical treatments have been directly compared, the former have been found to be more effective (Tavendale et al., 1986). In contrast to plasma techniques, the electrolyte boiling point limits the temperature range of electrochemical methods, although several hundred degrees Celsius can be utilized for electrolytes like H3P04. 

A safety valve at the top of the cell is provided to allow excessive pressure to escape should the battery be abused to the point that the electrolyte boils. This condition might be caused by a severe overeharge where adequate heat dissipation is not provided. Under high temperature abuse (+100°C and above), the safety valve will open at approximately 45 psia over pressure allowing water vapor to escape. Electrolyte will not be expelled, even with the cell in an inverted position. When the cell is allowed to cool, the valve will reseal and the negative pressure cell will return to a normal operating condition. A reduction in cell capacity may be anticipated due to the loss of water from within the cell. 

Niobic Acid. Niobic acid, Nb20 XH2O, includes all hydrated forms of niobium pentoxide, where the degree of hydration depends on the method of preparation, age, etc. It is a white insoluble precipitate formed by acid hydrolysis of niobates that are prepared by alkaH pyrosulfate, carbonate, or hydroxide fusion base hydrolysis of niobium fluoride solutions or aqueous hydrolysis of chlorides or bromides. When it is formed in the presence of tannin, a volurninous red complex forms. Freshly precipitated niobic acid usually is coUoidal and is peptized by water washing, thus it is difficult to free from traces of electrolyte. Its properties vary with age and reactivity is noticeably diminished on standing for even a few days. It is soluble in concentrated hydrochloric and sulfuric acids but is reprecipitated on dilution and boiling and can be complexed when it is freshly made with oxaHc or tartaric acid. It is soluble in hydrofluoric acid of any concentration. 

Class C direct dyes are dyes of poor leveling power which exhaust well in the absence of salt and the only way of controlling the rate of exhaustion is by temperature control. These dyes have high neutral affinity where, resulting from the complexity of the molecules, the nonionic forces of attraction dominate. When dyeing with these dyes it is essential to start at a low temperature with no added electrolyte, and to bring the temperature up to the boil very slowly without any addition of electrolyte. Once at the bod the dyeing is continued for 45—60 min with portionwise addition of salt to complete exhaustion. 

Practical Processes. With acid leveling dyes no real problems exist because the dyes show good migration, electrolyte is added from the beginning, and rather like Class A direct dyes level dyeing is achieved by prolonging the times at the boil. 

Sulfamide, (H2N)2S02, can be made by ammonolysis of SO3 or O2SCI2. It is a colourless crystalline material, mp 93°, which begins to decompose above this temperature. It is soluble in water to give a neutral non-electrolytic solution but in boiling water it decomposes to ammonia and sulfuric acid. The structure (Fig. 15.50c)... 

Lead alloys Preferably use the electrolytic cleaning procedure just described. Alternatively, immerse for 5 min in boiling 1% acetic acid. Rinse in water to remove the acid and brush very gently with a soft bristle brush to remove any loosened matter. 

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