Secondary Standard of Emf

The secondary standard of Emf is the unsaturated Weston cell.

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Saturated Weston Cell (Primary Standard of Emf )

Unsaturated Weston cell 

Construction and Characteristics. The unsaturated cell is similar to a suturated cell except that it does not have Cadmium sulphate crystals ( CdSO4 ).

• It has the same electrodes but the solution is saturated at 4 degree Cecius , and therefore unsaturated at room temperature.
• In addition, a 'sputum' or retaining member (porous plugs in this case ) is used over each electrode to hold the material in place, and hence these cells are considered portable.
• After being moved from one place to another, the cell should be allowed a few days to settle so that the results obtained from measurements involving cells are accurate.
• The cells are enclosed in a bakelite case which is lined with 1.5mm thick copper to help keep all parts of the cell at a uniform temperature.
• Not all unsaturated cells have the same emf. In fact, saturated cells differ from one another.
• The emf of new unsaturated cells lies between 1.0190 to 1.094 absolute volt. Therefore each cell must be calibrated against the primary standard in order to know its exact emf. 
• The temperature at which this emf is measured should also be specified.

Note: The unsaturated cell becomes less stable with age, yet it is remarkably good. The decrease in emf is about 30 to 50 microVolt per year. It is recommended that the cells be checked by returning them to a standardizing laboratory once a year for certification.

• An unsaturated cell has a shorter life than a saturated type but some cells last even 20years. age of the cell life also depends on the kind of treatment the cell received.

The temperature co-efficient: it is less than that of a saturated cell. It is -10microVolt per degree Celcius. Thus can be used for most purposes without temperature correction., if the temperature is near to the calibration temperature.

The cell has a positive temperature effect on one electrode and a negative at the other. Therefore it is important to keep all the parts at the same temperature.

Precautions. 

• Same precautions are followed as saturated cells except for some difference that is cel is portable and can be shipped.
• cell should be not operated below 4 degree Celcius and below 40 degree Celcius 

Note: Below 4 degree Celcius the emf drops rapidly and eventually the electrolyte freezes. Above 40 degree Celcius the cell materials start to melt above voltage drops sharply around 55 degree Celcius . It has been mentioned earlier that unsaturated cells lose emf, at an average rate of 30-50 microVolt per year at normal temperature of use and storage. At higher temperatures, the decrease is much more rapid and is about 240 microVolt per year 60 degree Celcius.