PASCO Specialty & Mfg. CI-6734 User Manual download pdf (Page 9)

012–06615A Sodium Ion Selective Electrode

Measurement

Measuring Hints

• All samples and standards should be at the same temperature for precise measurement, preferably ambient

temperature. A difference of l °C in temperature will result in about a 2% measurement error. The electrodes

should not be used above 70 °C.

• Constant, but not violent, stirring is necessary for accurate measurement. Magnetic stirrers can generate

sufficient heat to change the solution temperature. To counteract this effect, place a piece of insulating

material, such as a styrofoam sheet, between the stirrer and the beaker.

• Always rinse the electrodes with electrode rinse solution from a wash bottle between measurements. Use

a clean, dry tissue to prevent cross contamination. Never use distilled water.

• Store the electrodes in electrode storage solution between measurements. Do not store in air or distilled

water. Always soak new electrodes overnight in electrode storage solution prior to first use. When making

low level sodium measurements, use a dilute sodium chloride storage solution. Add 1 ml of ISA to 100 ml

of dilute storage solution.

• Plastic lab-ware should be used for low level measurements (< 1 ppm).

• All measurements should be made in basic solution. All samples and standards should be adjusted to a pH

> 9 with ISA.

• For samples with high ionic strength, prepare standards whose composition is similar to the sample.

• Always check to see that the membrane is free from air bubbles after immersion into standard or sample.

Sample Requirements

• The sample measuring range is pH 9–12. Use the ISA recommended to adjust the pH for best accuracy.

Make sure that the samples and standards are at the same temperature. The glass electrode sensing bulb will

not be attacked by most organic solvents.

Units of Measurement

Sodium concentrations are measured in units of parts per million, equivalents per liter, soles per liter, or any other

convenient concentration unit. Table 1 indicates some of the concentration unit conversion factors.

TABLE 1 Concentration Unit Conversion Factors

ppm Na

moles/liter Na

229.90 1.0 X 10

-2

22.99 1.0 X 10

-3

2.30 1.0 X 10

-4

1 2 3 4 5 6 7 8 9 10 11 12 13 14 ... 21 22

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PASCO Specialty & Mfg. CI-6734 User Manual Page 9