Conductance of Boiler Water
Specific Conductance
The specific conductance of water is a measure of its ability to conduct an electrical current. This property is of no consequence in itself with respect to water treatment. However,from a control standpoint, the conductivity test is important as a direct measure of the total ionizable (dissolved) solids in the water. The conductivity test provides an accurate measurement of steam purity as well as a simple control for boiler water solids. Conductivity may also be used for blow down control in recirculating cooling water systems. Specific conductance is inversely proportional to electrical resistance. Pure water is highly resistant to the passage of an electric current and therefore has a low specific conductance. However, if the water contains ions, the water becomes a better conductor of electricity and the specific conductance is increased. Inorganic compounds such as sodium chloride and sodium sulfate dissociate into positive and negative ions, which will conduct electricity in proportion to the number of ions present. The conductivity test therefore is not specific for any one ion, but rather a measure of the total ionic concentration.
The basic unit of electrical resistances is the ohm; since electrical conductivity is the reciprocal of resistance, the term “mho” (ohm spelled backwards) was chosen as the basic unit of conductivity. The conductivity test measures small amounts of electrical conductance, so the instrument is usually calibrated in micromhos (1 micromho = 1 millionth of a mho). Often a conductivity meter is calibrated to read directly in ppm-dissolved solids (or some specific ion or compound). This is not recommended for control testing, since the conversion factor from micromhos of specific conductance to ppm will vary slightly with different waters. The conductivity test provides an accurate, simple method of blowdown control. However, in alkaline samples, the hydroxide ion has disproportionately high conductance in comparison to other ions present. Thus, it must be neutralized before the conductivity test, and this is accomplished by the use of gallic acid. Excess gallic acid only slightly dissociates, and thus will not contribute to the conductivity.
Theory of Test
Ionizable solids in boiler water have the ability to conduct an electric current through a solution. This property of electrical conductance of solids makes it possible to accurately measure the quantity of solids in solutions by suitable conductance equipment. One such conductance apparatus, typical of commercial units, is shown in figure 58-1. It has a permanent cell-cup built into the meter and the electrodes never need replatinizing. While earlier meters had various forms of manual temperature compensation (the temperature co-efficient for conductance measurements is about 2 per cent per degree C), this meter has an automatic temperature compensator.
Apparatus Required
Conductivity Meter (variable range) - 1
Glass Flask - 1
Measuring Dipper, brass - 1
Stirring Rod, Glass - 1
Chemicals Required
Conductivity Standard Gallic Acid
Phenolphthalein Indicator
Procedure for Test
Pour approximately 40 ml of settled or filtered boiler water cooled to 60 to 150F (16 to 66c) into a rinsed 50 ml flask or cylinder. Add two dippers of gallic acid (0.2 gram). Stir if a small amount of the gallic acid remains undissolved; the accuracy of the test is not affected adversely. Rinse the permanent cell cup built into the meter at least three times with the sample to be tested. Fill the cell cup with sample to above the disc electrode, and read the specific conductance (in micromhos) directly off the meter scale. As an example, if the meter reads 5.4 with the range selector switch at 1000, the neutralized conductivity of the sample equals 5400 mircomhos.
NOTE : Two dippers of gallic acid neutralize approximately 1300 parts per million (ppm) of phenolphthalein alkalinity. With some highly alkaline boiler waters, additional gallic acid may be required. A desirable precaution is to first add approximately four drops of phenolphthalein indicator to the sample, and then add gallic acid until the pink color imparted by the indicator disappears.
If recalibration of the instrument is needed, a commercially available conductivity standard of known conductance can be used. If, for example, a standard solution has a known conductance of 4600 micromhos at 77F (25C), the calibration control trimmer can be slowly adjusted until the meter reads 4600 micromhos. Calibration checks are recommended at least once every three months.
Calculation of Results
Specific conductance in micromhos is read indirectly from the calibrated scale as indicated by the needle movement.
The relationship between specific conductance and the dissolved solids content of a boiler water depends on the characteristics of each individual boiler water and therefore may be slightly different for each plant. Using the gallic acid neutralization method, an average value determined over a wide range of operating conditions is that one micromho is equivalent to 0.9 ppm dissolved solids. This value is sufficiently accurate for the average industrial plant.
The exact relationship between micromhos and solids can be individually established for each plant by determining both the conductance and solids content of a serious of approximately 10 samples taken over a two-week period.
Limitations of Test
The conductance method affords a rapid means of checking the dissolved solid contents of a sample. The effect of hydroxide in causing high conductivity is minimized by the gallic acid neutralization, thereby securing a consistent relationship between solids and conductance. The conductance method does not measure non-electrolytic solids such as organic matter, and to express result in terms of parts per million of boiler water solids, it is necessary to use a conversion factor as described above.
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