CONDUCTIVITY

 

Equipment:      -YSI Model 32 Conductance Meter

                        -YSI 3403 Conductivity Cell  (Cell Constant = 1.0)

                        -YSI 3220 Temperature Sensor

 

Preparation:    -Bring all samples and standards to 25 C (+/- 1 C) in a water bath.

                        -In addition to the lake water samples, you should have standards of 1400 µmho/cm, 10 µmho/cm, and 100 µmho/cm, and samples of water from the Trout Lake Station water purification systems.

-Connect the conductivity cell leads to the terminals marked "CELL".

  (The lead ends may go on either terminal.)

                        -Plug the temperature probe into the proper socket.

 

1.  Determine the cell constant:

-Shake the bottle of 1400 µmho/cm standard to ensure solution homogeneity.

 

                        -Rinse the plastic sample tube, the conductivity cell, and the temperature probe twice with the standard.

 

                        -Fill sample tube and place the conductivity cell in sample, making sure that the vent slots are submerged and there are no air bubbles trapped inside the conductivity cell.

 

                        -Dial the FUNCTION knob to 'TEMPERATURE' and read the sample temperature to a tenth of a degree.  Allow enough time for the meter to stabilize before recording the temperature.

 

                        -Dial the FUNCTION knob to the first 'CONDUCTANCE' position and record the sample conductivity.  Conductivity should be read on the lowest range possible.  If the conductivity reading is followed by a 'u', switch to a higher range.

 

                        -Take temperature and conductivity readings on three more aliquots of the same standard. 

 

                        -Calculate the 'CA' value for each aliquot, and use the average CA to calculate the cell constant.  Report the cell constant to .001.  The cell constant should be between 0.98 and 1.02.

 

                        -Rinse the sample tube, conductivity cell, and temperature sensor three times with milli-Q after running the standard.

 

2.  Measure the conductivity of the remaining standards and samples:

                        -Run the station RO and deionized water samples first, followed by the 10 µmho/cm standard, followed by the samples in order of increasing conductivity.  Run the 100 µmho/cm standard last.

 

                        -Shake each sample and standard before measuring conductivity.

 

                        -Rinse the sample tube, conductivity cell, and temperature probe with two aliquots of the standard or sample to be measured.

 

                        -Place conductivity cell in sample, and record temperature and conductivity in the same manner as for the 1400 µmho/cm standard

 

                        -Pour a second aliquot and record temperature and conductivity as for the first.  If the second conductivity value differs from the first by more than 0.5 µmho/cm, continue to run additional sample aliquots until two consecutive readings are within 0.5 µmho/cm of each other.

 

                        -Always run three aliquots of the standards and purified water samples even if the first two are in agreement.

           

3.  Calculate corrected conductivity values from the measured temp. and cond. values:

                        -Use the conductivity calculation worksheet on the Station server to calculate final conductivity values.  Fill in the measured temperature and conductivity values.  The 'average conductivity' column contains the values that will be entered into the LTER database.

 

4.  Clean-up:

                        -Turn off the meter and store it in the chemistry lab.

                        -Rinse the conductivity cell well with milli-Q and store it in milli-Q.

                        -Rinse sample and standard bottles three times with milli-Q.  Place bottles in the hood or on drying rack in the chemistry lab to dry

 

5.  Additional Comments:

                        -Samples should be stored in the refrigerator before analysis.  They should contain enough air space to allow for mixing.

 

                        -Conductance of some samples may drift, especially those from the bog lakes.  In this case, the procedure is to measure the sample as quickly as possible after shaking and pouring, and to record the conductivity immediately after agitating the sample at the cell.

 

                        -The coating of platinum black on the electrodes of the conductivity cell should be inspected before and after use for signs of flaking or loss of material.  If the coating seems poor, cleaning and replatinization is required as described in the manufacturer's instructions.

 

 

EQUATIONS FOR CALCULATING CELL CONSTANT AND SAMPLE CONDUCTIVITY

 

Conductivity is routinely reported as a value corrected to 25 C.  This correction is represented in the following equations as:   (CM)(25 - T)(0.0191)

                        where:             CM = measured conductivity

                                                T = measured temperature (C)

 

Calculation of Cell Constant:

                        K = ST/CA                     where:    K = cell constant

                                                                              ST = true conductivity of standard

                                                                              CA = calculated conductivity of standard

 

                        CA = CM+(CM)(25-T)(0.0191)

 

Calculation of Sample Conductivity:      CA = K{CM+(CM)(25-T)(0.0191)}

                                                                                                                       

 

(reviewed 1/05 pkm)