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Lab / experiment Grade 12 Science

Conductivity as a Salinity Proxy: A Calibration Lab

A grade 12 lab building a calibration line between salt concentration and a meter reading, then estimating a fictional reservoir sample's salinity from it.

Objectives

  1. Students will prepare salt standards of known concentration and measure each with a meter.
  2. Students will build a calibration relationship between concentration and meter reading.
  3. Students will use the calibration to estimate the salinity of an unknown sample.

Vocabulary

Total dissolved solids (TDS)
Total dissolved solids is a measure of everything dissolved in water that you cannot see or filter out, like minerals and salts. Water with very high TDS can taste salty or bitter, even though it might look perfectly clear.

Also: conductivity, calibration, salinity, proxy measurement

Teacher overview

Water conducts electricity in proportion to the ions dissolved in it, which is why an inexpensive meter can estimate total dissolved solids without a chemistry lab. In this investigation students prepare salt standards of known concentration, measure each with a pocket meter, and build a calibration relationship they can use to read the salinity of an unknown sample. The classroom meter is not a certified instrument, so the results are a teaching analog rather than an official measurement, but the method mirrors how a technician calibrates a field probe before trusting its readings.

Conductivity as a Salinity Proxy: A Calibration Lab — Grade 12 Science
Name: ____________________________ Date: ______________

Procedure

Step 1. Safety: the solutions are only salt water and are not hazardous, but do not taste or drink any sample, keep the electronic meter away from spills, and wipe up water promptly to avoid a slip hazard.

Step 2. Prepare five standards in labeled cups at 0.0, 0.5, 1.0, 2.0, and 4.0 grams per liter. The scale cannot weigh the small amounts of salt the low standards need, so make them by dilution. First make a 4.0 grams-per-liter stock: weigh 2.0 grams of non-iodized salt into the stock container, add 500 mL of distilled water measured with the graduated cylinder, and stir until the salt fully dissolves. Pour 100 mL of stock into the 4.0 cup. For the 2.0 cup, mix 100 mL of stock with 100 mL of distilled water. For the 1.0 cup, mix 100 mL of the 2.0 standard with 100 mL of distilled water, and for the 0.5 cup, mix 100 mL of the 1.0 standard with 100 mL of distilled water. Fill the 0.0 cup with 100 mL of distilled water. Use the dropper to bring each volume exactly to the line, and rinse the cylinder with distilled water after each salt solution. Note that 1.0 gram per liter equals 1,000 milligrams per liter.

Step 3. Rinse the meter probe with distilled water and blot it dry before the first reading, so no leftover salt biases the result.

Step 4. Measure the 0.0 grams-per-liter distilled-water cup first and record the reading. This baseline shows what the meter reports with almost no dissolved salt.

Step 5. Measure the remaining standards from lowest to highest concentration, rinsing and drying the probe between cups, and record each reading in the data table.

Step 6. Obtain the unknown sample, labeled the Trellick Reservoir sample, from your teacher. Measure it with the same rinse-and-dry routine and record its reading.

Step 7. Plot the four salt standards and the baseline as points, with concentration in grams per liter on the horizontal axis and meter reading on the vertical axis, then draw a straight line of best fit through the points.

Data & Analysis

Standard (grams per liter) Meter reading
0.0
0.5
1.0
2.0
4.0
Trellick Reservoir sample

  1. Describe the relationship between salt concentration and meter reading shown by your plotted points.

  2. Does your line of best fit pass near the origin? Explain what the distilled-water baseline reading tells you about the meter.

  3. Using your calibration line, estimate the salt concentration of the Trellick Reservoir sample from its meter reading. Describe the steps you used to read it off the line.

  4. Suppose another group measured your 2.0 grams-per-liter standard and got a different reading. Give one likely reason two groups would disagree on the same standard.

  5. Explain why conductivity is only a proxy for salinity and cannot tell you which specific salt or ion is present.

  6. A drinking-water utility uses a quick field meter and also sends samples to a full laboratory. Explain why the utility needs both the fast proxy reading and the detailed lab analysis.

From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.

Answer Key

  1. Answer: A positive, roughly straight-line relationship.
    As the salt concentration rises, the meter reading rises in a nearly proportional way, so the points climb from lower left to upper right in an approximately linear pattern.

  2. Answer: The line should pass close to the origin.
    The distilled-water cup should give a reading near zero, because almost no ions are present to carry current. A near-origin baseline confirms the meter responds to dissolved salt rather than to something in the water itself.

  3. Answer: Accept any estimate read correctly from the student's own line.
    Students should locate the sample's meter reading on the vertical axis, move across to the line of best fit, and drop down to read the matching concentration in grams per liter. The value should be consistent with where the sample reading falls among the standards.

  4. Answer: Accept any one reasonable source of variation.
    Reasonable answers include an uncalibrated or differently calibrated meter, water at a different temperature, salt that was not fully dissolved, a probe that was not rinsed between cups, or a weighing or volume error while mixing.

  5. Answer: Conductivity responds to all dissolved ions together.
    The meter reports the combined effect of every ion in solution, so it measures how much dissolved material is present, not which salt it is. Two different salts at the same concentration could give similar readings.

  6. Answer: The two tests answer different questions.
    The field meter gives a fast, cheap signal that dissolved solids have changed, which is useful for daily monitoring. The laboratory analysis identifies specific contaminants and confirms compliance, which the proxy reading cannot do on its own.

Differentiation

Support

Prepare the five standards ahead of time so groups begin at the measuring step, and provide graph paper with the axes already labeled and scaled for the plot in step 7.

Extension

Have students prepare and measure a sixth standard at 3.0 grams per liter that they did not use to draw the line, then check how closely their calibration predicts its known concentration and discuss the size of the error.

ELL supports

Provide picture cards for each material and a labeled diagram of the meter probe, and give a sentence frame for item 5 that contrasts "how much is dissolved" with "which salt is dissolved."

Extensions

Have students collect water samples from a tap, a rainwater barrel, and a natural water body, measure each with the calibrated meter, and rank them by estimated dissolved solids, discussing why the samples differ.