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pH, conductivity and TDS

Two measurements often made on water matrices are pH and conductivity. These are usually categorized as physical tests, though they are strongly dependent on the chemical characteristics of an aqueous solution. The tests are easy to conduct and are usually performed shortly after obtaining a sample. Constant values from day to day give some indication that conditions are stable; and correspondingly changes in these parameters suggest that underlying conditions are also changing.

pH

pH is a measure of the hydrogen ion concentration in water. What does that mean? Water can be ionized by the following reaction:

H2O <=> H+ + OH-

Mathematically:

pH = -log[H+]

What this means is that for every tenfold change in hydrogen ion concentration, there is a one unit change in pH. The pH scale is usually said to run from 1 to 14, though the pH of say, six molar sulfuric acid is less than zero.

In pure water, [H+] = [OH-] = 10-7 M. So the pH of pure water is pH 7. Adding acids or bases to water shifts this balance. Acids add H+, so adding acid raises the [H+] and lowers the pH. If we add just enough hydrochloric acid to pure water to bring the [H+] to 10-6 molar, what would the pH be?

The pH of water must be close to neutral (pH 7) for fish or other aquatic organisms to survive. Similarly, the pH of water in the pore spaces of soil must be close to 7. The soil pH may also affect the availability to plants of nutrients in the soil. Water with a low pH (below about 6.5) is corrosive to metal surfaces (eg, copper pipes, steel tanks).

Many lab tests require samples to be adjusted to a particular pH by adding a buffer. Likewise, media used to grow or assay microbes usually need to be adjusted to a certain pH range. A buffer is a solution that has a high ability to absorb acid or base without changing pH. Borate, citrate, phosphate, phthalate and other multivalent anions are often used to make buffer solutions. Buffers can be made quite precisely for calibrating pH measurements.

We measure pH using indicator dyes, pH test strips, or a pH meter. Dyes are organic compounds with absorbances in the visible range. Some of these, such as methyl orange or phenolphthalein, will shift their conformation slightly in the presence or absence of hydrogen ions. This shift causes a change in the absorbance maximum of the compound and hence a change in color. As a solution drops below about pH 4, methyl orange turns from yellow to orange. As a solution goes above about pH 9, phenolphthalein goes from clear to pink. Mixed indicators show a range of colors in response to pH changes.

You can get pH indicator strips. These are strips of paper or other material on which dyes have been fixed. When wetted these will show a particular color corresponding to the pH of the solution. A color chart is used to read the strip.

A chemical cell consisting of an acid-permeable glass membrane separating two solutions will develop a voltage related to the difference between the hydrogen ion activities in the two solutions. (Chemical activity is closely related to concentration). The voltage is related to the activities (or concentrations) by the Nernst equation:

E = E0 - (2.303RT/nF)log([H+]in/[H+]out)

But, donít worry: Most people who measure pH do not have any idea how to use this equation. It is built into the circuitry of the pH meter, which measures and amplifies the millivolt-sized output of the pH electrode.

Conductivity

Conductivity is a measure of how well a solution conducts electricity. Water with absolutely no impurities (which really does not exist) conducts water very poorly. In real life, the impurities in water increase its conductivity. Because of this, if we measure the conductivity of water, we have some estimate of the degree of impurity. The current is actually carried almost entirely by dissolved ions. The ability of an ion to carry current is a functions of its charge and its mass or size: Ions with more charge conduct more current; larger ions conduct less.

To measure conductivity we use a machine called a conductivity meter. The actual amount of electricity that a given water solution will conduct changes with how far apart the electrodes are and what temperature the water is. This quantity is expressed in units called mhos (the unit of resistivity is the ohm; mho is ohm spelled backwards). The meter has a probe with two electrodes, usually 1 centimeter apart. Most of the modern ones sense the temperature as well and electronically correct for its effects. Since the meter gives a reading which is corrected for temperature and electrode separating distance, the number is called "specific conductance," expressed in mhos per centimeter at 25į C. The SI unit of conductivity is the siemen (S) named after the French physicist and equivalent to the mho. Thus 1 microsiemen per meter (mS/m) is equivalent to 100 mmho/cm. Very often, a meter will read out in mS/cm or mS/cm (or just mS or mS which are assumed to be per centimeter).

Laboratory pure water has a specific conductance of about one millionth of a mho/cm. What is the conductivity of our distilled water? Wells and lakes in Connecticut usually have a specific conductance of about 50 to 500 times that. To make these number easy to write, we usually use units of micromhos per centimeter (mmhos/cm). Thus laboratory pure water is around 1 mmho/cm; tapwater is usually around 50 to 500 mmhos/cm.

Using the meters

To use either the pH meter or the conductivity meter, the idea is the same. You put the probe into a solution with a known pH or conductivity and set the meter to the known value. Then you put the probe in the unknown solution you are trying to measure. Meters are usually set once a day, or more often as necessary.

Using the "set" knob, the pH meter is set at pH 7.0 in a suitable buffer. Then the electrode is placed in either a pH 4, pH 10 or some other buffer and the meter is adjusted to the right pH using the "slope" knob. Different meters may have the calibration knobs labeled differently, but the principle is the same.

To use the conductivity meter: Rinse the probe. With the meter set on the highest range, put the probe in the sample and lower the range until it is at the lowest setting that leaves the meter on-scale. Select a standard solution in this range if one is available. Rinse the probe and transfer it to this standard. Set the meter to the known value using the set knob. Measure the conductivity of the sample. The electrode or probe must be rinsed between samples and standards to prevent cross contamination.

The electrode or probe must be rinsed between samples and standards to prevent cross contamination.

Conductivity and TDS

Conductivity is often used as an estimate of total dissolved solids (TDS) content of water samples. Using sodium chloride solutions with known TDS content, we will determine a relationship between TDS and conductivity

Total dissolved solids is one part of the total solids in an aqueous solution. The total solids content of a water sample is just what it sounds like--the total amount of solid matter left when we take away the water. It is often divided into two fractions: The Total Suspended Solids (TSS) is the material trapped on a filter. The material which goes through the filter is called Total Dissolved Solids (TDS). If the TSS content of the water is low, the Total Solids is very close to the TDS.

Measuring TDS

One procedure that can be used to determine total dissolved solids is as follows:

    1. Record everything neatly in your lab observations notebook.
    2. Weigh a clean dry ~500 mL beaker to the nearest milligram or better. Once you have obtained a tare weight, do not handle the beaker with your bare hands (a fingerprint can weigh a milligram or so).
    3. Remove the solids from at least 300 mL of a water sample by filtering through a glass-fiber filter into a clean, dry side arm flask.
    4. Measure approximately 300 mL of filtered sample into the beaker. Record the volume to the nearest milliter. Donít fill the beaker more than about 60-70% full.
    5. Cover the beaker loosely to prevent dust from falling in. Allow water to evaporate in a drying oven (this will take a few days).
    6. When the beaker is dry, let it cool in a dessicator and reweigh.

Calculate the TDS using this formula:

Total Dissolved Solids (mg/L) = 1,000,000*(BD - BT)/W

BD = mass of beaker after water evaporated, grams
BT = tare weight of beaker, grams
W = volume of water sample, milliliters

LAB EXERCISE

You will make up various solutions and measure their pH and conductivity values. You will make a series of sodium chloride standards and measure their conductivity values. For a water sample, you will measure pH, conductivity and TDS.

  1. Make the following solutions by weighing out an appropriate quantity of dry chemical and dissolving in water. Make 250 mL of Solutions A-D and 1000 mL of Solution S. Calculate the mass of salt you need to weigh out for each solution on the left hand page of your lab observations notebook. Record the actual weighings on the right hand page.
  2. Solution A: 0.01 M sodium chloride (NaCl)
    Solution B: 0.01 M sodium carbonate (Na2CO3)
    Solution C: 0.01 M potassium dihydrogen phosphate (KH2PO4)
    Solution D: 0.01 M sucrose (C12H22O11)
    Solution S: 0.0141 M sodium chloride solution

  3. For solutions A through D, measure both pH and conductivity. Calculate the total dissolved solids based on the mass of salt you dissolved in water. For example, if you dissolve 0.010 g of Na2CO3 in 100 mL of water, the TDS is 0.010g/100mL * 103 mL/L * 103 mg/g = 100 mg/L. Record the results in your observations notebook.
  4. Using the 0.0141 M sodium chloride solution (824 mg/L), make a series of five sodium chloride solutions with 50 to 500 mg/L of sodium chloride using the volumes listed in Table 1. Measure the pH and conductivity of these solutions and record the results in your laboratory observations notebook.
  5. For one or more water samples, measure TDS. Measure the conductivity and pH for the sample(s) that you run TDS on. Record these measurements in your laboratory observations notebook.
  6. Get the TDS, pH, and conductivity values for a number of samples run by students in the class. Record these values on a left hand page in your observations notebook.

Table 1

Solution #

mL of 0.0141 M NaCl solution

mL of de-ionized H2O

TDS, ie
NaCl content (mg/L)

1

6 mL

94 mL

50

2

12 mL

88 mL

100

3

24 mL

76 mL

200

4

43 mL

57 mL

350

5

61 mL

39 mL

500

Required in Lab Report #2

Title Page; Include

Procedure

Results

Graph #1

Graph #2 (and maybe #2A)

Discussion; Answer and Explain:

  1. Does there appear to be a relationship between TDS and pH for sodium chloride or for the water samples?
  2. Does there appear to be a relationship between TDS and conductivity for sodium chloride? Which solutions (of A through D) fall close to the line and which do not? Why?
  3. Do the water samples fall on the trendline for the sodium chloride standards? Why not? How does the trendline for just the samples (graph 2A) compare with the line for the standards?
  4. Under what conditions would conductivity be a good estimate for TDS?

Notebook Pages (samples attached)

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Anthony Benoit abenoit@trcc.commnet.edu