Environmental Chemistry/Fresh Water

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Fresh water is a topic for Environmental Chemistry in the 2022 season (and likely 2023).

The Competition

In the event, a team of two is given fifty minutes to complete any combination of hands-on activities, questions, experiments, data analysis, all of which relate to freshwater environmental chemistry. Participants are also expected to have read and studied several pages of the Clean Water Act and Wastewater Operator Certification Manual, both of which describe proper water-handling procedures. Scoring is largely based on the chemistry portion of the event (60% of total score), but answering questions about interpreting data (20% of total score), and answering questions about the Clean Water Act and Wastewater Operator Certification Manual (20% of total score) also make up a large portion of the participants' score.

Parameters

Teams may bring pencils, a ruler (12-15 in.), one three-ring binder of any size containing information in any form and from any source (sheet protectors are allowed), and two stand-alone, non-graphing calculators. Pages may not be removed from the binder during the event. Teams should also bring the Recommended Lab Equipment for Division C Chemistry Events, posted on soinc.org.

Additionally, participants must use goggles, an apron or lab coat, and their skin must be covered from the neck down to the wrist and toes. Pants must be loose-fitting, and shoulder length or longer hair must be tied back.

Standard Curves

An important part of the event is being able to generate standardized curves.

What is a standard curve?

Standard curves are also known as calibration curves. They typically have the concentration of the solution on the x-axis, and absorbance at a specific wavelength on the y-axis. By measuring the absorbance of a solution at various known concentrations and plotting these on a graph, we can calculate a line of best fit for the graph, allowing us to calculate an what an unknown concentration is, given a specific absorbance.

What is concentration?

Concentration is the amount of solute in a solution divided by the total volume of the solution. One common unit of concentration is the molar (M), which is the number of moles of solute (m) divided by the number of liters of solution (L). Another unit of concentration is parts per million (ppm), which is also equivalent to milligrams per liter (mg/L). The unit parts per thousand (ppt) is equivalent to grams per liter (g/L).

What is absorbance?

Absorbance is how well a substance absorbs light of a specific wavelength. It is also known as optical density.

To calculate absorbance of a certain material at a specific wavelength, shine a light of that wavelength through the material. Measure the intensity of the light before it passes through the material and after it passes through the material. The absorbance is:

[math]\displaystyle{ A = \log(I_0/I) }[/math]

where:

  • A = absorbance
  • I0 = intensity of the incident light (before it passes through a material)
  • I = intensity of the transmitted light (after the light passes through a material/substance)


The Beer-Lambert Law relates the attenuation of light to the properties of the material that the light is traveling through:

[math]\displaystyle{ A = εbC }[/math]

where:

  • A = absorbance
  • ε = molar attenuation coefficient of the material, aka molar absorptivity. This is a proportionality constant (1/M cm)
  • b = length of light path, which is usually the width of the cuvette (cm)
  • C = molar concentration of the sample solution

Since ε is a proportionality constant and the length of the light path (b) also remains constant, this means there is a direct, linear relationship between the absorbance and the concentration of the solution. This is why the standardized curves have a linear line of best fit.


Another important thing to note is that transmittance is the opposite of absorbance-- percent transmittance represents the fraction of incident light transmitted rather than absorbed. The relationship between absorbance and percent transmittance is as follows:

[math]\displaystyle{ A=2-\log(T) }[/math]

where:

  • A = absorbance
  • T = percent transmittance


Finally, you can also calculate percent transmittance based on the intensities of the incident and transmitted light:

[math]\displaystyle{ T = I/I_0 \times 100 }[/math]

pH

pH is a measure of the concentration of hydrogen ions (H+) in a solution. It ranges from 0-14 (although solutions can occasionally exceed this range). A higher pH corresponds to a more acidic solution, while a lower pH corresponds to a less acidic (aka more basic, or more alkaline) solution. The pH scale is logarithmic, meaning that each one increase on the scale represents a solution that is ten times more acidic than the previous, and each one step decrease on the scale represents a solution that is one-tenth of the acidity of the previous one.

pOH is the opposite of pH-- it measures the concentration of hydroxide (OH-) ions in a solution. This scale also ranges from 0-14 and is logarithmic. However, larger values now represent a more basic/alkaline solution, while smaller values represent a more acidic solution.

pH Calculations

The relationship between pH and pOH is as follows: [math]\displaystyle{ pH + pOH = 14 }[/math]. This makes it easy to convert between pH and pOH.

To calculate pH from based on the concentration of hydrogen ions: [math]\displaystyle{ pH = -\log{[H^+]} }[/math]

Similarly, to calculate pOH from based on the concentration of hydroxide ions: [math]\displaystyle{ pH = -\log{[OH^-]} }[/math]

pH and Water Quality

U.S. EPA water quality criteria for pH in freshwater suggest a range of 6.5 to 9.

Under low pH conditions (acidic):

  • Metals tend to dissolve
  • Water corrodes pipes
  • Certain chemicals' toxicity may increase

Under high pH conditions (basic/alkaline):

  • Concentration and toxicity of [math]\displaystyle{ NH_3 }[/math] increases
  • Overgrowth of plants and algae

Total Dissolved Solids (TDS)

Total dissolved solids is a measure of the total concentration of dissolved substances in the water.

  • Freshwater: TDS < 1000 ppm
  • Brackish Water: TDS = 1000-10000 ppm
  • Saline Water: TDS = 10000-35000 ppm
  • Hypersaline Water: TDS > 35000 ppm

For drinking water, the EPA recommends no more than 500 mg/L (500 ppm).

Salinity

Salinity refers to the concentration of salts in water. The most common ions in saltwater are sodium and chlorine.

Chlorinity measures just the concentration of halide ions (mostly chlorine and bromine), which can be used to calculate the salinity (unless the salinity is very low) using this formula: [math]\displaystyle{ S = 1.80655 [Cl^-] }[/math] (units are in ppt)

Because dissolved ions in water affect both the salinity and conductivity, another way to measure salinity is to measure the conductivity of the water and convert it to salinity using the formula [math]\displaystyle{ 0.4665 \times C^{1.0878} = S }[/math] where C = conductivity in milliSiemens per cm and S = salinity (ppt or g/L).

Salinity Units and Scales

Salinity is typically measured in ppt = g/L = ‰ (permil).

Salinity calculated from chlorinity is known as Knudsen salinities.

The practical salinity scale 1978 (PSS-78) was developed after people began to measure water salinity using conductivity. The term "practical salinity unit" or PSU is sometimes incorrectly used to denote practical salinity scale even though PSS has no units.

The thermodynamic equation of seawater 2010 (TEOS-10) was developed to replace practical salinity with absolute salinity.

All of these scales are fairly close to one another.

Water Conductivity

As discussed in the salinity section, since dissolved ions in water affect both the salinity and conductivity, a way to measure salinity is to measure the conductivity of the water and convert it to salinity using the formula [math]\displaystyle{ 0.4665 \times C^1.0878 = S }[/math] where C = conductivity in milliSiemens per cm and S = salinity (ppt or g/L).


Water Hardness

Water hardness is the amount of calcium and magnesium dissolved in the water. It is measured in ppm, and the equation to calculate hardness is:

Hardness = [math]\displaystyle{ 2.497 [Ca^{2+}] + 4.118 [Mg^{2+}] }[/math]

All units in the above equation are in ppm or mg/L. However, hardness is also measured in other units, including grains per gallon or degree of hardness (dH). To convert from ppm or mg/L to grains per gallon, divide by 17.12.

A hardness (ppm or mg/L) of less than 17.1 is considered soft; 17.1-60 is slightly hard, 60-120 is moderately hard, 120-180 is considered hard, and >180 is considered very hard.

"as CaCO3"

Water hardness is also expressed as "mg/L as CaCO3" which is calculated as if all the calcium and magnesium were present only as calcium carbonate.

For this, 0-60 mg/L is considered soft, 61-120 mg/L is moderately hard, 121-180 mg/L is hard, and >180 mg/L is considered very hard.

However, do keep in mind that different sources have slightly different thresholds for the boundaries between different water hardness categories.

Residual Chlorine

Residual chlorine is also known as chlorine residual, free chlorine residual, and free chlorine.

During the process of potable water treatment, chlorine is added to the water as a disinfectant. Of the chlorine that is added, some of the chlorine will react with a variety of compounds in the water prior to disinfection. The chlorine that is used up in this process will NOT be used for disinfection and is the called chlorine demand of the water. Next, the remaining amount of the chlorine, or the total chlorine, is made up of the combined chlorine and the residual/free chlorine. The combined chlorine is combined with nitrogen compounds in the water so it also can't be used for disinfection. However, the residual/free chlorine CAN be used for disinfection.

Resources

https://www.epa.gov/

https://www.cdc.gov/