Learning Objectives Analyze ppm and ppb values to determine toxicity. Recall basic metric prefixes and conversion factors. Calculate ppm and ppb concentrations if provide mass and volume amounts. Compare calculated values to EPA primary standards. Classify contaminant as being primary or secondary. If contaminant is primary, then recognize it as being inorganic, organic, microbial, radionuclide, disinfectant, or disinfectant byproduct. Parts Per Million (PPM) Many of the EPA's primary contaminants are regulated in ppm concentrations. Recall from section 8.2, the meaning of ppm in terms of the metric system. These are very small concentrations of solutions. To visualize how tiny these units are, watch the video below. In aqueous solutions, 1 ppm is essentially equal to 1 mg/L \[\text{1 ppm}= \dfrac{\text{1 mg Solute}}{\text{1 L Solution}} \nonumber \] The equation represents the allowable milligram toxin amount per liter of drinking water. A small ppm value correlates to lethal substances. Click here to view the EPA MCL primary drinking water concentrations. You will note that many of them are represented by ppm amounts. For parts per billions, 1 ppb corresponds to 1 µg/L. \[\text{1 ppb} = \dfrac{1\; \mu \text{g Solute}}{\text{1 L Solution}} \nonumber \] A simple method of converted a ppm concentration to ppb units is to multiply by 1000. When given a ppb value, simply dividing that number by 1000 will produce a ppm amount. Example \(\PageIndex{1}\) When calculating ppm, remember to employ the metric system. Often, the mass of the contaminant may be given in grams or another metric unit. This mass should be converted to milligrams before it can be placed into the ppm equation. It is common to make laboratory volume measurements in milliliters. These too, must converted to liters before inserting into the equation above. Check your learning management system (moodle) for video footage of your instructor working ppm/ppb problems. Your drinking water contains .00055 grams of chlorite per 200 mL of water. Does this exceed the EPA drinking water standard? Also, which type of primary contaminant is this chemical (hint: use the hyperlink above and scroll until you see the classification)? Solution \[\begin{align} .00055\, \cancel{g} \times \dfrac{1000\,mg}{1\,\cancel{g}} &= .55\,mg \\[5pt] &= 5.5\times 10^{-1}\,mg \end{align} \nonumber \] \[\begin{align*}200\,\cancel{ml} \times \dfrac{1\,L}{10^{3}\,\cancel{ml}} &= .200\,L \\[5pt] &= 2.0\times 10^{-1}\,L \end{align*} \] \[\text{2.75 ppm}= \dfrac{\text{.55 mg Solute}}{\text{.2 L Solution}} \nonumber \] This is larger than the EPA drinking water standard for chlorite (MCL= 1.0 ppm). This contaminant is a byproduct of disinfection. Remember to use the EPA's National Primary Drinking Water Regulations handout as a reference guide to assist you in learning more about a specific contaminant. Example \(\PageIndex{2}\) Based upon MCL values which contaminant is more toxic: Diquat or Thallium? Also, classify Diquat and Thallium as being one of the six primary drinking water contaminants. Solution The MCL value for Diquat is .02ppm while allowed Thallium concentration is .002 ppm. The smaller the MCL concentration the more dangerous the contaminant is. Therefore, Thallium is the more toxic of these two chemicals. Looking at the EPA legend, Diquat is an organic chemical and Thallium is an inorganic element.