1. True. Organic material, one cause of high blanks, can be present even after distilling water and also can be leached from deionization columns.
2. False. Standard Methods 5210B says all samples taken after disinfection must be seeded. Furthermore, unless it is known for certain that sufficient viable bacteria are present in a sample, it must be seeded.
3. True. This is the basis for the CBOD test and the reason CBOD results should be lower than BOD results for the same sample when the sample includes nitrogen that can be converted to ammonia.
4. True. Standard Methods 5210B says valid bottles must deplete at least 2.0 mg/L DO while leaving a residual DO of at least 1.0 mg/L. It makes no exception for the seed control bottle. (The 21st edition of Standard Methods no longer says the seed control bottle must deplete between 0.60 and 1.00 mg/L of DO.)
5. c. See Standard Methods Sec. 5210A(1). Concentration of organic material (a) is approximated by the total organic carbon (TOC) test, organic material and reduced forms of nitrogen (b) by the TOC, ammonia and nitrite tests, and dissolved oxygen (d) by the DO test.
6. b. Aging encourages growth of nitrifying bacteria, which would bias BOD and ultimate BOD results. For CBOD analysis, however, nitrifying bacteria are inhibited, so their presence would not have an effect on the test results.
7. d. Sodium sulfite is the best choice. While, sodium thiosulfate and sodium sulfite both remove unwanted chlorine, both also have an oxygen demand that, if added in excess, would bias the BOD result on the high side. Because sodium sulfite’s oxygen demand is less than that of sodium thiosulfate, sodium sulfite is the better choice. Sulfuric acid and sodium hydroxide both interfere with the BOD test.
8. a. Deionization columns including an activated carbon filter that removes impurities that might have made it through the distillation process. Drinking water usually contains additives to give it “taste.” Filtering removes microscopic solid material but not dissolved organic and inorganic material. Water downstream from a wastewater treatment plant (or elsewhere in a stream) would most likely contain materials that would interfere with the BOD test.
9. b. Temperature, atmospheric pressure, and salinity all affect the saturation point of dissolved oxygen in water. Even though the elevation of the lab has an effect on atmospheric pressure, knowing the lab’s elevation is not enough, because atmospheric pressure changes from hour to hour and day to day.
10. b. Dilution factors for bottles 23, 24, and 25 are 60, 30, and 20, respectively. Since use of 2 mL of seed in the seed control bottle depleted 2.0 mg/L of DO, 1 mL of seed would cause a depletion, or seed correction, of 1.0 mg/L. Total depletion for Bottle 23 is 8.80 mg/L – 4.50 mg/L = 4.30 mg/L. Depletion after seed correction is 3.30 mg/L. BOD for Bottle 23 is 3.30 mg/L times the dilution factor of 60 = 198 mg/L. Total depletion for Bottle 24 is 8.80 mg/L – 1.80 mg/L = 7.00 mg/L. Depletion after seed correction is 6.00 mg/L. 6.00 mg/L times dilution factor of 30 = 180 mg/L. Bottle 25 did not leave a residual of 1.0 mg/L or more and is an invalid bottle. The average of the two valid bottles is (198 + 180)/2 = 189 mg/L.