How to Measure Organic Matter in Water

Measuring organic matter, or the remnants of organic compounds in the water, can help you track the health of your waterbody and the organisms living within it. If you have access to lab equipment, you can measure the total biodegradable dissolved organic carbon (BDOC) or the amount of chemical oxygen demand (COD) in your water. If you don’t have access to lab equipment, consider sending a sample to a lab in your area.

[Edit]Steps

[Edit]Collecting a Sample

1. Pick an area of the water that is slow or still. Water that is fast-moving is harder to sample because the organic matter is swirling throughout it. Find an area of water that is either slow-moving or still to get an accurate sample.^[1]
   
   • If there aren’t any areas of slow-moving or still water nearby, you can take a sample from a fast-moving area. However, it may not be as accurate.
2. Drop a weighted bottle into the water. Attach a weight or a rock to an glass bottle and attach it to a length of twine. Lower the bottle into the water until it hits the bottom.^[2]
   
   • A lot of sampling containers have been tried out and discarded, and while a bottle isn’t perfect, it is the most universally available.
3. Wait for about 2 minutes to allow the water to settle. If you are standing near the bottle, keep still so you don’t kick up sediment particles. Wait for about 2 minutes, leaving the bottle in the water the entire time.^[3]
   
4. Bring the bottle back up slowly and put a lid on it. Pull on the length of twine that the bottle is connected to and raise it up out of the water. Screw a lid on it so the sample is airtight and it won’t spill.^[4]
   
   • If you’d like to, take a few samples so you can get an average throughout the entire body of water.
5. Keep the sample in the fridge until you can test it. Make sure your fridge is set to so the sample stays cool, but not frozen. Ideally, you should take your sample right before you test it, but if you need to store it, you can put it in the fridge.^[5]
   
   • The sooner you test your sample, the more accurate your results will be.

[Edit]Measuring BDOC

1. Pour of the sample through a 2-um-pore-size filter. Measure out of your total sample, and set the rest aside to use later. Set a filter over a glass jar and pour your sample through the filter. Make sure all of the large pieces of organic matter are removed from the sample before you start testing it.^[6]
   
   • Make sure your filter is completely clean before you use it to avoid contamination.
   • Large pieces of sediment can contaminate your sample and disrupt the results.
   • BDOC is the total biodegradable dissolved organic carbon in the water, and it can help you determine how much bacteria is living in your waterbody.
2. Add of autochthonous bacteria to the sample. Autochthonous bacteria is bacteria that was already present in the water sample. After you strain your sample, add of the unfiltered water back into the glass jar.^[7]
   
   • Putting the bacteria back into the water will tell you how much biodegradable carbon there is, not just carbon in general.
3. Take of water from the sample. Now, measure out of the inoculated sample and separate it into two subsamples. Put the water into glass jars that have a lid on them for future storage. This will make testing easier and on a smaller scale.^[8]
   
4. Measure the samples with a Dohrman 80 Total Carbon Analyser. Set the samples upright into the carbon analyser and shut the plastic lid. Turn the machine on and watch the display screen to see when it starts it measurements. Wait for the numerical reading on the front of the machine to record your first dissolved organic carbon, or DOC, reading.^[9]
   
   • The Dohrman 80 Total Carbon Analyser works by heating up the samples to an extreme temperature and then using UV light to measure the carbon levels.
   • You can also use this machine to test total organic carbon, or TOC.
   • To get more accurate results, test each sample 2 times and then take the average of the readings.
5. Keep the samples at in the dark for 4 weeks. Place the sealed samples upright in the fridge that’s set to . Keep the door shut so the samples stay in the dark for 4 weeks, or 28 days.^[10]
   
   • The darkness will allow the bacteria to eat through the carbon to give you your results.
6. Measure each sample again with a Dohrman 80 Total Carbon Analyser. Take your samples out of the fridge and put them back into the Carbon Analyser. Turn it on and wait for the reading to give you your final DOC number for each sample.^[11]
   
   • If you’d like to, you can take 2 readings of each sample again and average them out.
7. Subtract the final DOC from the initial DOC to get BDOC. Take the average of all 4 of your readings to get the most accurate results. Your BDOC will be measured in parts per million, or mg/liter.^[12]
   
   • For example, if the initial DOC reading was 9.6 and the final was 7.8, the BDOC is 1.8 parts per million, or 1.8 mg/liter.
8. Analyze your results based on what kind of waterbody you have. The BDOC level in water can depend on numerous factors, but if there’s a lot of bacteria in your water breaking down carbon, it’s probably not suitable for drinking without further filtration. High BDOC, or BDOC above 1 part per million, isn’t necessarily bad in all bodies of water.^[13]
   
   • If you’re concerned about the levels of biodegradable carbon in your water, talk to a wastewater management expert.

[Edit]Testing COD

1. Separate of your sample into a test tube. You won’t need your entire water sample to determine the chemical oxygen demand. Set the rest of your sample aside in case you want to use it for more tests.^[14]
   
   • You can also test multiple tubes of samples at a time and average the results for more accuracy.
   • COD is the total chemical oxygen demand, and it can show you how much oxygen is being used in your waterbody.
2. Prepare a test tube with of mineral water. Make sure the water is pure mineral water without any additives. You’ll use this test tube along with your sample tube to figure out the final equation.^[15]
   
   • The mineral water tube is also called the “blank.”
   • You can find mineral water at most grocery stores.
   • You need to do the same steps to the sample and the mineral water to have a comparison for your final equation.
3. Add 1 g of magnesium sulfate, then let the mixture stand for 5 minutes. Take a small pipette and slowly add the magnesium sulfate to the sample and mineral water tube. Take your time, and make sure you don’t overload either tube. Leave your mixture on the counter for 5 minutes to let the solution dissolve.^[16]
   
   • You have to go slowly so that the magnesium sulfate can be absorbed into the water.
4. Add in 1 g of silver sulfate and a few glass beads. Heat up 3 to 4 glass beads at for 1 hour. Add 2 beads to each test tube as you add the 1 g of silver sulfate to each tube.^[17]
   
   • The glass beads give the liquid in each tube a large surface area so they can cool and heat rapidly.
5. Cool the sample in ice water, then add 75 mL of sulfuric acid. Dunk the tubes into ice water quickly, being careful not to get any water inside the tubes. Leave them there for 3 to 5 minutes until they are cooled to room temperature, then add in the sulfuric acid.^[18]
   
   • Use caution as you put in the sulfuric acid, and try not to get it on your skin.
   • If you do get sulfuric acid on your skin, rinse it off with cool water right away.
6. Add in 25 mL of potassium dichromate. Use a clean pipette to drop potassium dichromate into each test tube. You don’t need to go slowly, so you can put in all 25 mL at once.^[19]
   
   • Your sample may change colors at this point, which is normal.
7. Place the tubes onto a condenser and leave it for 2 hours. A condenser cools hot gas into liquids. Put all of your test tubes into the condenser and leave it on for 2 hours to transform your sample.^[20]
   
   • Make sure you keep track of time. If you leave your samples on the condenser for too long, it could skew your results.
8. Add mineral water to each tube, then cool them to room temperature. Turn the condenser off and let them cool enough so you can touch them, or for about 2 minutes. Add enough mineral water to each test tube to create of total liquid in each tube. Leave the test tubes out on the counter for 10 to 15 minutes until they are completely cool.^[21]
   
   • If your test tubes are too small, transfer the liquid to a larger glass container.
9. Titrate the sample with Ferroin indicator solution. Use a pipette to slowly add Ferroin indicator solution 1 drop at a time. Keep track of how many drops you use for both the mineral water tube and your actual sample tube.^[22]
   
   • It’s very important to count how many drops you use in each, because you’ll use that number in your final calculation.
10. Stop titration when the color turns reddish brown. The goal is to turn your sample from a green/blue color to a red/brown color. Once the sample changes, stop the titration and write down how many drops of indicator solution you used.^[23]
    
11. Determine COD by comparing your sample tube to the mineral water. First, subtract the amount in mL of indicator solution you used on the actual sample from the amount of indicator solution you used on the mineral water. Then, multiply that number by 0.1, which is the normality for the indicator solution you used. Multiply that sum by 8,000, then divide the number by to get your amount of chemical oxygen demand in parts per million, or mg/Liter.^[24]
    
    • For example, if you used 4 mL in your actual sample and 20 mL in your mineral water, subtract 20 - 4 to get 16. Then, multiply that by 0.1 to get 1.6. Multiply 1.6 x 8,000 to get 12,800, then divide that by 50 mL (your sample size) to get 246 mg/L.
12. Aim for a COD of 10 to 30 mg/L for an unpolluted waterbody. If the COD is 10 to 30 mg/L, it’s not polluted. If the COD is 25 to 50 mg/L, it’s mildly polluted. If the COD is 250 mg/L, it’s sewage water.^[25]
    
    • If you are concerned about the amount of COD in your water, contact a wastewater management specialist.

[Edit]Tips

• Test your sample as soon as possible for the most accurate results.

[Edit]Warnings

• Always read the manuals for all lab equipment before using it.
• If you are not familiar with lab equipment or the chemicals used in the testing process, send your sample to a lab for a professional evaluation.

[Edit]Things You’ll Need

[Edit]Collecting a Sample

• glass bottle with lid
• Weight
• Twine

[Edit]Measuring BDOC

• Filter
• Glass jars
• Dohrman 80 Total Carbon Analyser

[Edit]Testing COD

• Test tubes
• Mineral water
• Pipette
• Magnesium sulfate
• Silver sulfate
• Glass beads
• Sulfuric acid
• Potassium dichromate
• Condenser
• Ferroin indicator solution

[Edit]References

1. ↑ https://pubs.usgs.gov/twri/05a03/report.pdf
2. ↑ https://pubs.usgs.gov/twri/05a03/report.pdf
3. ↑ https://pubs.usgs.gov/twri/05a03/report.pdf
4. ↑ https://pubs.usgs.gov/twri/05a03/report.pdf
5. ↑ https://pubs.usgs.gov/twri/05a03/report.pdf
6. ↑ https://aem.asm.org/content/aem/55/10/2732.full.pdf
7. ↑ http://www.seas.ucla.edu/stenstro/j/j62
8. ↑ https://aem.asm.org/content/aem/55/10/2732.full.pdf
9. ↑ https://www.ru.nl/science/gi/facilities-activities/elemental-analysis/toc/
10. ↑ https://aem.asm.org/content/aem/55/10/2732.full.pdf
11. ↑ https://aem.asm.org/content/aem/55/10/2732.full.pdf
12. ↑ https://aem.asm.org/content/aem/55/10/2732.full.pdf
13. ↑ https://aem.asm.org/content/aem/55/10/2732.full.pdf
14. ↑ https://pubs.usgs.gov/twri/05a03/report.pdf
15. ↑ https://pubs.usgs.gov/twri/05a03/report.pdf
16. ↑ https://pubs.usgs.gov/twri/05a03/report.pdf
17. ↑ https://pubs.usgs.gov/twri/05a03/report.pdf
18. ↑ https://pubs.usgs.gov/twri/05a03/report.pdf
19. ↑ https://pubs.usgs.gov/twri/05a03/report.pdf
20. ↑ https://pubs.usgs.gov/twri/05a03/report.pdf
21. ↑ https://pubs.usgs.gov/twri/05a03/report.pdf
22. ↑ https://pubs.usgs.gov/twri/05a03/report.pdf
23. ↑ https://pubs.usgs.gov/twri/05a03/report.pdf
24. ↑ https://pubs.usgs.gov/twri/05a03/report.pdf
25. ↑ https://pubs.usgs.gov/twri/05a03/report.pdf

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