AOAC 973.47 Organic Carbon in Water - Infrared Analysis Method

The standard “AOAC 973.47 Organic carbon in water - Infrared analysis method” developed by the Association of Official Analytical Chemists (AOAC) describes a procedure for determining carbon in a water sample.

AOAC International is a non-profit scientific association headquartered in the United States. It was founded in 1884 as the Association of Official Agricultural Chemists and became AOAC International in 1991.

When applying the method defined in this standard, in principle, the carbonaceous material of the water sample is oxidized to oxygen (O950) in a catalytic combustion tube at 2 degrees or to carbon dioxide (CO2) in the air stream. The calibrated infrared (IR) analyzer measures the carbon dioxide.

The method is valid for 1-150 mg of organic carbon in surface and saline waters and domestic and industrial wastes. Pretreatment of the sample defines the type of carbon measured:

• Soluble, non-volatile organic carbon (e.g. natural sugars)
• Soluble, volatile organic carbon (e.g. mercaptans)
• Insoluble, partially volatile carbon (e.g. oils)
• Insoluble, particulate carbonaceous materials (e.g. cellulose fibers)
• Soluble or insoluble carbonaceous substances adsorbed or trapped on insoluble inorganic suspended matter (e.g. oily matter adsorbed on silt particles).

Since the usefulness of the method is to assess the potential O2 demanding load of organic matter, carbon trioxide (CO3) and bicarbonate (HCO3) carbon should be removed before analysis or subtracted from the final result. The results for full increments of oxidizable organic compounds of distilled water (H2O) analyzed in various laboratories are included in the table titled “Statistical results for water containing full increments of oxidizable organic compounds”.

When preparing samples, glass bottles are the preferred storage containers, but polyethylene and Cubitainers can be used if tests have not shown carbon addition to the samples. Keep the interval between collection and analysis to a minimum, store at 4°C and protect from light and O2. If samples cannot be analyzed within 2 hours, acidify with hydrochloric acid or sulfuric acid to a pH of not more than 2.

The equipment used in these tests includes:

• Organic carbon analyzer: Includes air pump, purification train, flow controls, special non-dispersive type IR flow analyzer for CO2 and recorder.
• Syringes:
  • 0-50L, needle opening about 150m
  • 0-500L, needle opening about 400m
  • 20 or 200 L size push-button syringes ensuring uniformity of injection speed
• Reagents:
  • Water: For dilution of samples and preparation of standards, blanks and reagents. Use CO2-free, double-distilled water, not ion-exchange purified water.
  • Organic carbon standard solutions: Stock phthalate solution or working solutions.
  • Carbonate carbon standard solutions: Stock solution, working solutions.
  • Packing for total carbon tube: No forced packing. When completed, the packing length should be about 5-6cm. Test the packed tube by measuring the flow rate of gas passing through it at room temperature and 950 degrees. The ratio should be less than 20 percent.
  • Packing for carbonate tube (double channel instrument): A small piece of quartz wool or asbestos is placed near the outlet of the carbonate evolution tube. 10-6 porous quartz chips are added to a length of 12 cm from the inlet end. While holding the tube vertically, phosphoric acid is added and excess water is allowed to drain.

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