Water Quality Monitoring: Total Phosphate

Total Phosphate

  • Phosphorus is usually present in natural water as phosphates (orthophospates, polyphosphates, and organically bound phosphates).
  • Phosphorus is a plant nutrient needed for growth and a fundamental element in the metabolic reactions of plants and animals (hence its use in fertilizers).
  • Sources of phosphorus include human and animal wastes (i.e., sewage), industrial wastes, soil erosion, and fertilizers.
  • Excess phosphorus causes extensive algal growth called "blooms," which are a classic symptom of cultural eutrophication and lead to decreased oxygen levels in creek water.

Test Equipment

  • Hach Total Phosphate Test Kit (Cat. No. 2250-01)
  • Graduated cylinder and cap for square mixing bottle
  • Alcohol lamp, matches, and tripod
  • Wash bottle containing demineralized water
  • Gloves and goggles
  • Waste container


If turbidity is low (less than 5 jtu), it is possible to skip the total phosphate measurement and proceed directly to orthophosphate, since very little phosphate will be attached to suspended solids in the water. In the summer, when turbidity is often low, algae is prevalent, and the total phosphate measurement will include phosphates bound up in the algae. This phosphate is not available for use by other plants and should not be included in the measurement.

The color comparison performed at the end of the procedure can be difficult if the sample water is colored or if the reagents produce a hue that does not closely match the color wheel. Cerrito Creek is often slightly yellow, for example. The treated sample can be compared to one or more of several reference points including distilled water, untreated sample, and reagent blank (i.e., treated distilled water). Please note on the field data sheet which reference point is used and any difficulties in comparison. If comparing to a reagent blank, please also compare the reagent blank to distilled water and record the offset.

Total Phosphate

  1. Put on gloves and goggles. To avoid contamination, thoroughly rinse the graduated cylinder with sample water three times. Hold the graduated cylinder face down and perpendicular to the creek at arm's length. Submerge it straight down into the water, rotate it to point upstream, then tilt it slightly up until it fills with water. Pour out water until 20 ml remain. Pour the sample into a clean 50 ml erlenmeyer flask.
  2. Open on potassium persulfate powder pillow. Add the contents of the pillow to the flask. Swirl to mix.
  3. Add 2.0 ml of 5.25N sulferic acid solution by twice filling the dropper to the 1.0 ml mark and discharging the contents into the flask. Swirl to mix.
  4. Set up the boiling apparatus. Unfold the cookit (2206-00) and place it upside-down under the tripod. Place the alcohol lamp on top of the cookit, the support (2179-00) on top of the tripod, and the sample on the support. Start the lamp with a match.
  5. Boil the sample for 30 minutes. Add a little demineralized water occasionally to keep the volume near 20 ml, but preferably below 18 ml, because 2 ml of another solution will be added in step 7 . Do not bring the volume above the 20 (preferably 18) ml mark near the conclusion of the 30 minute period. Do not boil to dryness. While the sample is boiling, it may be possible to conduct other measurements.
  6. Extinguish the flame. Allow the sample to cool.
  7. Add 2.0 ml of 5.0N sodium hydroxide solution by twice filling the dropper to the 1.0 ml mark and discharging the contents into the flask.
  8. Rinse the graduated cylinder with the demineralized water from the wash bottle and pour the rinse water into the waste container. Return the sample to the graduated cylinder. If the volume is less than 20 ml, add demineralized water (from the Hach kit) to return the volume to 20 ml. Proceed with the orthophosphate test, except read the ppm phosphate as total phosphate.


  1. Fill the square mixing bottle to the 20 ml mark with the water to be tested.
  2. Open one PhosVer 3 phosphate reagent powder pillow. Add the contents of the pillow to the bottle, cap, and swirl to mix. Allow at least two but not more than 10 minutes for color development. If phosphate is present, a blue violet color will develop.
  3. Insert the lengthwise viewing adapter into the comparator.
  4. Fill one sample tube to the line underlining "Cat. 1730-00" with the prepared sample.
  5. Place the tube containing the prepared water sample into the inside comparator opening.
  6. Fill the other sample tube with untreated water (or distilled water or reagent blank) to the line as described in step 4. Insert this tube into the outside comparator opening. The color of this sample will serve as a reference point.
  7. Orient the comparator with the tube tops pointing to a window or light source. View through the openings in the front of the comparator. When viewing, use care to not spill samples from unstoppered tubes.
  8. Rotate the disc to obtain a color match. Read the concentration of the measured parameter through the scale window. Make a note in the comments section of the field data sheet explaining which reference point was used.
  9. Divide the reading from the scale window by 50 to obtain the ppm phosphate (i.e., total phosphate if beginning with total phosphate procedure).


  1. Pour prepared and untreated water samples into the waste container.
  2. Return all glassware and chemicals to their boxes.
  3. When safe to do so, remove gloves and goggles.

Water Quality Index

WQI for TP
Note: If total phosphate is greater than 10 ppm,
the quality index equals 2.


  1. Convert total phosphate (ppm) to water quality index.

        Total phosphate:  (ppm)

    Water quality index:

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