Phosphate present in natural water or wastewater as compound orthophosphate, polyphosphate and organic phosphate. Each phosphate compound is present in dissolved form, suspended or bound in the cells of organisms in the water. orthophosphate is an inorganic phosphate, is one form of phosphorus (P) dissolved in water. Orthophospate is benttuk phosphor that can be directly utilized by plant organisms (phytoplankton and water plants).
In acidic solution, orthophosphate reacts with ammonium molybdate to form ammonium phosphomolybdate complex compound. With a reduction reagent (Method Stannous chloride), molybdenum in the complex compounds can be reduced to a blue compound. Blue intensity is increased by the amount of dissolved phosphate levels that exist. The amount of orthophosphate concentration in the water sample can be measured using the principle spektrofotomerik conducted in labs.
In order to be readable by a machine spectrophotometer, orthophosphate in 10 ml water sample that has been filtered should be treated in advance with several chemical compounds. However, this reaction must be running under a pH of 8.3. Therefore, the water sample is given 1 or 2 drops of phenolphthalein indicator as the pH indicator. When they appear pink after a given indicator (ie pH> 8.5), then the pH of the water sample is derived by adding dilute H2SO4 until the color changed to clear (pH <8.3).
After that the water sample is reacted with 1.6 ml combine reagent comprising 5 N H2SO4, tartat antimonil potassium, ammonium molybdate, and ascorbic acid. Then sealed and allowed to stand for 10 minutes. Then the color of the water sample absorbance (blue) are measured with a spectrophotometer at a wavelength of 880 nm. The blue color is caused due to formation of ammonium compounds fosfomolibdat reduced. Then Absorbance water sample absorbance adjusted with distilled water (blank) and a constant calculation (APHA, 1989).
Ways of working :
a. Taking water samples using a pipette 2.0 that has been filtered and put into a test tube.
b. Add 2.0 ml of H3BO3 1%, and stirred.
c. Add 3.0 ml pengoksid phosphate (a mixture of 2.5 M sulfuric acid, ascorbic acid and ammonium molybdate) and stirred. And let an hour, so that reaction was perfect.
d. Make a reference solution of 2.0 ml of distilled water. By performing the procedure b and c
e. Choosing a measurement program phosphate spectrophotometer
f. Entering into the cuvette reference solution that has been made and then insert the tool into a spectrophotometer cuvette and then press the “Zero”
g. Afterwards insert cuvette containing water samples have been prepared and then pressing the “Read”
h. Noting the value of phosphate obtained in mg / L
Figure 33. Measurement of water quality by using a spectrophotometer