October 17, 2013
Department of Civil Engineering
CIV1319 Chemistry and Analysis of Waster and Water
TOTAL PHOSPHORUS AND ORTHOPHOSPHATE
(TP – PO4-3)
Student Number: 1000475121
Lab. # 4
This practice aims to introduce and learn Phosphorus analysis, and its environmental impact. SPECIFIC OBJECTIVES
Understanding the importance of phosphorus controls and its environmental impact. Understanding the different form of phosphorus in water, and the possible sources of contaminations. Understanding the colorimetric Stannous method for measuring Orthophosphate Understanding the acid digestion method to convert organic phosphorus and condensate into orthophosphate ions. Build a calibration curve using a the colorimetric method.
Phosphorus is an essential nutrient for all aquatic plants and algae. Only a very small amount is needed, however, so an excess of phosphorus can easily occur. Excess phosphorus is usually considered to be a pollutant because it can lead to eutrophication, a condition where an overabundance of nutrients, cause increased plant and algal growth, lowering the level of dissolved oxygen in water affecting the many aquatic organism.
Most phosphorus in surface water is present in the form of phosphates, however there are four classifications: Orthophosphates: sometimes referred as “reactive phosphorus” which is inorganic forms of phosphates such as PO4-3, HPO4-2 and H2PO4-1. These are form of phosphates used heavily in fertilizer and are often introduced to surface water through runoff. Organically bound phosphate is phosphate bound to plant or animal tissue. Organic phosphates are formed by biological processes. They are contributed to sewage by body waste and food residues. Condensed phosphates (pyro-, meta- and other polyphosphates) are sometimes added to water supplies and industrial processes to prevent formation of scaling and to inhibit corrosion. In water, polyphosphate are unstable and will eventually convert to orthophosphate. Total phosphates are the sum of the aforementioned forms.
The EPA water quality criteria state that phosphates streams/river 0.1mg/L, streams entering lakes 0.05 mg/L and lakes/reservoirs 0.025 mg/L. The separation between dissolved and particulate fractions is achieved by filtrations through 0.45 m membrane filters. Stannous Chloride Method
In this colorimetric determination of phosphate concentration, ammonium molybdate reacts under acid conditions to form a heteropoly acid, molybdophosphosric acid. PO4-3 + 12(NH4)2MoO4 + 24H+ == (NH4)3PO4.12MoO3 + 21NH4+ +12H20 The molybdophosphate complex is then reduced by stannous chloride to intensely coloured molybdenum blue. The concentration of the orthophosphate is then determined colorimetrically. (NH4)3PO4.12MoO3 + Sn+2 = (Molybdenum blue) + Sn+4
Normally the analysis of total phosphorus involves two steps. First the digestion or conversion of the phosphorus to dissolved orthophosphate and then the colorimetric determination of dissolved orthophosphate. Acid hydrolysis at boiling water temperature converts dissolved and particulate condensed phosphate to dissolved orthophosphate. The phosphate fraction that is converted to orthophosphate by oxidization of organic matter effectively is considered organic or organically bound phosphate.
To prepare a series of standards solutions with orthophosphate it was used a Stock PO4-P solution with a concentration 1.0 mL = 0.05 mg PO4-P.
As the stock concentration had a high concentration in relation with the desired standards concentration, the volume required to prepare the series of standards from the stock solution was less than 1 mL. Small deviation in the volume can results in high difference in concentration. For this reason, it was prepared an intermediate solution in order to avoid and diminish the impact of possible...
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