Aim
This practice has as objective learn the flow injection analysis technique (FIA) and determine the filterable reactive phosphorus (FRP) in Yarra River’s water samples in order to assess the trophic status of the Yarra River’s sites.

Methods A phosphate calibration curve was made from a stock solution provided. The samples and the QC solution was measured using the FIA technique and the FRP was obtained using the peak height from the samples. The QC solution was diluted 2:5. Sample Calculations x= y-0.29781.137 equation 1
% relative error=|x-x||x| (equation 2) x= Measured value of QC and x = Known value of QC

Results

Calibration curve

Sample | Peak | FRP | Mean | SD | %RSD | | Height | (µg/L) | | | | | 85.8 | 75.2 | | | | Dight Falls | 85.0 | 74.5 | 74.9 | 0.4 | 0.5 | | 85.5 | 74.9 | | | | | 37.0 | 32.3 | | | | Chirnside | 37.0 | 32.3 | 32.0 | 0.5 | 2 | | 36.0 | 31.4 | | | | | 27.5 | 23.9 | | | | Unknow | 27.3 | 23.7 | 23.7 | 0.2 | 0.9 | | 27.0 | 23.5 | | | | | 35.5 | 31.0 | | | | QA | 36.0 | 31.4 | 31.4 | 0.4 | 1 | | 36.5 | 31.8 | | | |
Table 1: Peak Height, FRP and statistics of the samples and QA.

Questions 7.1
The QC has a concentration of 100 µg/L, but it was diluted in a 2:5 ratio for the measurements, so the expected QC concentration was 40 µg/L. The found value for the QC concentration using the FIA is 31.4 ± 0.4 µg/L. The % relative error, calculated using the equation X above, is 21.5%. Therefore, we can observe that this measurement is not accurate. The possible source of error is that the FIA only measure the dissolved form of phosphorus and in the water might have particulate forms as well. Also, a poor dilution can be occurred or a wrong calibration.
7.2
The concentration of phosphorus obtained don’t indicate a problems, since they are lower than 0.08 mg/L (0.0749 ± 0.0004 mg/L, 0.0320 ± 0.0005 mg/L and 00.0237 ± 0.0002 mg/L). Despite the possibility of have particulate phosphorus on the water, the dissolved inorganic phosphorus is the most bioavailable form of phosphate in waters, so the FRP provides a good estimate of total phosphorus in these water samples.
7.3
Since the pH measured for the Dights Falls on experiment 1 is around 7, the DO is about 7 g/L as well and the Eh is 221 µS cm-1, the dominant orthophosphate form might be HPO42-
7.4
The term “bioavailable” means that the nutrient is available for the organisms (algae and others plants). This underestimation may occur when the inorganic phosphorus concentration is low or below detection (below 0.1 mg/L). Since the Yarra River samples have a concentration below 0.08 mg/L, this underestimation can be applied to them.
Conclusion
With this experiment we could learn how to do a phosphorus analysis using the FIA technique. This technique is not strongly useful when the inorganic phosphate concentration is under 0.1 mg/L. However, when the phosphate concentration is higher than this value, this technique can be useful to indicate if there is a problem with the phosphorus concentration, since the FRP measurements is easier than the total phosphorus measurements.

References * Laboratory Manual * Experiment 1 report * Dodds, W. K. (2003). “Misuse of inorganic N and soluble reactive P concentrations to indicate nutrient status of surface waters”, Journal of the North American Benthological Society, 22, 171 – 181.