Determination of dissolved inorganic phosphate

Murphy, J., and J. P. Riley The Storage of Sea-Water Samples for the Determination of Dissolved Inorganic Phosphate. Anal. Chim. Acta 14, 318 (1956). 

Analytical methods 175 10.2.6 Determination of dissolved inorganic phosphate in the presence of arsenic... 

Determination of dissolved inorganic phosphate by an extraction procedure (high-sensitivity method)... 

Proceed as described in Section 10.2.5.5, determination of dissolved inorganic phosphate (note the change of reagent sequence or additional ascorbic acid addition ), and in Section 10.2.12.2, calibration and calculation of total and organic nutrients. 

Murphy, J. and Riley, J.P., The storage of sea-water samples for the determination of dissolved inorganic phosphate, Analytica Chimica Acta 14,318,1956. 

The absorbance of the blue phosphorus complex reaches a maximum in a few minutes and stays almost constant for many hours. However, the absorbances should be read within 10-30 min, using a wavelength of 880 nm. This determination, on an unfiltered sample, gives the amount of dissolved inorganic phosphate ions in true solution and probably also includes a small fraction of those ions that are adsorbed onto particles and subsequently dissolved by the acid in the mixed reagent. The latter fi-action is not included when a filtered sample is analysed. 

Equation 8.4 predicts that aerobic respiration should release dissolved inorganic nitrogen and phosphorus into seawater in the same ratio that is present in plankton, i.e., 16 1. As shown in Figure 8.3, a plot of nitrate versus phosphate for seawater taken from all depths through all the ocean basins has a slope close to 16 1. Why do both plankton and seawater have an N-to-P ratio of 16 1 Does the ratio in seawater determine the ratio in the plankton or vice versa Current thinking is that the N-to-P ratio of seawater reflects a quasi steady state that has been established and stabilized by the collective impacts of several biological processes controlled by marine plankton. 

Table 1.1 shows the proportion of the several constituents found in the dissolved, diffusible state. Actually, phosphate is present in five classes of compounds inorganic dissolved, inorganic colloidal, water-soluble esters, ester-bound in caseins, and lipid. These can be determined by making the following analyses ... 

An alternative interference removal step was developed by Ledo de Medina et al. who developed an IC method for phosphate in natural waters in the presence of high concentration of sulphates. This interference was avoided by first precipitating sulphate as lead sulphate prior to 1C analysis. Samples with high iron content were investigated by Simon. Interferences caused by the precipitation of iron hydroxides from air oxidation of ferrous iron in anoxic water samples and from the alkaline eluent used in IC, were found to affect the determination of phosphate and other inorganic anions in riverine sediment interstitial water samples with high concentrations of dissolved iron (0.5 to 2.0 mmol/1). To eliminate this interference the complexation of iron with cyanide was used prior to IC analysis. ... 

Chemical methods for the detection and estimation of phosphorus in a compound are many and varied, but they usually require the phosphorus to be first obtained in the form of inorganic orthophosphate. For this, a prior oxidation and/or hydrolysis may have to be carried out. Insoluble phosphates are dissolved in mineral acids, with boiling if necessary. In some cases, fusion with alkaline fluxes (e.g. boric acid and sodium carbonate at lOOO C) is needed, followed by water or acid extraction of the soluble phosphates produced. Cation exchange resins may sometimes be used to convert insoluble into soluble phosphate salts. Numerous extraction and clean-up procedures have been prescribed for determination of the P content of compounds of bio origin. Pre-treatment of the sample by chemical methods is sometimes a necessary procedure before examination by physical methods (e.g. spectrophotometty), can be undertaken. 

Orthophosphate and condensed phosphate are a measure of inorganic phosphoms. The latter is also termed as acid-hydrolyzable phosphate. However, during mild acid hydrolysis, a small amount of phosphoms from organic phosphoms compounds may be released. To determine suspended and dissolved forms of phosphoms, the sample should be filtered through a 0.45 pm membrane filter, and the filtrate and the residue analyzed separately. 

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