1 / 21

Phosphorus Measurements

Phosphorus Measurements. The Technique Detection Limits Wallastonite. Ascorbic Acid Technique. Ammonium molybdate and antimony potassium tartrate react in an acid medium with orthophosphate -phosphorus to form an antimony-phospho-molybdate complex.

nadalia
Download Presentation

Phosphorus Measurements

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Phosphorus Measurements • The Technique • Detection Limits • Wallastonite

  2. Ascorbic Acid Technique • Ammonium molybdate and antimony potassium tartrate react in an acid medium with orthophosphate-phosphorus to form an antimony-phospho-molybdate complex. • This complex is reduced to an intensely blue-colored complex by ascorbic acid. • The reaction is slow and the complex is not stable and thus analysis must be performed after 10 minutes and before 30 minutes.

  3. Interference • Barium, lead, and silver interfere by forming a precipitate. • The interference from silica, which forms a pale-blue complex is small and can usually be considered negligible. • Arsenate is determined similarly to phosphorus and should be considered when present in concentrations higher than phosphorus.

  4. Sample Preparation • No pretreatment • Measures orthophosphates • Sulfuric acid treatment • Measures hydrolyzable and orthophosphates • Persulfate digestion (strong oxidant) • All phosphorus converted to orthophosphates • Measures total phosphorus

  5. Detection Limits • What controls our ability to measure small concentrations of phosphorus? • How could we determine if the answer we get is meaningful? • Expected analytical range is 10 mg/L to 1 mg/L as phosphorus

  6. Types of Detection Limits • Instrument detection limit (IDL) • instrument noise • Method detection limit (MDL) • instrument noise • sample preparation • Practical quantitation limit (PQL) • routinely achievable detection limit with reasonable assurance that any reported value greater than the PQL is reliable • 5 times MDL Which dominates?

  7. Instrument Noise for a Spectrophotometer • What measurements are involved in obtaining a concentration reading from a spectrophotometer? • _____________ • _____________ • _________________________________ • _________________________________ • _____________ • _____________ Reference (P0) Lamp intensity Absorbance of reference solution Absorbance of cuvette Standards sample preparation Sample

  8. What are the limitations at low concentrations? • Po - _________ light intensity • P light intensity after passing through sample • As C  0 P __ • Describe the journey after light leaves sample to computer • ______________________________________ • ___________________ • ________________________________ Reference P0 Photons strike diode and produce a voltage response Voltage is digitized Digital Calculations → absorbance

  9. Minimum Detectable Absorbance • Suppose a 12 bit Analog to Digital Converter is used. What is the smallest absorbance that can be measured? 12 bit ( ) means _____ intervals 4096 What if P0 is digitized into 200 intervals?

  10. Additional Instrument Limitations • Differences in ___________ • Fluctuations in ______ intensity • Power supply • Warm up time • Repeatability of Cuvette ___________ • Sample carryover if using sipper cell Cuvettes Lamp alignment

  11. Method Detection Limit • "Method detection limit" is the smallest concentration that can be detected above the noise in a procedure and within a stated confidence level. • What is C such that I can be 99% confident that C > 0?

  12. Measuring the MDL • Make a standard that is near the MDL • Divide it into at least 7 portions. • Process each portions through all sample preparation and analysis steps • Calculate the MDL using the equation n is the sample size, s is the standard deviation, a=0.01 is generally the required confidence, t is the student t distribution

  13. Is the MDL > IDL? • Are sample preparation errors significant? • Variability in reagent blank (reference sample) • Results in a calibration curve with nonzero intercept • Sample contamination • Ultra pure water • Acid washed plastic or glass ware • Airborne contamination

  14. Decreasing the IDL • May or may not decrease the MDL • How can you improve an estimate of a parameter? • Use more ________! • How could you use a section of the spectrum? • Use standards to determine _________ ________ ______ • Take an average of all the predicted concentrations? diodes Arrays! extinction coefficient array

  15. Maximum Detection Limit • Chemistry • reagent limitations (stoichiometry) • reaction by-products • Instrument limitations • Maximum detection limits are easily surmounted by __________ dilution

  16. Wallastonite • Wallastonite (calcium metasilicate mixed with ferrous and aluminum metasilicate) tailings can be used to effectively remove phosphorus from solution. • These tailings are waste products generated during wallastonite ore mining in Northern New York.

  17. Wallastonite Column Results 5 mg phosphorus/L influent • Why are long retention times needed? • What is the mechanism?

  18. Wallastonite Research (Proposal) • Quantify phosphorus removal as a function of time in batch tests • Phosphorus concentration (100 mg/L) • Wallastonite concentrations (0, 10, 30, 100, 300, 1000) mg per 7 mL phosphorus solution • Batch contact times (1, 5, 15, 30, 60, 90) minutes

  19. Expectations

  20. Prelab • You will be creating 1 mL standards by diluting a stock of 100 mg P/L (1, 3, 10, 30, 100 mg P/L) • Reagent dilution problem

  21. Spectral Analysis • The initial extinction coefficient arrays are obtained from the slope of the linear regression line for A(l) = f(c) • Uses general least squares regression to add multiples of extinction coefficient arrays for each component to obtain the best curve fit for the sample • A better estimate of the extinction coefficient is obtained by interpolating between adjacent standards • Repeat least squares regression analysis

More Related