In the Laboratory
Determination of Phosphates by the Gravimetric Quimociac Technique Lee Alan Shaver Department of Chemistry, Washburn University, Topeka, KS 66621;
[email protected] Many gravimetric methods of analysis performed by students are tedious, laborious, and may not be in wide use in non-academic laboratories. However, students acquire improved laboratory technique skills and learn interesting chemistry from experiments involving gravimetric methods. Laboratories throughout the world utilize the gravimetric technique when relatively high accuracy and precision are required. Phosphates are commonly found in fertilizers, cleaners, cement, and as food and feed additives. Some phosphoruscontaining compounds such as phosphorus sulfides are commonly processed into lubrication oil additives, pesticides, and ore flotation products. Rapid, simple, accurate, and precise phosphorus-content results are required for the production and sale of many of these compounds (1, 2). The so-called gravimetric quimociac technique is a classic method proven to meet this need in industrial and government labs. It has worked very well as an experiment in our undergraduate analytical chemistry laboratory. The term “quimociac” is the shortened name for the quinoline molybdophosphoric acid precipitate formed in the test method. The method has the advantages of producing a predictable, stable, high molar mass precipitate that students find easy to form, filter, dry, and weigh. The group of students performing this experiment enjoyed it much more than the groups doing the more tedious classic filtration of gelatinous iron hydrous oxide in the gravimetric determination of iron listed by Harris (3). Principle This method consists of first converting all the phosphoruscontaining species in the sample to soluble orthophosphate (PO43‒) ion by oxidation and hydrolysis in acid solution. Then an acidic quimociac reagent is added to the prepared orthophosphate sample and a bright yellow precipitate forms. The resulting precipitate is filtered, dried and weighed. Precipitation follows the reaction:
Validation of the gravimetric quimociac method was the subject of several studies, especially for fertilizer analysis (7, 8). This is a good opportunity to introduce students to the interesting chemistry involved in the gravimetric quimociac method. In acidic solutions, molybdate ions react with orthophosphate ions to form the classic heteropoly cage-like Keggin structure of 12-molybdophosphate (9–11). Twelve molybdate ions surround a single phosphate ion (Figure 1). This heteropoly anion accepts three acidic protonated quinoline cations to form the insoluble quinoline molybdophosphoric acid. Experimental The quimociac reagent is prepared a day or more in advance. Ammonium molybdate tetrahydrate is dissolved in deionized water. In a separate beaker citric acid monohydrate is dissolved in dilute nitric acid. After cooling, the molybdate solution is added to the citric–nitric acid mixture with agitation. In a separate beaker, synthetic quinoline is added to deionized water. Then the quinoline solution is added to the molybdate– citric–nitric acid solution. After mixing, the solution is allowed to stand overnight. The solution is filtered through fine porosity paper then acetone and deionized water are added. Then the solution is mixed and stored in a polyethylene bottle. The mixed reagent is generally stable for up to one month. Sample preparation depends on the type of sample of material to be tested and may be done in advance. Students may be provided with both known and unknown samples. For example, solid inorganic phosphates are oven dried overnight a day or more in advance to remove moisture. For typical samples of this type, between 0.5 to 1 g of sample is weighed to the nearest 0.1 mg. The sample is transferred to a beaker; deionized water is added, along with concentrated nitric and hydrochloric
H3PO4 12H2MoO4 3C9H7N (C9H7N)3·H3PO4·12MoO3·H2O 11H2O
When the precipitate is dried, the water of hydration is removed, leaving a stable, anhydrous, yellow product with a molar mass of 2213 g/mol (4). It is essential to convert all the phosphorus in the sample to soluble orthophosphate (PO43‒) ion. Examples of treatments are oxidation and hydrolysis by heating in acidic bromine or in nitric acid solution. Inorganic polyphosphates (pyro-, tripoly-, and glassy phosphates) are hydrolyzed to orthophosphate by boiling in acid solution. Citric acid and acetone are added to the quimociac reagent to eliminate interference from silicate and ammonium ions, respectively, that are present in some types of samples (5, 6).
Figure 1. Cage-like Keggin structure of 12-molybdophosphate acid: the orange circle is the P atom; the pink circles are the Mo atoms; and the red circles are the O atoms.
© Division of Chemical Education • www.JCE.DivCHED.org • Vol. 85 No. 8 August 2008 • Journal of Chemical Education
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In the Laboratory
acids. The beaker is covered with a watch glass and the solution is boiled gently on a hot plate in the fume hood for about 15 minutes (one hour for glassy phosphates or phosphate rock). More deionized water is added if necessary so that the solution does not go dry. The solution is cooled, filtered if necessary, then transferred quantitatively to a volumetric flask. The prepared sample solution is then diluted to volume with deionized water and mixed. Sample analysis is normally completed during the scheduled lab period. An aliquot of the prepared sample solution containing up to 70 mg as P2O5 is quantitatively transferred by pipet to a beaker then diluted to about 100 mL with deionized water. Quimociac reagent is added to the sample solution with gentle swirling. The beaker is covered with a watch glass and heated to a boil for one minute. At this time a yellow precipitate forms. The solution is cooled in a water bath to about room temperature, then vacuum filtered through a clean, dry, tared, medium-porosity fritted-glass filter crucible. The yellow precipitate is washed with small portions of deionized water. The precipitate on the fritted-glass filter is dried for 20–30 minutes at 200–230 °C. The crucible containing the precipitate is cooled in a desiccator then weighed to the nearest 0.1 mg. The fritted-glass filter crucibles may be cleaned before and after the test by brief soaking in 10% sodium hydroxide or concentrated aqueous ammonia followed by dilute acid and water rinses. The crucibles are dried at 200–230 °C for an hour or more then cooled and stored in a desiccator. The heated and cooled crucibles are weighed then heated and cooled again until a constant weight is obtained. Calculations are relatively simple and it is a good exercise for the students to compute the molar mass of the precipitate from the stoichiometry given in the precipitation reaction then set up the calculation equation. It is common practice to report phosphorus content either as percent of P2O5 or as percent of P. The calculation for percent of P2O5 is percent P2O5
sample g volume 100% mol P2 O 5 ratio g mol ppt mass 22213 2 mol ppt mol P2 O5 sample
mass ppt 141 . 9
sample volume sample solution volume volume sample aliquot ratio
Hazards Nitric and hydrochloric acids are corrosive and dangerous if inhaled or if they come into contact with any part of the body. Quinoline, citric acid, acetone, and ammonium molybdate are all irritants. Quinoline is a cancer suspect agent. Acetone is volatile, flammable, and dangerous if inhaled. Work in the hood and wear protective gloves when preparing the quimociac reagent and when treating samples with acids. Sodium hydroxide and aqueous ammonia are irritants. Aqueous ammonia is an inhalation hazard and should be used in the hood. Student Results Overall student results were within 6% (relative) of certified values and the relative standard deviation was less than 1098
3%. The unknowns that the students tested contained 10 to 11% P2O5. This accuracy and precision are typical for samples containing this level of P2O5 when compared to the results of a collaborative study where multiple analysts tested 29 different samples ranging from 0.02 to 61% P2O5 (12). The greater the percent of P2O5, the better the accuracy and precision. Accuracy and precision also improve with experience. One sample can be analyzed in about two hours, of which 25 minutes is working time. Six samples can be analyzed in about three hours, of which one hour is working time. The quimociac reagent and filter crucibles need to be prepared prior to the laboratory session. Samples may need to be dried and prepared in advance. Summary Reported here is a practical, accurate, and precise gravimetric technique for the determination of phosphates as a lab experiment for undergraduate analytical chemistry laboratories. The experiment requires some preparation of reagents and samples in advance, but students should find it easy to complete the lab within a single three-hour lab period. In addition to practicing laboratory gravimetric technique, students should also learn about the dissolving and hydrolysis of phosphatecontaining compounds as well as the formation and structure of 12-heteropoly molybdophosphoric acid. Literature Cited 1. Hoffman, W. M.; Ferretti, R. J. J. AOAC Int. 1962, 45, 40–46. 2. Solomon, S.; Lee, A.; Bates, D. J. Chem. Educ. 1993, 70, 410–412. 3. Harris, D. C. Quantitative Chemical Analysis, 7th ed.; W. H. Freeman and Company: New York, 2006; Experiment 3 from text Web site; http://www.whfreeman.com/qca7e (accessed Mar 2008). 4. Wendlandt, W. W.; Hoffman, W. M. Anal. Chem. 1960, 32, 1011–1012. 5. Wilson, H. N. Analyst 1954, 79, 535–546. 6. Dahlgren, S. E. Z. Anal. Chem. 1962, 189, 243–256. 7. Hoffman, W. M.; Breen, H. J. J. AOAC Int. 1964, 47, 413–419. 8. Johnson, F. J. J. AOAC Int. 1973, 56, 1084–1086. 9. Pope, M. T. Heteropoly and Isopoly Oxometalates; Springer-Verlag: New York, 1983; pp 23–31. 10. Moffat, J. B. Metal-Oxygen Clusters; Kluwer Academic/Plenum Publishers: New York, 2001; pp 5–40. 11. Cotton, F. A.; Wilkinson, G.; Murillo, C. A.; Bochmann, M. Advanced Inorganic Chemistry, 6th ed.; John Wiley and Sons: New York, 1999; pp 920–945. 12. Caudill, P. R. J. AOAC Int. 1969, 52, 587–592.
Supporting JCE Online Material
http://www.jce.divched.org/Journal/Issues/2008/Aug/abs1097.html Abstract and keywords Full text (PDF) Links to cited URLs and JCE articles Supplement
Student handout including laboratory questions
Instructor notes
Journal of Chemical Education • Vol. 85 No. 8 August 2008 • www.JCE.DivCHED.org • © Division of Chemical Education
