A Novel Trace Elemental Analysis of Potassium Phosphates
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A Novel Trace Elemental Analysis of Potassium Phosphates A thesis submitted to the Graduate School of the University of Cincinnati in partial fulfillment of the requirements for the degree of Master of Science in the Department of Chemistry of the College of Arts and Sciences by Josh Rohman B.S., University of Cincinnati June 2008 Committee Chair: William Heineman, Ph.D. Committee Chair: Julio Landero Figueroa, Ph.D. Abstract In the chemical manufacturing industry, it is critical to have an understanding of not only the major components in the product of interest but also the contaminants or trace components. Potassium phosphates are common to the food and industrial markets, so trace metals are of particular concern in order to protect people as well as processes and equipment from their effects. This can be a particularly difficult task due to the samples having a high dissolved solids content which can cause issues with ICP analyses. In this study, a method is developed to digest anhydrous tetrapotassium pyrophosphate and analyze for trace metals by inductively coupled plasma optical emission spectrometry (ICP- OES). The analytes of interest were As, Cr, Fe, Mn, Ni, Pb, and Zn. Reproducibility was performed; however, spike recovery proved to be a better measure of success due to the majority of the analytes being near or below the method detection limit. The recoveries for all analytes ranged from 96.9-102.4%. Although the recovery was successful, the reproducibility of the analytes above the method detection limit had relative standard deviations mostly greater than 10%, which could be a sign that the solid sample was not homogenous since prior studies with liquid tetrapotassium pyrophosphate had relative standard deviations below 5%. ii iii Acknowledgements I would like to recognize the late Dr. Joseph Caruso for his guidance as I have pursued my advanced degree and accepting me into his research group. His support, encouragement, and insight into the academic world provided me the keys to success in pursuing my degree. I would also like to thank my committee members Dr. William Heineman, Dr. Julio Landero Figueroa, Dr. Hairong Guan, and Dr. Thomas Ridgway for their time and support in finishing this project. I would also like to thank my family for supporting me throughout my college career. Everyone supported me by providing me the time needed to finish this thesis by watching my sons. My industry mentor also deserves a tremendous amount of recognition; Dr. Yonghua Xu provided me an unlimited amount of support while on my Co-op assignment and eventual employment at Sun Chemical. His vast knowledge of analytical chemistry and method development were critical to my project and my current career success. I would like to thank PotashCorp for allowing me to use their laboratory to perform my experiments. Lastly, my wife deserves the most thanks. She provided me the support I needed to finish this project. She pushed me to get through the toughest parts of my college career and celebrated all of my successes with me as well. She always found a way to get me the time I needed to work on my thesis and was extremely understanding of the nights I had to stay late in the lab to finish an analysis. iv Table of Contents Abstract…………………………………………………………………………………………… ii List of Tables……………………………………………………………………………………...vi List of Figures………………………………………………………………………………….... vii Introduction……………………………………………………………………………………….. 1 Phosphate Production……………………………………………………………………...1 Uses of Phosphates……………………………………………………………………….. 2 Trace Metals in Potassium Phosphates…………………………………………………… 2 Experimental……………………………………………………………………………………… 4 Instrumentation…………………………………………………………………………… 4 Reagents and Standards…………………………………………………………………... 5 Sample Preparation……………………………………………………………………….. 5 Results and Discussion…………………………………………………………………………… 7 Quality Control…………………………………………………………………………… 7 Digestion………………………………………………………………………………….. 7 Method Validation………………………………………………………………………... 9 Discussion………………………………………………………………………………...12 Conclusions and Future Work……………………………………………………………………17 References……………………………………………………………………………………….. 18 v List of Tables Table Page 1. ICP-OES parameters and conditions…………………………………………………………... 4 2. Digestion parameters…………………………………………………………………………... 6 3. Comparison of digestion acids…………………………………………………………………. 8 4. LOD and LOQ results………………………………………………………………………… 10 5. Wavelengths used during analysis……………………………………………………………. 11 6. Validation results for linear and full fit calibration…………………………………………... 13 vi List of Figures Figure Page 1. Chemical reactions that produce TKPP……………………………………………………....... 1 2. As 449.423 nm calibration curve…………………………………………………………… 12 3. Linear versus full fit calibration curves for manganese………………………………………. 15 vii Introduction Phosphate Production Potash in industry today typically refers to potassium salts from underground mining, primarily composed any or all of potassium chloride, potassium sulfate, and potassium nitrate [1]. It has been used in many applications throughout time, but today it is used primarily in fertilizer and as a feedstock for other industrial chemicals, such as potassium hydroxide. Potassium hydroxide, or caustic potash, is a strong alkali available in both solution and solid forms [2]. It has many uses in industry; almost all use it for its reactivity towards acids. Phosphoric acid is produced through two main techniques: thermal and wet. Thermal phosphoric acid is produced from burning elemental phosphorus and the wet method uses phosphate rock, in the form of tricalcium phosphate, and react it with sulfuric acid to yield phosphoric acid. It can be further purified to a food-grade material which is the form used to make potassium phosphates. Potassium phosphates are produced by reacting phosphoric acid and potassium hydroxide. The potassium phosphate produced depends on the mole ratio of potassium to phosphorus and produces monopotassium phosphate with a 1:1 ratio, dipotassium phosphate with a 2:1 ratio, or tripotassium phosphate with a 3:1 ratio. Potassium phosphates with a 2:1 molar ratio of K:P are the focus of this study, Figure 1 shows the reactions to create dipotassium phosphate (K2HPO4 or DKP) and tetrapotassium pyrophosphate (K4P2O7 or TKPP). Using Figure 1. Chemical reactions that produce TKPP H3PO4 + 2KOH K2HPO4 + 2H2O 2K2HPO4 + Heat K4P2O7 + H2O 1 dipotassium phosphate as a feedstock, tetrapotassium pyrophosphate is produced by spraying it into a rotating kiln heated above 400°C. The dehydration process provides a white granular solid that can be easily used in industry as a solid or dissolved and used as a liquid [3]. Uses of Phosphates Phosphoric acid salts such as sodium phosphates and potassium phosphates have many uses. The largest consumer is the food industry who uses them for a multitude of applications from texture-modifying and surface tension modifying to pH control and adding the essential nutrient, phosphorus, to food. Other uses include in drugs as a bowel cleansing agent, in fertilizer to supply phosphorus and/or potassium to the crops, as a paint additive, and in water treatment [4, 5, 6]. Potassium phosphates were widely used for many years, prior to their ban, in laundry detergents to help bind up hard water minerals, such as calcium and magnesium. This allowed detergent manufacturers to use less surfactant and keep the same cleaning power [6]. There use in foods is typically the same as sodium phosphates; however they are used less frequently due to the cost. A major reason to use potassium phosphates over their sodium counterparts in food products is due to the concerns over sodium content and its’ effects on the cardiovascular system [7]. Using the same ideology as laundry detergents, potassium phosphates are used in industrial water treatment for inhibiting scale from calcium and magnesium as well as inhibiting corrosion of metal surfaces [8]. Trace Metals in Potassium Phosphates In the following method development, the analytes of interest are As, Cr, Fe, Mn, Ni, Pb, and Zn. As and Pb were chosen because they are regulated compounds for food grade potassium phosphates based on the Food Chemicals Codex [9]. The remaining compounds are measured 2 due to a likelihood of being present in the manufacturing process. Rotary kilns for producing tetrapotassium pyrophosphate are lined with cast iron, stainless steel, or both and galvanized steel is heavily used in industrial manufacturing due to its corrosion resistance. Fe is monitored for the cast iron uses, Zn is monitored for the galvanized steel uses, and Cr, Mn, and Ni are monitored for stainless steel. Since the feedstock used is made from phosphoric acid and potassium hydroxide, any excess of either can create an even more corrosive environment than the already basic dipotassium phosphate used at a pH of 9.0-9.5. This possible corrosion at elevated temperatures could cause leaching of the components of the various steels [10, 11]. The goal of this method development is to create an efficient and cost effective method for production quality control laboratories to use to evaluate metals in dipotassium phosphate and tetrapotassium pyrophosphate while still maintaining high accuracy and precision. All of these aspects are analyzed to determine the practical method conditions for future uses. Beyond this study, an evaluation of the same extent could be applied to 1:1 and 3:1 molar ratios of K:P to extend this method to the entire potassium phosphate manufacturing community. 3 Experimental