POLYPHOSPHORIC ACID ASSAY

Transcription

1 POLYPHOSPHORIC ACID ASSAY Introduction This Technical Information Report describes the sampling technique and method of analysis recommended to perform rapid and reliable polyphosphoric acid assays for both inventory and quality control requirements. The analytical method presented is referred to as the "chain length method". It has been tested against the classical ammonium phosphomolybdate precipitation method and the widely used titration method, both of which depend on hydrolysis of the polyphosphoric acid to its ortho equivalent. The results of these tests, which demonstrate the validity of the chain length method, are presented. Theory and calculations Every polyphosphoric acid is a mixture of molecules of different chain lengths or number of phosphorus atoms per molecule. Within a given sample, the distribution of different chain lengths tends to equilibrate according to the sample strength (P 2 O 5 content) and temperature. References 1 and 2 offer some quantitative data on this process. While the distribution may vary, the average chain length within a sample remains constant. This is true whether or not the sample is at equilibrium. Further, the average chain length between samples is a singlevalued function of P 2 O 5 content. Thus, if the average chain length is determined, the P 2 O 5 content or assay is known. This forms the basis of the chain length method. An unhydrolyzed acid sample is titrated with caustic to two end points or inflections. The first inflection corresponds to strongly acid protons of which there is one per phosphorous atom. The second corresponds to weakly acid protons of which there are two per chain, one at each end. For V1, the volume of titrant to the first inflection, the total number of phosphorus atoms is proportional to V1. For VT, the total titrant to the second inflection, the number of chains is proportional to (VT V1)/2. The average chain length, n, is the total number of phosphorus atoms divided by the number of chains, and since the proportionality constant in both above relations is the same, V1 2V1 N= = (VT V1) /2 (VT V1) The average chain of length n can be visualized as which has a formula weight of 80n The equivalent amounts of H 3 PO 4 with formula weight 98 and P 2 O 5 with formula weight 142 are given by 98n and (142/2)n = 71n, respectively. Thus, Equiv. %H 3 PO 4 =

2 98n 71n X X 100% and Equiv. % P 2 O 5 = 100% 80n n + 18 From these equations, the relationship between n and %H 3 PO 4 or %P 2 O 5 is constructed (Table 1). Validation The chain length method depends upon titration of an unhydrolyzed sample. Any hydrolysis will create new chains which may invalidate the results. On the other hand, sample weight and titrant normality, which are contained in the proportionality constant in the derivation above, cancel out of the calculations and are only important to the extent they control titrant volumes. Polyphosphoric acid is highly hygroscopic and rapidly absorbs moisture when exposed to air. Besides making accurate weighing of a sample difficult, the absorbed moisture begins the hydrolysis process. When water is added to the sample, the hydrolysis process is free to proceed to completion. Quantitative data on rate of hydrolysis is scarce. At room temperature the rate is slow. For those analytical methods based on titration or reaction of a totally hydrolyzed sample (orthophosphoric acid), the sample must be boiled in water for 30 minutes to an hour or more. To determine the significance of hydrolysis effects on the validity of the chain length method, each of six polyphosphoric acid samples ranging between 113 and 118% H 3 PO 4 was analyzed in four different laboratories, and in most cases by more than one analyst in each laboratory. Chain length, phosphomolybdate, and titration methods were employed. The results, as presented in Table 2, demonstrate the validity of the chain length method. Table 1 Relation of average chain length to % H 3 PO 4 and % P 2 O 5 Chain Length % H 3 PO 4 % P 2 O

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4 Table 2 Results of Comparison of Methods for Analysis of Polyphosphoric Acid Lab Sample A B C D E F Chain Length Method ( % H 3 PO 4 ) Mean Std. Dev (+/) Phosphomolybdate Method (% H 3 PO 4 )

5 Mean Std. Dev (+/) Hydrolysis Method ( % H 3 PO 4 ) Mean Std. Dev (+/) Sampling procedure equipment A sample dipper or pipet with a handle several feet long is required. It should be constructed of 316 stainless steel. A pipet design suitable for cars, trucks or drums is given in Figure 1. The sample container should be a 16 oz. Wide mouth bottle with no recesses which are difficult to clean or rinse. Nalgene or similar material is recommended to avoid breakage form thermal or mechanical shock. The sample bottle is best handled in or over a polypropylene tray or bucket to contain spills and drips resulting from the sampling operation. The viscous acid will cling to the equipment, then run off rapidly as it is diluted by atmospheric moisture. Techniques and safeguards CAUTION: Polyphosphoric acid is unloaded and transferred hot to reduce its viscosity. When sampling, heavy padded rubber gloves must be worn to protect the hands from chemical and thermal burns. Samples should always be taken through the top of a truck or car and well into the bulk liquid. Drums are heated to 200 to 230 F (90 to 115 C) and sampled through the bung and midway into the drum. Samples taken from unloading connections or anywhere that the acid has been exposed to atmospheric moisture are not reliable. The sampling device should be clean and dry. It is first filled and drained back into the truck or car and then wiped with a clean gloved hand or other device as the sampler is withdrawn. The bottle is contained in or held over the tray or bucket. The sample bottle is capped immediately.

6 All sampling procedures should be carried out as rapidly as possible. The gloves and other safety equipment required for safe handling of the acid hinder rapid manual manipulation of the sampling equipment. Some practice is required to develop speed in handling without sacrificing safety. Any polyphosphoric acid spilled on clothing or skin should be removed immediately. Where skin contact occurs, follow the instructions provided in the Innophos Material Safety Data Sheet on polyphosphoric acid. Chain length method procedure This procedure assumes manual titration using a ph meter. An automatic titrimeter is equally suitable. Apparatus Buret 50ml with Teflon plug. Beakers 250ml. Graduated cylinder 100ml. magnetic stirrer with stirring bar on or a glass stirring rod. ph meter or automatic titrimeter. Figure 1. Polyphosphoric acid sampling pipet. Reagent Sodium hydroxide solution (approximately 0.5N). dissolve 20 grams NaOH, reagent grade, in about 150ml of deionized water and transfer to a 1 liter container. Fill to volume, cool, refill to volume and mix thoroughly. Procedure 1. Calibrate the ph meter with ph 4 and ph 7 buffers 2. Mount the buret beside the ph meter and fill with the sodium hydroxide solution. 3. Weigh approximately 1 gram of room temperature sample directly into a clean 250ml beaker using a top loading balance. 4. Add 100ml of deionized water and a stirring bar. 5. Place the beaker on the magnetic stirrer and under the buret, and immerse the ph electrodes.

7 6. Add 20ml of sodium hydroxide solution in 5ml increments, recording ph after each addition. Continue by slowly adding 1ml increments and recording ph until near the first inflection (ph 4.0 to 4.5). At the inflection, reduce the increments to 0.5ml. 7. Following the first inflection, add two 10ml increments of solution and then reduce to 1ml increments, recording ph as before. Reduce to 0.5 increments at the second inflection (ph 9.0 to 9.5). 8. Plot the ph versus milliliters of sodium hydroxide on suitable graph paper and determine the volumes of sodium hydroxide solution to the two inflection points. For V1, the volume to the first, and VT, the volume to the second, the average chain length is calculated a n in N = 2V1 (VT V1) 9. Obtain the % H 3 PO 4 or % P 2 O 5 by interpolation from Table 1 or by calculation, where 98n 71n % H 3 PO 4 = X 100% or % P 2 O 5 = X 100% 80n n + 18 Notes Care should be taken that the magnetic stirrer does not heat the solution being titrated. It is often helpful to make a preliminary rapid titration to locate the inflection points and then follow with the detailed procedure. References Huhti, AnnaLiisa, Gartagamis, Phodbus A., Canadian Journal of Chemistry 34:790, Toy, Arthur D.F., Phosphorus Chemistry in Everyday Living, American Chemical Society (1976) NL Baroid, Houston, TX, private communications. TRADEMARKS: Nalgene is a registered trademark of Nalge Co. And Teflon is a registered trademark of E.I. dupont denemours & Co., Inc. TIR7 September 1991 See your Innophos Sales Representative or call our Order and Sales Customer Service Department at for more information about products discussed in this Technical Information Report. Innophos believes all information given in this report is accurate. It is offered in good faith, but supplied without consideration or guarantee. Innophos assumes no obligation or liability for the accuracy or sufficiency of the information given or the results obtained, all such information being given or accepted at user s risk. The use(s) referred are listed for purposes of illustration only and the user is urged to investigate and establish the suitability of application of such use(s) in every case. Nothing herein contained is to be construed as a recommendation for uses which infringe valid patents or as extending a license under valid patents or as advising or authorizing practice of any patents or patent applications owned by Innophos or others.