determination of phosphate by titration
Even though phosphorus is an essential element for the human body, an excess intake of phosphates from food and drinks can lead to health problems, especially for people with kidney dysfunctions. Speed of Reduction.Incidental to the reduction with K2Mn2O8, I noted the effect of the speed of reduction and the rate of oxidation of the reduced solution when exposed to the air. 2014; xmp.iid:051FD897AFAFEA1182C1EDBE25262AE8 volume347,pages 6366 (1993)Cite this article. The results from 0.5 grammes of phospho-molybdate were: K2Mn2O8, 74.65, 74.65, 74.60; average, 74.63. 1) The shape of a polyprotic acid titration curve, revealing stepwise removal of protons. General chemistry/analytical chemistry experiments, Acid-Base titrations, Alkalimetric titrations, Phosphates determination. Pergamon Press, New York, Lurie J (1989) In: Spravotchnik po analyticheskoy himii. Correspondence to: MariaMichela Salvatore, Dipartimento di Scienze Chimiche, Universit degli Studi di Napoli Federico II, Complesso Universitario Monte S. Angelo, Napoli, Italy. Sodium phosphate is used in mouthwash and dental gels as a buffer in an acidulated phosphate fluoride (APF) solution, an agent that helps to prevent dental caries. As there is rapidly increase in demand for industrial phosphate, the needs for fast and reliable phosphate determination become more and more important. The resulting determinations of phosphorus were as follows. Sulfate ions can be determined by titrating with a solution of barium chloride. solvent blank and sample blank). Use this graph to determine the phosphate (as P) concentration in each water sample (remember to allow for the dilution of the sample in your calculations) and subtract the sample blank reading from the corresponding sample. default Temperature.To determine whether all the phosphorus was precipitated at 25 C., whether the ratio of phosphorus to molybdic acid remained constant regardless of the temperature of precipitation, and whether the presence of Fe2(NO3)6 had any effect on the composition of the phospho-molybdate, 0.100 P was added to a steel solution and precipitated at 25 C. and 85 C., the proper checks being made on the steel at those temperatures. 3) The first equivalence point is when all H 3 PO 4 present (both dissociated and non-dissociated) has been converted to H 2 PO 4 1-. Beer Lambert law no longer applies above an absorbance of 1. Measurements can easily be repeated and there should not be any significant difference between the readings (<0.003 absorbance). In the presence of vanadium, yellow vanadomolybdo-phosphoric acid is formed. H2SO4 from twelve minutes to two minutes, I found the results to be coincident. 34 Matched cuvettes or the same cuvette should be used throughout the series of analysis. For the purpose of comparing these results with Emmertons, the following determinations were made, as in Emmertons original paper, using 20 grammes of zinc for reduction. To avoid any possible phosphate contamination from detergents, it is very important that you use only the acid-washed glassware supplied in the cupboard for this experiment. To have a complete text for a Method for the Determination of Phosphorus we must first discuss the interaction with other elements: The Effect of Arsenic.A question involving the temperature of precipitation of ammonium phospho-molybdate, which was brought to my attention some time ago, led to the following experiment: A known quantity of arsenic was added to a steel solution, and the yellow salt was precipitated at varying temperatures, with the following results: These results show the possibility of contamination of the phospho-molybdate by arsenic if the temperature exceeds 25 C. While differing degrees of dilution, etc., seem to affect the percentage of arsenic precipitated, at 25 C. I have never found it to exceed the limits of error of a phosphorus determination. This application note describes the determination of phosphate in MAP fertilizer and in Mouthwash thermometrically using Mg(NO3)2 as titrant and the Thermotrode. PO43-+ Bi(NO3)3 BiPO4+ 3 NO3- (sample titration), Bi(PAR) + EDTA Bi-EDTA + PAR (color change). Instrument needs to be calibrated using a set of standards of differing concentration for quantitative analysis. 2023 Springer Nature Switzerland AG. The intensity of the yellow color is proportional to phosphate concentration. The formation of insoluble MgNH 4 PO 4 is a strong exothermic reaction that is monitored by thermometric titration. Even though phosphorus is an essential element for the human body, an excess intake of phosphates from food and drinks can lead to health problems, especially for people with kidney dysfunctions. The vanadate-molybdate reagent used in this practical is in strong acid solution. With greater speed the reduction was less complete, so that in one case, where the time was thirty seconds, nearly 10 per cent, escaped reduction. uuid:831af223-9ac0-4627-919d-63f113514298 The end-points are detected visually (back EDTA-titration of the non-bonded Bi3+-ions), or potentiometrically (direct titration of phosphates with bismuth by a specially prepared Bi/Bi2O3-electrode). Further drying for fourteen hours, at 180 C., gave no additional loss, and, notwithstanding the ammoniacal solution was slightly blue in color, titration with a dilute solution of K2Mn2O8 showed that no perceptible reduction of molybdic acid had taken place. Measure the absorbance of each standard, sample and blank at 470 nm. This bulletin deals with the determination of phosphate in phosphoric acid and granular fertilizers such as MAP (mono-ammonium phosphate), DAP (di-ammonium phosphate) and TSP (triple superphosphate). Within this context, a laboratory experiment, based on the alkalimetric titration of solutions obtained by dissolving the mixtures in an excess of strong acid, is presented and its significance is fully examined. adobe:docid:indd:fd40b441-6f1d-11e8-9454-b88292973359 Where practicable use a more specific technique such as ion chromatography. 4, Plenum Press, New York, 1976. = .006141 iron = .0001 P. If the MoO3 were reduced to Mo2O3, we should have MoO3 = iron x 85.714. 1. & Nikolov, T. Titrimetric determination of phosphates and monofluorophosphates in tooth pastes. Here titration of phosphate ions with a magnesium nitrate, Mg(NO3)2 solution produces an insoluble precipitate of MgNH4PO4 at alkaline pH. The titrations are performed without separating the precipitate. In a back titration an excess of titrant is added to the sample. In many cases samples may need to be diluted so that the measured absorbance is between the lowest and highest standard on the calibration curve. Hence, it is important to know the exact concentration of phosphate ions in the mentioned products. Titer Determination of Barium Chloride and Blank Value by Thermometric Titration, Literature: White Papers, Guides, Brochures. There was an error retrieving our menu. The reduced solution, after standing exposed to the air for thirty minutes, showed no oxidation. The Ratio of Phosphorus to Molybdic Acid in the Ammonium Phospho-Molybdate.A quantity of ammonium phospho-molybdate was carefully prepared at 25 C. by the addition of microcosmic salt to an ammonium molybdate solution. Cuvettes should always be placed the same way round to so subtle differences in the cuvette shape are avoided. This gives molybdic acid = iron x 90.814 as against the theoretical value of 90.76. SUMMARY OF METHOD 2.1 2.2Ammonium molybdate and antimony potassium tartrate react in an acid medium with dilute solutions of phosphorus to form an antimony-phospho-molybdate complex. They are based on the precipitation of PO 3 4-ions (directly or after the hydrolysis of PO 3 F 2-ions) as BiPO 4 in nitric acid (pH 0.1-0.5). The formation of the insoluble barium sulfate precipitate is a strong exothermic reaction that can be followed by thermometric titration. Determination of potassium with an ion-selective electrode. HWr:|Wi$2o"'%i}n%BD>;/$VL1`381/Ne"J|obIW6P@|FA2{IL S!5(\uhR,b)&zm 8GV P){.}?LWY7w%Jf6! The instrument should be allowed to warm up before use. Results are reported as percentage of P and P2O5. The recommended frequency of use for these dental products depends on the concentration of the phosphate and fluoride ions in them. This application note describes the determination of sulfate by thermometric titration with barium chloride (BaCl2) for two different samples: sulfuric acid of unknown concentration and phosphoric acid contaminated with sulfate. This article discusses the issue of determining the composition of mixtures of sodium and/or potassium monobasic, dibasic and tribasic phosphate salts with the purpose of addressing, in an educational environment, intriguing features of Acid-Base chemistry. Copyright 2012-2021 911Metallurgist | All Rights Reserved. The resultant solution is diluted with distilled water and filtered. Cite this paper: Maria Michela Salvatore, Francesco Salvatore, Analysis of Phosphate Salts Mixtures by a Simple Visual Alkalimetric Titration, Journal of Laboratory Chemical Education, Vol. From this must be sub- tracted a correction of 0.12 mg. for the phosphate in the trichloroacetic acid, as well as one of 1.6mg. of phosphorus per 100 cc. Qualitative identification of UV/Vis is very crude but it is ideal for quantification using a colored complex in this experiment. proof:pdf Adobe PDF Library 10.0.1 We also acknowledge previous National Science Foundation support under grant numbers 1246120, 1525057, and 1413739. Phosphates are also used as additives in carbonated soft drinks and processed foods. Figure: titration curve of a standard sample, Literature: White Papers, Guides, Brochures. Leave all prepared solutions (including calibration standards) to stand for at least 10 minutes for the color to develop fully before taking absorbance readings. To give evidence in court you have to show that the analytical method is producing valid results. The yellow salt, dried at 150 C., lost 0.99 per cent, of water. Phosphate is the conjugated base of phosphoric acid and is naturally occurring form of element phosphorus Phosphate ions have spread their roots in various industrial applications namely fertilizer, feed, food, detergents, personal hygiene, and construction, etc. enthalpy change) is monitored in the course of a titration. In a back titration an excess of titrant is added to the sample. There was an error retrieving our menu. Varying the speed of reduction of a 200 c.c. A3: Determination of Phosphate by a Colorimetric Method. Please reload the page. In this experiment, you will determine the phosphorus content of various samples of waters. In a dilute orthophosphate solution, ammonium molybdate reacts under acid conditions to form the heteropoly acid, molybdophosphoric acid. Sample may require degassing before analysis to remove air bubbles. Fresenius' Journal of Analytical Chemistry, access via Aquatic plants are depen-dent on a certain amount of phosphorus in order to survive; however, excess phosphorus leads to eutroph-ication, the over enrichment of a water body with nutrients. The end-points are detected visually (back EDTA-titration of the non-bonded Bi 3+-ions), or potentiometrically (direct titration of . Shake the bath water sample to ensure thorough mixing, then transfer a 25-ml aliquot to each of two 50-ml volumetric flasks. Vanadate-molybdate reagent: a mixture of ammonium molybdate [(NH4)6Mo7O24] and ammonium metavanadate [NH4VO3] in HCl. It can be fully automated with an autosampler and the thermometric sensor. 3.3: : Determination of Phosphate by a Colorimetric Method is shared under a not declared license and was authored, remixed, and/or curated by LibreTexts. -ions (directly or after the hydrolysis of PO3F2-ions) as BiPO4 in nitric acid (pH 0.10.5). Connectivity Software solutions Data flow from one instrument to another, Productivity (released time from the team) Uptime Optimize processes, Reduction in operating costs Total cost of ownership Maintenance cost reduction, Shift to the next level measurement process (from manual to automated, or to on-line measurement; automation), Sustainable design (futureproof modular design long lasting), Time reduction quick adjustment and scalability of methods, User skills and knowledge (theory of applications, technologies, methods, tips and tricks), Machinery for Transportation and Logistics, Metal, Plastic and Electronics Components, Electronics and Electromechanical Devices, Standards, Legislation and Regulatory Bodies, Synthesis Workstations and Reaction Calorimeters, Parallel Synthesis MiniBlock and MiniBlock XT, Scale Indicator and Scale Controller Systems, Weigh Modules, Load Cells, Weight Sensors, Good Weighing Practice (GWP) and Validation, TOC Analyzers and Real-Time Microbial Detection, Sensor Housings and Sensor Cleaning Systems, Sodium, Silica and Chloride Sulfate Analyzers, Vision Inspection Systems for Print, Label and Packaging, Truck Scale Weighbridge Management Software. There was an error retrieving our menu. This was carefully washed by decantation with water, until the final decantation contained less than .00001 per cent, of the original solution, then transferred to a porcelain evaporating-dish and dried at 85 C. to 90 C. for three days, with no contamination by organic matter. PubMedGoogle Scholar, Borissova, R., Debouki, A. A back titration is used, where an excess of bismuth nitrate is added to the sample, bismuth phosphate is then precipitated and unreacted bismuth ions are titrated against EDTA. 2020-06-18T18:15:21+02:00 Determination of silicate, phosphate, and sulfate by calcium atomization inhibition titration. Wh8AT1,(9yvT%&Z,/M^nk B_+dlQj[ your institution. There was an error retrieving our menu. Part of Springer Nature. For more information on complexometric and back titrations see the Titration Fundamentals guide. This is an interesting point, and I should be glad if other members of the Institute felt disposed to make comparative tests. With a K2Mn2O8 solution (1 c.c= 0.006703 iron), I obtained the following determinations of K2Mn2O8, corresponding to 0.5 grammes of phospho-molybdate, viz. A double beam spectrophotometer permits the reference beam to have a cuvette containing solvent. Laboratory Testing Consulting & Engineering Process Equipment. Notice that one of these is the conjugate acid form, and the other is conjugate base form of phosphate. xmp.did:C0A493F6AEAFEA11814ADE23F4E2E7FA Are you certain that any sample pre-treatment has not invalidated the analysis? Cuvettes must be scrupulously clean especially in the region where the light beam passes through. // Halogens - fluoride, bromide, iodide . UV/visible spectrophotometer & glass cells. Aluminum determination by atomic emission spectrometry with calcium atomization inhibition titration. This application note (download below) contains the method and procedure tips for determination of phosphates in selected soft drink and mouthwash samples by complexometric titration. Add 10 ml of the vanadate-molybdate reagent to one of these flasks - the second acts as a sample blank - and dilute each to the mark with de-ionised water. application/pdf PO43-+ Bi(NO3)3 BiPO4+ 3 NO3- (sample titration), Bi(PAR) + EDTA Bi-EDTA + PAR (color change). Solid material must be removed from the sample before analysis. Adobe InDesign CS6 (Windows) for the phosphate added in the course of the analysis. Sodium phosphate is used in mouthwash and dental gels as a buffer in an acidulated phosphate fluoride (APF) solution, an agent that helps to prevent dental caries. This experiment can smoothly be integrated within first-year undergraduate general chemistry/analytical chemistry laboratory, but might as well be interpreted as a virtual experiment for classroom discussion. This article discusses the issue of determining the composition of mixtures of sodium and/or potassium monobasic, dibasic and tribasic phosphate salts with the purpose of addressing, in an educational environment, intriguing features of Acid-Base chemistry. Akademiai Kiado, Budapest, Borissova R, Gertchev A (1986) Izchislenia v analiticheskata himia s pomochta na personalni computri, Sofia, Department of Analytical Chemistry, Sofia University of Technology, Bulgaria, Department of Physical Chemistry, Sofia University of Technology, 8 Kliment Ohridski, BG-1156, Sofia, Bulgaria, You can also search for this author in Figure: titration curve of a standard sample, Literature: White Papers, Guides, Brochures. 2) The % dissociation of H 3 PO 4 initial vs. the % dissociation of H 2 PO 4 1-at the 1st equivalence point. However, due to potential interference from ferric ions at low wavelengths, 470 nm is normally chosen. No sample pre-treatment is required, except for dilution and addition of the colorimetric reagent. Following reaction takes place during the titration . Hence the reduction seems to be at an intermediate point between Mo2O3 and Mo12O19, the formula quoted by Emmerton from Werncke for the sulphuric acid and zinc reduction. This method is developed using an Excellence Titrator with the Thermotrode, suitable for the measurement of small temperature differences induced by the chemical reaction. 90-96. doi: 10.5923/j.jlce.20140205.03. The difference between the added amount of the first and second titrant then gives the equivalent amount of the analyte. Titrimetric methods for a fast determination of phosphates and monofluorophosphates in tooth pastes are described. Adobe InDesign CS6 (Windows) Determination of Sulphate using T7 Autotitrator, Connectivity Software solutions Data flow from one instrument to another, Productivity (released time from the team) Uptime Optimize processes, Reduction in operating costs Total cost of ownership Maintenance cost reduction, Shift to the next level measurement process (from manual to automated, or to on-line measurement; automation), Sustainable design (futureproof modular design long lasting), Time reduction quick adjustment and scalability of methods, User skills and knowledge (theory of applications, technologies, methods, tips and tricks), Machinery for Transportation and Logistics, Metal, Plastic and Electronics Components, Electronics and Electromechanical Devices, Standards, Legislation and Regulatory Bodies, Synthesis Workstations and Reaction Calorimeters, Parallel Synthesis MiniBlock and MiniBlock XT, Scale Indicator and Scale Controller Systems, Weigh Modules, Load Cells, Weight Sensors, Good Weighing Practice (GWP) and Validation, TOC Analyzers and Real-Time Microbial Detection, Sensor Housings and Sensor Cleaning Systems, Sodium, Silica and Chloride Sulfate Analyzers, Vision Inspection Systems for Print, Label and Packaging, Truck Scale Weighbridge Management Software, Determination of Sulphate by Thermometric Titration. converted Absorbance may be measured at 400, 420 or 470 nm depending on the sensitivity required, the absorbance at 400 nm being ten times that at 490 nm for a given P concentration. ; average of all, 1.626 per cent. Anyone you share the following link with will be able to read this content: Sorry, a shareable link is not currently available for this article. Assuming the salt to contain 1.63 per cent, of phosphorus, and the ratio of phosphorus to molybdic acid to be 1.794, molybdic acid= iron x 87.847. Activity 1: determination of pKa of phosphate by titration: You are to prepare 10 different samples of K2HPO4 and KH2PO4 mixture as directed in the table. It can be fully automated with an autosampler and the thermometric sensor. For more information on complexometric and back titrations see the Titration Fundamentals guide. your institution. 80 + 38.0 = 2011mg. 286 Titration of Inorganic Phosphate character of calorimetric and nephelometric analyses, particularly when they are applied to complex mixttires . Mouth pipetting is forbidden. 2020-06-18T18:15:20+02:00 The reduction seems to be more perfect than with the addition of zinc to a sulphuric-acid solution, as used in Emmertons original method. A number of determinations were made with varying quantities of molybdic acid, and all ranged from 87 to 88. ; by (2), 1.626 and 1.631 per cent. Wavelength scan should normally be performed to determine the absorbance maximum ("max) however, the determination was not carried out at the "max due to the potential interference from iron (Fe). Copyright 2014 Scientific & Academic Publishing Co. All rights reserved. Theory partially dried comminuted meat sample is digested by heating with a mixture of nitric and hydrochloric acids. They are based on the precipitation of PO from application/x-indesign to application/pdf 5, 2014, pp. Smith, R.M., Martell, A.E., Critical stability Constants, Vol. j29>.GxByeUTX{3 a )%$6 {-W{!>5}e:(=x),OG_v]zvrk"a_lD@zpS$#`d`kvDG4wB. Hence, it is important to know the exact concentration of phosphate ions in the mentioned products. The standards thus diluted contain 0, 5, 10, 15 and 20 !g P per ml (the one containing no phosphate acts as a reagent blank). Are you confident that the data you have obtained can discriminate between the samples? The assay of official calcium phosphates for calcium can be brought into the general method for calcium determination by titration with EDTA using hydroxynaphthol blue indicator by dissolving the sample in a strictly limited amount of hydrochloric acid, passing an aliquot of the solution through a column of anion-exchange resin in the chloride form, washing out the calcium ion with water, and . 4-(2-Pyridylazo)resorcinol (PAR) is used as the color indicator and the color change at the endpoint of the reaction is detected with a photometric sensor. An indirect microdetermination of phosphate via EDTA titration is described, which can be applied to minerals, soils, fertilizers, biological samples, drugs and organo-phosphorus compounds. https://doi.org/10.1007/BF00326046. Legal. Please reload the page. The molybdic acid was determined by precipitation of lead molybdate PbO,MoO3 and lead phosphate Pb3,(PO4)2 by lead acetate in an acetic acid solution, washing by decantation with boiling water, filtering on a weighed paper, and drying at 100 C. to 120 C. The final decantation showed no trace of lead, proving the mixed salts to be insoluble in boiling water. : 77.14, 77.15, 77.10, 77.15, 77.20; average, 77.15 K2Mn2O8. 1 0 obj <>>> endobj 2 0 obj <>stream An ultrasonic field is applied to accelerate the destruction of the dentifrices. Please reload the page. The calculation follows. Connectivity Software solutions Data flow from one instrument to another, Productivity (released time from the team) Uptime Optimize processes, Reduction in operating costs Total cost of ownership Maintenance cost reduction, Shift to the next level measurement process (from manual to automated, or to on-line measurement; automation), Sustainable design (futureproof modular design long lasting), Time reduction quick adjustment and scalability of methods, User skills and knowledge (theory of applications, technologies, methods, tips and tricks), Metal, Plastic and Electronics Components, Phosphate Determination by Complexometric Back Titration. Analytica Chimica Acta 1976, 87 (1) , . Benches must be kept clean and tidy. The current application is a titrimetric adaptation of a classical gravimetric procedure. 2(5):90-96, Maria Michela Salvatore, Francesco Salvatore, Dipartimento di Scienze Chimiche, Universit degli Studi di Napoli Federico II, Complesso Universitario Monte S. Angelo, Napoli, Italy. 4-(2-Pyridylazo)resorcinol (PAR) is . In thermometric titration, the change in reaction heat (i.e. Fresenius J Anal Chem 347, 6366 (1993). And method files (.lmt and .xml) for direct import and implementation on the instrument, Literature: White Papers, Guides, Brochures. The high molybdenum-phosphorus binding rate ranks this method among very sensitive methods of quantitative analysis of phosphates in water. After 9 minutes, the corrected colorimeter reading was 38.0mm. After a sufficiently long waiting time, this excess is then titrated with a second titrant. 2020-06-18T18:15:21+02:00 An aliquot of filtrate is further diluted with distilled water. 2020-06-18T18:15:20+02:00 This application note (download below) contains the method and procedure tips for determination of phosphates in selected soft drink and mouthwash samples by complexometric titration. A back titration is used, where an excess of bismuth nitrate is added to the sample, bismuth phosphate is then precipitated and unreacted bismuth ions are titrated against EDTA. solution acidified with 10 c.c. Eating, drinking and smoking are not allowed in the laboratory. False endstream endobj 9 0 obj <> endobj 3 0 obj <> endobj 11 0 obj <>/Font<>/ProcSet[/PDF/Text/ImageB]/XObject<>>>/Thumb 21 0 R/TrimBox[0.0 0.0 595.276 841.89]/Type/Page>> endobj 12 0 obj <>/Font<>/ProcSet[/PDF/Text]>>/Thumb 23 0 R/TrimBox[0.0 0.0 595.276 841.89]/Type/Page>> endobj 13 0 obj <>/Font<>/ProcSet[/PDF/Text]>>/Thumb 25 0 R/TrimBox[0.0 0.0 595.276 841.89]/Type/Page>> endobj 14 0 obj <>/Font<>/ProcSet[/PDF/Text/ImageC]/XObject<>>>/Thumb 28 0 R/TrimBox[0.0 0.0 595.276 841.89]/Type/Page>> endobj 15 0 obj <>/Font<>/ProcSet[/PDF/Text]>>/Thumb 31 0 R/TrimBox[0.0 0.0 595.276 841.89]/Type/Page>> endobj 16 0 obj <>/Font<>/ProcSet[/PDF/Text]>>/Thumb 33 0 R/TrimBox[0.0 0.0 595.276 841.89]/Type/Page>> endobj 32 0 obj <>stream Lab coats and safety spectacles must be worn at all times. After this treatment, less than 0.02 per cent, of molybdic acid remained in the solution. Analytical Chemistry 1978, 50 (9) , 1253-1256. . A wavelength check should be carried out to check the instrument is correctly set up. Approximately 20-30 samples per hour can be analyzed. // Pesticides, fertilizers // pH Ion DO conductivity measurement // APHA Method 3500-K. The Thermotrode is suitable for the measurement of small temperature differences induced by the chemical reaction and thus gives precise and accurate results. Copyright 2014 Scientific & Academic Publishing. the titration of the non-bonded bismuth with 0.01 tool/1 EDTA, in the presence of 10-4-10 -5 mol/1 of PAR, is possible. Here titration of phosphate ions with a magnesium nitrate, Mg(NO 3) 2 solution produces an insoluble precipitate of MgNH 4 PO 4 at alkaline pH. Fresenius' Journal of Analytical Chemistry By method (1), 1.622 and 1.625 per cent. After a sufficiently long waiting time, this excess is then titrated with a second titrant. By Hundeshagens method of freeing the solution from molybdic acid before precipitation of the ammonium magnesium phosphate, by the saturation of the ammoniacal solution with H2S, acidulation with HCl, and filtering from the precipitated MoS. The formation of insoluble MgNH4PO4 is a strong exothermic reaction that is monitored by thermometric titration. Dunod, Paris, Beck M, Nagypal I (1989) Chemistry of complex equilibria. In the given method, the temperature rise corresponds to the exothermic formation of the barium sulfate precipitate. Prepare a series of calibration standards by transferring 0, 5, 10, 15 and 20 ml aliquots of the standard phosphate solution to 50 ml volumetric flasks, adding 10 ml of the vanadate-molybdate reagent to each and diluting to the mark with deionized water. The difference between the added amount of the first and second titrant then gives the equivalent amount of the analyte. Learn more about Institutional subscriptions, Knig H, Walldorf E (1975) Fresenius Z Anal Chem 289:177, Knig H, Walldorf E (1989) Fresenius Z Anal Chem 335:216, Masing L, Young L (1962) Can J Chem 40:903, Ikonopisov S, Andreeva L, Nikolov T (1973) J Electrochem Soc 120:717, Selmer-Olsen A (1961) Acta Chem Scand 15:2052, Karadakov B, Ivanov H (1973) J Anal Him 28:525, Sochevanov V, Volkova G (1969) J Neorg Him 14:118, Niklos I, Czegedi R (1961) Acta Chim Acad Scient Hung 26:365, Battari E, Augerg G (1967) Helv Chim Acta 50:2349, Bhatt T, Krishna R (1965) Z Anorg Allg Chem 335:331, Bishop E (1972) In: Indicators.