Method For Determination Of Iron Research Articles

Features of the iron determination in dust emissions of gas purification of electrometallurgical production

Features of the iron determination in dust emissions of gas purification at electrometallurgical facilities are considered. The elemental and phase compositions of dust emissions were determined using energy dispersive x-ray fluorescence analysis (EDXRF) and inductively coupled plasma atomic emission spectrometry (ICP-AES). Sample preparation conditions were optimized with allowance for the analytical methods used. The phase composition of dust emissions represented mainly by zincite, zinc ferrite, halite and sylvite was preliminary determined by X-ray phase analysis. Model mixtures were prepared and used to construct calibration dependences taking into account the results of diffractometric studies. A technique for rapid x-ray fluorescence determination of iron in dust emissions of gas purification has been developed. The methods for iron determination tested on real samples of dust emissions of electrometallurgical production demonstrated a satisfactory convergence of the results obtained by EDXRF (Sr = 1.3%) and ICP-AES (Sr = 2.6%).

Feasibility of digital image colorimetric methods for iron determination in river sediment

The current study describes the development of simple, low-cost, and high-throughput digital image colorimetric methods to determine the total iron concentration in river sediment using the spot-test reactions of iron with 1,10-phenanthroline and thiocyanate. The colorimetric assay was done on 96-microzone plates, and a flatbed scanner was applied to acquire the images. The proposed methods offered a linear range from 0.2 to 14.0 mg/L, with a detection limit of 0.11 mg/kg for the 1,10-phenanthroline method, and, for the thiocyanate method, the linear range comprises 2.0–10.0 mg/L, with a detection limit of 0.28 mg/kg. It was observed that both proposed digital image colorimetric methods (1,10-phenanthroline and thiocyanate) yielded statistically similar results to the reference procedures at a 95% confidence level. A standard reference material (NIST 8704) also was utilized for accuracy assessment and the results were statistically equivalent to the certified values within the 95% confidence level. The digital image colorimetric methods can be an alternative method for iron determination in sediment samples, allowing fast sample screening at a low cost.

A Portable Measurement Device Based on Phenanthroline Complex for Iron Determination in Water.

In this work, a newly developed self-contained, portable, and compact iron measurement system (IMS) based on spectroscopy absorption for determination of Fe2+ in water is presented. One of the main goals of the IMS is to operate the device in the field as opposed to instruments commonly used exclusively in the laboratory. In addition, the system has been tuned to quantify iron concentrations in accordance with the values proposed by the regulations for human consumption. The instrument uses the phenanthroline standard method for iron determination in water samples. This device is equipped with an optical sensing system consisting of a light-emitting diode paired with a photodiode to measure absorption radiation through ferroin complex medium. To assess the sensor response, four series of Fe2+ standard samples were prepared with different iron concentrations in various water matrices. Furthermore, a new solid reagent prepared in-house was investigated, which is intended as a "ready-to-use" sample pre-treatment that optimizes work in the field. The IMS showed better analytical performance compared with the state-of-the-art instrument. The sensitivity of the instrument was found to be 2.5 µg Fe2+/L for the measurement range established by the regulations. The linear response of the photodiode was determined for concentrations between 25 and 1000 µg Fe2+/L, making this device suitable for assessing iron in water bodies.

Application of an Economic USB Digital Microscope for Microvolume Colorimetric Determination of Iron in Supplements with a Guava Leaf Extract

The method for iron determination by using the microvolume of chemicals coupled with employing natural reagent was proposed towards green analytical chemistry. A simple, portable, and economic digital microscope was used to magnify the drop of 30 μL of product, that was mixed between sample and guava leaf extract reagent, to monitor the analytical signal as primitive color, red/ green/blue or RGB. The optimum conditions for the reagent extraction and the RGB detection were investigated. The calibration graph with linearity in the range of 40-120 mg L-1 Fe(III) was obtained with the detection limit at 10.8 mg L-1. High precision was achieved with %RSD less than 1.8%. The developed method was successfully applied to the assay of iron in supplement samples and found to be in good agreement with the results obtained by the FAAS method at the 95% confidence level.

RGB color sensor for colorimetric determinations: Evaluation and quantitative analysis of colored liquid samples.

In this work, a red, green, blue (RGB) color sensor was used for quantitative optical analysis of colored solutions. The capability of the sensor to respond to different colored solutions was critically evaluated to better understand which spectral bands are filtered and processed by each sensor channel. The effective capability of the RGB sensor, defined as its ability to illuminate and detect electromagnetic radiation reflected by the samples, was observed in the range of 415-564, 440-600 and 510-750nm for blue, green and red channels, respectively. These results can help understand the interaction between the light emitted by the sensor and the signals obtained by the RGB channels for different quantitative determinations. In order to investigate the interaction between the RGB sensor and colored substances, and thereafter achieve quantitative optical analysis, different colored dyes were chosen to evaluate the RGB sensor capability, thus covering a wide range of colors. The analytical performance of the RGB sensor yielded a linear range of 5.0-50.0μmolL-1 for dye solutions. The accuracy of this sensor was demonstrated by the thiocyanate method for colorimetric determination of iron in soil and supplement samples. Such RGB sensor achieved analytical performance similar to that obtained with the commercial spectrophotometer, without requiring the use of computers for image processing so as to gather RGB values. Additionally, this sensor also contributes to meeting the requirements of Internet of Analytical Things (IoAT) for the quantitative analysis of colored solutions.

Sequential injection method for bi-parametric determination of iron and manganese in soil leachates.

The aim of this work was to develop a sequential injection (SI) method for the determination of the micronutrients iron and manganese, in soil leachates, as a tool to assess potential groundwater contamination. The described sequential injection method was based on the reaction of iron with chelator MRB12, a greener alternative chromogenic reagent, and the reaction of manganese with zincon, within a single manifold. The developed SI method enabled the determination of iron in the range 0.10-1.00 mg L-1, and manganese in the range 0.25-2.5 mg L-1 with a limit of detection of 0.08 mg L-1 for iron and 0.24 mg L-1 for manganese. The determination of both parameters was made in 6 minutes, in triplicate. The application to monitor laboratory scale soil core columns (LSSCs), as a simulation of the soil leaching process, proved its efficiency to assess potential contamination of ground waters. Iron and manganese contents were effectively analysed in two different scenarios to mimic the leaching process with rainwater and fertilizer.

Spectrophotometric determination of iron (III) catalyst in organic compound chlorinations

In the science or industrial practice of chemical processes, iron (III) is sometimes used as a catalyst in organic compound chlorinations due to its effectiveness and low cost. Thus, a fast and easy method of determination in the system is useful especially when metallic iron is used as a precursor which is readily converted into iron (III) chloride by the gaseous chlorine used in the chlorination reactor. In the latter case, the determination of the produced catalytically effective iron (III) is a prerequisite for controlling the kinetic progress of chlorination. In this work, a method for the spectrophotometric determination of iron (III) chloride in organic media after complexation with methyl ethyl ketone is investigated. The formation of a strong o-complex of iron (III) with methyl ethyl ketone allows direct determination of iron at 360 nm.Beer’s law is valid up to absorbance 2.42, where the iron (III) concentration is 20.7 mg·L-1, with molar absorptivity (e) equal to 6.532×103 L·mol-1·cm-1 and Sandell’s sensitivity 8.5×10-3 ug·cm- 2. Standing time for color development is of the order of a few seconds and stability of color measurements exceeds 12 months. The method may be used among other applications in organic compounds chlorinations catalyzed by iron. These systems are complicated due to the coexistence of various complexes. Nevertheless, the method proposed being simple, fast, and not depending on the composition of the chlorination mixture and the amount of methyl ethyl ketone added was found to be suitable.

Field-deployable method for iron analysis using a simple preconcentration procedure and a 3D portable spectrophotometric system

In this paper, a field-deployable method for the spectrophotometric determination of iron after a simple preconcentration procedure in column is proposed. A 3D-printed portable system, housing a small spectrophotometer with a CCD detector and a LED as homemade light source is used for spectrophotometric detection. The entire system is powered through two USB outputs of a notebook. The Fe (III) preconcentration is based on a solid phase extraction with a column packed of Chelex-100 chelating resin. The Fe (III) eluted in the cuvette was reduced to Fe (II) and the iron concentration was determined using the o-phenanthroline method. Preconcentration factors between 9.7 and 98 were obtained. Under optimized conditions, linear calibration curves without and with preconcentration were constructed (R2 > 0.9992). Low LD (5 µg L−1) and LQ (10 µg L−1) were achieved, and good values for intra-day and inter-day precision were obtained (RSD ≤ 2.9% and RSD ≤ 9.6%, respectively). Recoveries of spiked iron in local tap water, sea water, well water, leachate and supernatant of anaerobic digester were ranged from 93 to 106%. The validation of method was also carried out by successfully analyzing a river water reference material SLRS-4.

Comparison of Spectrophotometric Methods for Determination of Iron in Acid Cleaning, and Passivating Stainless Steel Solution by UV-VIS, ICP/MS, and FAAS

Generally, more sophisticated techniques such as, flame atomic absorption (AAS), inductively coupled plasma spectrometer (ICP-MS), and UV/Visible Spectrophotometer were used to determine trace iron metals in pickling (acid cleaning), and passivating stainless steel solutions. Simple and novel spectrophotometric methods are described for simultaneous determination of iron. While these techniques provide excellent sensitivity and selectivity. The results were found to be in satisfactory agreement (shows no significant difference) with those acquired by the flame atomic absorption spectrophotometer (FAAS), UV-Vis Spectrometer, and ICP/MS techniques. The results shows that the concentration of iron are very close each other (less than 5.0%) in the pickling and passivating solutions.

Carbon dots doped by nitrogen and sulfur for dual-mode colorimetric and fluorometric determination of Fe3+ and histidine and intracellular imaging of Fe3+ in living cells

The first dual-modality highly intensive fluorescent and colorimetric nanoprobe for Fe3+ ions and histidine isreported. The carbon dots doped by nitrogen and sulfur (N,S-CDs) prepared by the one-pot hydrothermal method have an excitation/emission wavelength of 320/420nm with 56% quantum yield. N,S-CDs exhibit strong visible fluorescence with high stability at pH ~ 7.0. The fluorescence intensity of the N,S-CDs is quenched in the presence of Fe3+ ions which are recovered upon the addition of histidine. The addition of Fe3+ ions also induces a color change from yellow to red.Using colorimetric determination, Fe3+ and histidine exhibited linearity in the range 75-675 and 100-375μmolL-1, respectively, while with fluorometric determinations the dynamic range was 0.1-275 and 0.1-3μmolL-1 for Fe3+ and histidine, respectively. The limits of detection were 19nmolL-1 and 0.03μmolL-1 usingfluorometry and 20μmolL-1 and 24.2μmolL-1 usingcolorimetry, for Fe3+ and histidine respectively. The relative standard deviations (n = 5) for Fe3+ (10μmolL-1) and histidine (1μmolL-1) usingfluorometry were 4.6 and 7.3% and usingcolorimetry at 100μmolL-1 of Fe3+ and 150μmolL-1 of histidine were 3.2 and 5.6%, respectively. The developed fluorometric method was applied for the determination of Fe3+ and histidine in various foods and biological fluid samples as well as intracellular imaging of iron. The accuracy of the method for iron determination was confirmed by the analysis of certified reference materials (wheat flour, tomato leaves, and whole milk powder) and quality control materials (whole milk powder, serum, and urine), whereas for histidine, the accuracy was determined by recovery experiment and independent analysis. Good recovery values in ranges of 92-96% and 94-98% were achieved for Fe3+ and histidine, respectively. Graphical abstract.

Greener and wide applicability range flow-based spectrophotometric method for iron determination in fresh and marine water

A flow-based method for the spectrophotometric determination of iron in recreational waters, both fresh and marine (variable salinity content), was developed. For that purpose, 3-hydroxy-4-pyrydinone ligand functionalized with an ether function was synthetized and used as chromogenic chelator (1-(3′-methoxypropyl)-2-methyl-3-benzyloxy-4-(1H)pyridinone - MRB13) for iron quantification. This water-soluble reagent was previously reported as a greener alternative to quantify iron, due to its low toxicity and a more environmental friendly synthesis. Furthermore, it also displayed a high affinity and specificity for iron. With the main objective of quantifying iron in a variety of water types (different matrices and iron content), two strategies were developed, one of them including on-line solid-phase extraction (SPE), and the other without resorting to a SPE process. Water matrix clean-up and iron enrichment was achieved using a nitrilotriacetic acid resin column. The potential interference of metal ions usually present in water samples was assessed and no significant interference (<10%) was observed. The limits of detection were 11 and 2.9 μg L−1 without and with SPE, respectively. For one determination (three replicates), the corresponding consumption of MRB13 is 90 μg, sodium hydroxide is 1.4 mg, and boric acid is 5.6 mg. The method was applied to certified water samples and the results were in agreement with certified values. The developed method was also applied to fresh and marine water, and recovery ratios of 103 ± 4 and 101 ± 7 without and with SPE, respectively, were achieved.

Spectrophotometric Estimation of Iron (III) in Iron Polysaccharide Complex Capsule Formulation and its Accelerated Stability

Iron plays a pivotal role in human physiology, while its deficiency may prove fatal in severe cases. Analytical methods for the quantitative determination of iron are thus very important. Herein, we report the estimation of iron in iron Polysaccharide complex (IPSC) using raw material and formulations, through a spectrophotometric analytical method. IPSC capsules were formulated and their stability was studied by developing a simple and validated analytical method. The process is based on the acid hydrolysis of IPSC and the development of chromogen by reacting ammonium thiocyanate with IPSC, maximum absorption at 474 nm was observed. Beerand#39;s Lambert law (linearity response) was found in the range of 10-20 μg/ml with excellent correlation coefficient of determination (R = 0.998). The quantification and detection limits were established to be 0.45 mcg/ml and 0.14 mcg/ml correspondingly. The recovery of IPSC analysis was 99.25 to 102.28 %. Percentage assay of IPSC capsules showed results around 102.34 %. The formulated IPSC capsule was stable under accelerated conditions for 6 months (% assay andgt; 91.69). The dissolution profile over 60 minutes showed a better dissolution (94%) compared with the internationally marketed IPSC capsule (92%).

Spectrophotometric Estimation of Iron (III) in Iron Polysaccharide Complex Capsule Formulation and its Accelerated Stability

Iron plays a pivotal role in human physiology, while its deficiency may prove fatal in severe cases. Analytical methods for the quantitative determination of iron are thus very important. Herein, we report the estimation of iron in iron Polysaccharide complex (IPSC) using raw material and formulations, through a spectrophotometric analytical method. IPSC capsules were formulated and their stability was studied by developing a simple and validated analytical method. The process is based on the acid hydrolysis of IPSC and the development of chromogen by reacting ammonium thiocyanate with IPSC, maximum absorption at 474 nm was observed. Beerand#39;s Lambert law (linearity response) was found in the range of 10-20 μg/ml with excellent correlation coefficient of determination (R = 0.998). The quantification and detection limits were established to be 0.45 mcg/ml and 0.14 mcg/ml correspondingly. The recovery of IPSC analysis was 99.25 to 102.28 %. Percentage assay of IPSC capsules showed results around 102.34 %. The formulated IPSC capsule was stable under accelerated conditions for 6 months (% assay andgt; 91.69). The dissolution profile over 60 minutes showed a better dissolution (94%) compared with the internationally marketed IPSC capsule (92%).

Research on rapid semi-quantitative determination of iron in pyrotechnics uesd for fireworks and firecrackers

This study discloses a method for rapid semi-quantitative determination of iron in pyrotechnic powder for fireworks and firecrackers based on energy dispersive X-ray fluorescence spectrometer (EDXRF), including the following steps: preparation of samples, establishment of testing methods, determination of the characteristic line fluorescence intensity values of Fe element in samples, and according to the measured fluorescence intensity values, the iron content in the pyrotechnic powder samples can be semi-quantitatively determined. The method of the study has the advantages that: (1) the method is simple to operate, and the method can be repeatedly called for testing. Only one new test method needs to be built before the sample test. After the method is established, the test can be repeated at different times without re-establishing the test method for each test. After the first establishment of the new test method, the entire test process only includes three steps: sample preparation, sample loading into the sample cup and on-board testing. (2) The detection period is extremely short. After the sample is prepared, the entire measurement process takes only about 2 minutes. (3) Labor intensity is very low and the requirements for operators are not high. (4) The method has good stability, good repeatability and high credibility.

Cetyltrimethyl ammonium mediated enhancement of the red emission of carbon dots and an advanced method for fluorometric determination of iron(III)

Red-emissive carbon dots (CDs) were synthesized by one-step hydrothermal technique using citric acid (CA), and urea in N,N-dimethylformamide (DMF) solution. The CDs has an average diameter of 2.3nm, excitation/emission maxima at 553/606nm, and alow photoluminescence quantum yield (4%). Fluorescence is weakly quenched by the ions Fe3+, Hg2+, Cu2+, Co2+, Zn2+, Ca2+, Ni2+, and Pb2+. After addition of cetyltrimethyl ammonium ion (CTAB), electrostatic interaction between negatively charged CDs and CTAB causes the CDs to self-aggregate. The formation of CD/CTAB increases the average particle diameter to around 13nm and enhances the quantum yield to 24%. The hydrophobic segments of CTAB twined into a network structure can selectively trap Fe3+ and then interact with surface groups of the CDs to cause quenching. The CD/CTAB nanoprobe enables fluorometric determination of Fe3+ with a linear response in the 0.10-10μM concentration range and a 0.03μM limit of detection. The probe was utilized for determination of Fe3+ in human serum samples, and satisfactory results were obtained. Graphical abstractSchematic representation of fluorometric analysis of Fe(III) ion by cetyltrimethyl ammonium ion (CTAB) mediated red emission carbon dots (CDs). The hydrophobic segments of CTAB twined into a network structure can selectively trap Fe(III) and then interact with surface groups of the CDs to cause quenching.

Simple Spectrophotometric Method for Determination of Iron in Crude Oil

In this research article, spectrophotometric method for the determination of iron with 1,10-phenanthroline (phen) in two different crude oil samples from different oil fields in the Suez-Gulf region of Egypt has been proposed. The method is efficient, reliable and inexpensive where a cost-effective technique, along with commercially available spectrophotometric reagent, was utilized in this work. The method was based on decomposition of the organic matrix by combustion in a heating muffle furnace at 550°C. The inorganic residue was then dissolved in diluted nitric acid and the iron was reduced to the divalent state. The color was developed by the addition of 1,10-phenanthroline as chelating agent after adjusting the pH of the solution, then the absorbance of the solution was measured at approximately 510 nm after a short reaction period. The limit of detection (LOD) and limit of quantification (LOQ) obtained were found to be 0.017 and 0.051 μg/mL, respectively. The effect of interferences was studied and the accuracy of the method was evaluated by recovery experiment, analysis of oil reference material and by comparison of results with those obtained using flame atomic absorption spectrometer (FAAS) after dilution in an organic solvent for sample preparation.

Development of spectrophotometric method for iron determination in fortified wheat and maize flours

The determination of iron in fortified foods is mandatory by many global regulatory agencies. However, the spectroscopic techniques require elevated investments limiting their applicability especially in developing countries. Therefore, simple, viable and analytical methods with sufficient sensitivity can become an alternative. In this work, a sensitive, simple and viable spectrophotometry method to determine iron in wheat and maize flours was developed following a cloud point extraction (CPE) procedure. The analyte was first complexed with 2-(5-Bromine-2-pyridylazo)-5-diethylaminophenol (Br-PADAP) in the presence of the surfactant octylphenoxypolyethoxyethanol (Triton X-114). For the CPE optimization the variables: pH of the medium, stoichiometry of the complex, surfactant, and salt concentrations were evaluated. Linearity in the analytical blank was obtained by using the square root of absorbance (Abs) in order to adjust the residues of the curve. The precision was lower than 5% and the accuracy ranged from 97 to 101%. The limits of detection and quantification were 0.004μgmL−1 and 0.01μgmL−1, respectively. The method was applied to investigate the content of iron in 14 brands of fortified flours. The concentrations of iron varied from 0.435 to 3.62mg/100g and 0.570 to 3.15mg/100g in wheat and maize flour, respectively. The content of iron in all brands investigated in this study was approximately 10-fold lower than the value required by (ANVISA). The amount of iron in fortified foods was satisfactorily determined by using a simple, sensitive, and low cost spectrophotometric method.

Determination method of iron and its inorganic oxide in the air of workplace

Objective: To establish the method for determination of iron and its inorganic oxide in the air of workplace. Methods: The iron and its inorganic oxide was collected by MCE filter membrane and then digested by electric heating digestion apparatus. Atomic absorption spectrophotometer (AAS) or inductively coupled plasma optical emission spectrometry (ICP-OES) was used for the detection of iron and its inorganic oxide. Results: The sampling efficiency was higher than 97%; under the 372.0 nm wavelength, the linearity of AAS was good at the range of 1.0~150.0 μg/ml, the minimum quantitation concentration was 0.28 mg/m(3), the maximum quantitation concentration was 6.24 mg/m(3), the recovery was ranged from 99%~102%, the RSD of intra-and inter-batch precision were 0.5%~1.2% and 1.0%~2.2%, respectively; the linearity of ICP-OES was good at the range of 0.1~500 μg/ml, the minimum quantitation concentration was 0.28 mg/m(3), the maximum quantitation concentration was 20.8 mg/m(3), the recovery was ranged from 101%~103%, the RSD of intra-and inter-batch precision were 0.5%~1.0% and 1.5%~1.6%, respectively. Conclusion: The sampling method and determination method meet the requirements of analysis and apply to the collection and determination of iron and its inorganic oxide in the air of workplace.

Iron in fortified biscuits: A simple method for its quantification, bioaccessibility study and physicochemical quality

Iron deficiency is one of the most important nutritional problems in the world. The aims of this study were to determine the total concentration of iron in order to evaluate its bioaccessibility in biscuits produced with fortified flour, check the importance of its contribution to the iron intake and monitor physicochemical parameters such as moisture, acidity and peroxide value (PV) during 150days of storage. The simple and cheap method for iron determination was validated and proved to be adequate. Forty one samples of biscuits including salt water, cream cracker, cornstarch, and buttery biscuits were analyzed and their iron content were 5.3–7.8; 5.0–8.6; 2.5–6.8; and 3.7–5.7mg/100g, respectively. The in vitro assay results varied from 1.2 to 4.3mg/100g and from 0.2 to 2.1mg/100g to solubility and dialysis, respectively. There was significant difference in total, soluble and dialyzed iron content among the biscuit types analyzed. The intake of a biscuit portion can contribute from 5 to 32.5% of the recommended daily intake of iron, depending on the type of biscuit consumed. Lipid content varied from 9.8 to 18.0% for the biscuit types analyzed. In the end of storage time moisture levels increased 1.5% for the majority of samples, besides it was observed that most biscuits showed an increase (around 50%) of titratable acidity after 150days of storage. The highest PV was 27.8meq/kg of oil fat for salt and water biscuit (in 90days of storage), 23.3meq/kg of oil for cream cracker (in 120days of storage), 22.6meq/kg of oil for cornstarch (in 120days of storage) and 14.1meq/kg of oil for buttery biscuit (in 60days of storage), indicating lipid oxidation. Samples with the highest iron and moisture content also presented the highest peroxide value, indicating oxidation. The consumption of biscuits plays an important role in providing the daily requirement of iron intake. However, it is necessary to improve the stability and to provide the desired delivery of nutrients without causing damage to the quality of food and health of the consumers.

Capillary electrophoresis method for speciation of iron (II) and iron (III) in pharmaceuticals by dual precapillary complexation.

Pharmaceutical iron sucrose is an iron (III) replacement for the treatment of iron deficiency anemia in patients with chronic kidney disease. The drug product (injection) is a colloidal solution of ferric hydroxide in complex with sucrose, containing 20 mg/mL elemental iron; according to United States pharmacopoeia (USP), the limit of iron (II) is 0.4% w/v. A selective CE method for the simultaneous determination of iron (III) and its potential impurity iron (II), was developed by applying a dual precapillary complexation. In particular, 1,10-phenanthroline and 1,2-diaminocyclohexanetetraacetic acid were used for complexation of iron (II) and iron (III), respectively. Sample preparation was optimized to achieve mineralization of pharmaceuticals using HCl 6 M, by avoiding perturbation of the oxidation status of both iron species. Simple CZE conditions, involving a 60 mM (pH 9.3) tetraborate buffer at the constant voltage of 25 KV and 25°C, allowed fast separation of iron (II) and iron (III) complexes that were detected at 265 nm. Sensitivity for iron (II) determination was found to be 4.80 μM (LOQ) corresponding to 0.15% w/w with respect to the total iron test level. The method was validated by following International Conference on Harmonization guidelines for specificity, linearity, precision, accuracy, and robustness and it was applied to real pharmaceutical samples. The obtained results suggested that the method can be a useful alternative to the official USP and British pharmacopoeia polarographic method.