Accurate determination of trace tungsten in steel is essential for quality control and process management in steel production. The solvent extraction spectrophotometric method based on the formation of tungsten(V)–thiocyanate complexes with tetraphenylarsonium chloride, specified in Annex 3 of JIS G 1220, has long been used in routine analysis. However, several operational steps in this procedure are based on empirical practices, and their chemical validity has not been sufficiently examined. In this study, critical analytical steps affecting the analytical results were systematically investigated, with particular attention to the sulfuric acid fuming step after sample digestion, reduction of tungsten(VI) by tin(II), temperature control during complex formation, and time management between solvent extraction and absorbance measurement. The procedure examined in this study was applied to certified reference materials of alloy tool steel and high-speed steel, showing good agreement between measured and certified tungsten contents. These results provide a chemical basis for the procedure specified in Annex 3 of JIS G 1220 and support its application in routine tungsten analysis of steel samples.

Control of trace elements in steel is essential for maintaining material quality, and reliable analytical methods are required to determine their concentrations accurately. The molybdenum blue spectrophotometric method, standardized in JIS G 1212 based on ISO 4829, is widely used for the determination of silicon in steel. Although this method has been employed for many years, several procedural steps still rely on empirical knowledge, and the chemical basis underlying its prescribed conditions has not been fully clarified. In this study, we reassessed the JIS procedure with a focus on the formation of molybdosilicic acid, its reduction to molybdenum blue, and the influence of reaction temperature on analytical performance. UV–Vis measurements showed that β-type heteropolyacid species, exhibiting a single absorption maximum near 810 nm, are preferentially formed around 20°C, whereas higher temperatures promote conversion to α-type species with reduced absorbance. Calibration curves obtained at 20°C demonstrated excellent linearity. The validity of the optimized conditions was confirmed by analyses of certified reference materials, in which measured silicon contents agreed well with certified values. These findings provide a scientific rationale for the long-established JIS method and highlight the importance of strict control of reaction temperature and reaction time to achieve reliable and reproducible measurements.

Silicon is one of the five ubiquitous elements of steel, and the addition of silicon improves the performance of steel. Therefore, the silicon content in steel must be strictly controlled. This paper focuses on the silicon dioxide gravimetric method (JIS G 1212) specified in the Japanese Industrial Standards (JIS) as an absolute analysis method for silicon content in steel. The contamination derived from glassware affected the analysis values. The silicic acid (Si(OH)₄) in the digested solution is in equilibrium with both the silicon dioxide (SiO₂) in the sample and the SiO₂ on the glass surface. Therefore, it was found that the use of glassware in JIS G 1212 plays a crucial role in suppressing the dissolution of silicon from the sample. The elements contained in the steel also influenced the analysis values. Especially, molybdate ion (MoO₄²⁻) in the digested solution, originating from steel samples, not only inhibited the formation of hydrated silica (SiO₂·nH₂O) but also significantly accelerated the dissolution of Si(OH)₄ during rinse. From these results, it was found that molybdenum contained in steel samples reduces the silicon recovery in the silicon dioxide gravimetric method. Silicon recovery in the entire JIS G 1212 was approximately 100% regardless of the steel sample used. This indicates contamination derived from glassware does not affect the recovery of silicon in the total process of JIS G 1212.

The addition of cobalt to steels alters their properties, such as decrease in temperature of martensitic dislocation and degradation of hardenability. Quantitative recoveries of the precipitated bis(1-nitoroso-2-naphtolato) cobalt complex are essential to ensure the accuracy of the gravimetric analysis of cobalt in steel using 1-nitroso-2-naphthol as a precipitant. Factors affecting quantitative recoveries of the bis(1-nitoroso-2-naphtolato) cobalt complex as precipitates were investigated, including a concentration of 1-nitroso-2-naphthol, an aging time, and coexisting ions. We found that optimal 1-nitroso-2-naphthol concentrations for quantitative precipitation of the complex were regulated by initial amounts of cobalt in steel samples to be determined. Above the optimal concentrations, the recoveries of the complex decreased with increasing the 1-nitroso-2-naphthol concentrations. This decrease in the recoveries is rationalized by the formation of higher-order complexes of divalent cobalt and the rate-determining step to release the ligands from the oxidized trivalent cobalt complexes. Under the optimal concentrations of 1-nitroso-2-naphthol, the achievement of the quantitative recoveries of the precipitates required a much longer aging time than that specified in the standard method. The coexisting zinc ions after the separation of the iron matrix did not interfere with the quantitative recoveries of the cobalt complex. Quantitative recoveries of cobalt in a certified standard steel sample were yielded during the precipitation and filtration steps under the optimum conditions specified here.

Sulfur is known as one of the five ubiquitous elements of steel, and the addition of sulfur reduces the performance of steel. Therefore, the sulfur content in steel must be strictly controlled. In this study, we have been focusing on the iron separation–barium sulfate gravimetric method (JIS G 1215-1) specified in the Japanese Industrial Standards (JIS) as an absolute analytical method for the determination of sulfur in steel. In our previous paper, we demonstrated that the rinse process using hot water in JIS G 1215-1 causes dissolution of barium sulfate (BaSO₄), resulting in a significant decrease in recovery. Therefore, suppressing the dissolution of BaSO₄ during the rinse process specified in JIS G 1215-1 has been critical issue in this analytical method. In this paper, we aimed to establish a novel rinsing method to replace the conventional hot water rinsing. As a result, it was found that rinsing with an ethanol solution of acetic acid effectively removed excess Ba²⁺ and Cl⁻ without dissolving BaSO₄. With this novel rinsing method, the conventional problem of reduced recovery caused by the rinse process was resolved, and quantitative recovery of the BaSO₄ precipitate was achieved.

A simple photometric detector for titration was developed using a white LED and an RGB color sensor as the light source and detector, respectively. This detector could record the color change of the sample solution during titration. The color sensor has 12-bit resolution for each red, green, and blue channel. The detector included a magnetic stirrer, and the controller controlled the mixing of a sample solution. The RGB values were transferred to a PC via a USB port. They were automatically converted to absorbance and L*a*b* data, and the results were displayed as titration curves. However, the volume of titrant added to the sample must be manually entered into the PC. The detector is as compact as 125 mm × 90 mm × 90 mm. First, the analytical performance of the titrator was evaluated by standardizing the EDTA solution with calcium carbonate. Then it was applied to the determination of chromium ions in synthesized steel samples according to the method indicated in JIS G 1217:2017, Annex 1 (normative). This method uses ammonium peroxodisulfate oxidation followed by potassium permanganate titration. This detector's function is not only helpful for objectively identifying the endpoint of a titration but also valuable for transferring the veteran's skill in endpoint determination to a novice technician, since subtle color changes can be recorded as numerical data.

A microfluidic titration method using a cross-type microchannel device was developed for to the determination of total iron in steel samples. The redox titration reaction proceeded within the microchannel, and the titration endpoint was determined based on the differential value of grayscale intensity derived from blue-channel image analysis. The reaction behavior was visualized in real time, enabling quantitative evaluation of concentration-dependent color changes. Optimization of the flow rate and observation point revealed that a flow rate of 0.05 mL min⁻¹ and a detection position 1.5 cm downstream from the junction yielded the highest sensitivity. The obtained calibration plot exhibited a semi-logarithmic relationship with iron concentration. When applied to the certified reference material JSS 831-2 Taharoa iron sand, the total iron concentration determined by the microfluidic method (46.9 ± 1.9 mM) was statistically indistinguishable from that obtained by the conventional volumetric analysis based on JIS M 8212 (45.49 ± 0.18 mM), as confirmed by a t-test. These results demonstrate that the proposed system achieves analytical accuracy comparable to traditional wet chemical methods, while eliminating subjective endpoint determination, reducing sample and reagent consumption, and enabling automated and reproducible operation. The results demonstrate the feasibility of integrating redox titration within a microchannel as a new approach to miniaturized, automated wet analysis. The microfluidic titration platform offers a promising approach for miniaturized, automated wet analysis and has potential for extension to other volumetric techniques such as chelatometric and acid–base titrations.

In this study, we investigated the surface properties of iron subjected to corrosion in contact with frozen salt solutions, focusing on the effects of solution aeration and salt type on ferrous iron dissolution behavior. X-ray photoelectron spectroscopy (XPS) was used to examine the surface conditions after corrosion. Image analysis of the frozen media indicated that dissolved oxygen in freeze-concentrated solutions (FCS) plays a crucial role in the dissolution process. XPS analysis confirmed the formation of iron hydroxide and iron oxyhydroxide on the iron surface, suggesting a reaction mechanism similar to that observed under atmospheric conditions. Additionally, surface analysis revealed that specific salt ions—such as F⁻, Cl⁻, and Cs⁺—exhibit a tendency to adsorb onto the iron surface under conditions of pronounced dissolution. These hard anions form complexes with Fe (II) ions, thereby promoting their dissolution. Moreover, Cs⁺ ions readily adsorb onto FeOOH, creating a concentration gradient near and beyond the iron surface that further promotes iron dissolution.

Tamahagane is compared to the lance metal with multicolor after extremely low temperature blowing in a test converter using a multifaceted process engineering approach. The following conclusions were reached:

(1) The golden portion of tamahagane consists of an oxide film approximately 20 nm thick, which is composed of three layers. This multilayered oxide film gives tamahagane its “gorgeous” color.

(2) Tamahagane, a high-purity iron, is produced through a metallurgical reaction network consisting of three reactions: a hydrogen production reaction due to the decomposition of moisture in the blast, a reverse water-gas shift reaction, and a carburization reaction due to aging (long-term refining).

(3) The temperature at which tamahagane is formed through this metallurgical reaction network is around the iron point (720°C) of the intersection point between Fe–FeO equilibrium and Boudouard equilibrium and the eutectoid temperature of the iron-carbon system (A1 point: 723°C).

(4) In the case of a weight absolute humidity of 0.0055 (kg-H₂O/kg-dry air) at a base temperature of 10°C, and 0.0085 after adiabatic humidification (compression ratio of 1.1), the carburization time for an average carbon content of iron (total weight of 3 ton) is approximately 10.5 to 16.0 hours.

(5) Humidity of environments where Tamahagane is made is another important factor as well as charcoal reactivity and TiO₂ content in iron sands.