A strongly alkaline substance (caustic soda) that dissolves readily in water. In indigo vats it is used in place of wood-ash lye; in naphthol dyeing it dissolves the naphthol and keeps the bath alkaline. In its safety data sheet, the substance at 100% concentration is classified as category 1 for skin corrosion/irritation and for serious eye damage.
The classifications shown here are the safety data sheet categories for the substance at 100% concentration, not for a solution used in dyeing. For any individual product, see its label and safety data sheet.
The limitations noted for some sources are recorded in Japanese only — see the Japanese edition.
Sodium hydroxide is used in Ash-lye fermentation vat buildingcontextual
Scope: One example presented for beginners, using a 75-litre container
States that sodium hydroxide is used in place of ash lye when ash is unavailable, adding that although it needs careful handling and its use is not preferred, it is a convenient substitute for ash lye; also states that glucose is added and stirred in on the day after setup.
Source: 連載〈阿波藍5〉染料液の作り方（藍を建てる） （川人美洋子, 2010） ・P-287,「比較的簡易な発酵建て」の段落 ・practitioner's record ・ checked against the source by AI
Scope: Managing alkalinity in the ash-lye fermentation vat
States that slaked lime does not dissolve readily in water and so has lasting effect, whereas sodium hydroxide dissolves readily and lacks lasting effect; when pH keeps dropping sharply, the fast-acting sodium hydroxide is used, but slaked lime and ash lye are the normal choice.
Source: 連載〈阿波藍5〉染料液の作り方（藍を建てる） （川人美洋子, 2010） ・P-287,「管理する難しさ」の段落 ・practitioner's record ・ checked against the source by AI
Scope: One sentence in a paper introducing the dyes used for Indonesian batik. The original spelling is "costik soda". It does not give the concentration or other conditions for dissolving
this paper states naphthol is water-insoluble and needs caustic soda to dissolve it
States that NaOH treatment shrinks cellulosic fibre along its axis and increases its diameter, and is thought to raise water absorption and adsorption sites for deeper dyeing as crystalline structure breaks down and amorphous regions expand; for synthetic fibres it notes dyeability can sometimes improve if the diazonium salt is applied first instead.
Source: アゾ色素合成実験での二酸化窒素発生への注意 （杉山晴紀、大場茂, 2019） ・P14 6-4節直前の段落 ・textbook ・ checked against the source by AI
States that caustic soda is used to dissolve the naphthol and to keep the naphtholation bath at the proper alkalinity, and warns that if its alkalinity drops below the prescribed limit the naphthol may precipitate.
Source: Naphthols and Bases （hanu（ブログ「Dyeing and Processing / dyeing world」運営者名として表示）, 2010） ・Precautions / Role of Different chemicals節 ・practitioner's record ・ checked against the source by AI
Sodium hydroxide ・propertyThe government model SDS for sodium hydroxide (100% concentration) reports both skin corrosion/irritation and serious eye damage as “Category 1,” and specific target organ toxicity (single exposure) as “Category 1 (respiratory system),” with the signal word “Danger” and hazard statements including “causes severe skin burns and eye damage” and possible respiratory damage.contextual
Scope: Sodium hydroxide (caustic soda) itself, as a 100%-concentration substance
Classifies skin corrosion/irritation and serious eye damage both as Category 1 and specific-target-organ toxicity (single exposure) as Category 1 (respiratory), with signal word “Danger” and hazard statements including severe skin/eye damage and respiratory harm.
Source: 化学物質：水酸化ナトリウム（GHSモデルSDS情報） （厚生労働省, 2003） ・２．危険有害性の要約 ・standard definition ・ checked against the source by AI
Sodium hydroxide ・propertyFor skin sensitization, a human-volunteer test showed dose-dependent irritation but no enhanced reaction on rechallenge, and given the lack of reported cases of human skin sensitization despite long, widespread use, sodium hydroxide is reported to be considered not a skin-sensitizing substance and is classified as “not classified” for this endpoint.contextual
Describes a test inducing volunteers' backs with 0.063-1.0% solutions and rechallenging with a 0.125% solution after 7 days: dose-dependent irritation occurred but rechallenge did not enhance the reaction, and given the absence of reported sensitization cases despite long widespread use, the substance is classified as not a skin sensitizer.
Source: 化学物質：水酸化ナトリウム（GHSモデルSDS情報） （厚生労働省, 2003） ・１１．有害性情報／呼吸器感作性又は皮膚感作性 ・standard definition ・ checked against the source by AI
Chemical reduction of indigo (chemical vat) requires Sodium hydroxidecontextual
Scope: Industrial indigo dyeing of cotton
Explains that pairing a reducing agent (sodium hydrosulfite) with sodium hydroxide brings about a chemical change from the dye's insoluble blue form into a soluble one, and that this shift also gives the dye a very pale, greenish-yellow appearance for a time.
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