Sodium Thiosulfate - Sodium Thiolate

Na ₂ S ₂ O ₃ (158.11). Common name accepted by IUPAC. In characteristic group substitution nomenclature, it is sodium sulfurothioate. Commercially, it is available as a pentahydrate or anhydrous salt. The term "hypo" used in photographic fixation comes from the old misnomer sodium hyposulfite. The pentahydrate crystallizes when sulfur is added to an aqueous solution of _{Na 2} SO ₃ , and the solution is boiled, filtered, and concentrated. It can also be obtained by adding SO ₂ to an aqueous solution of Na ₂ S obtained by reacting an aqueous solution of NaOH with S. When the pentahydrate is heated, it becomes a dihydrate at 48 °C and an anhydrous salt at 68 °C. The pentahydrate is a monoclinic crystal. The anhydrous salt becomes β or γ forms at high temperatures, but the α form, which is stable at room temperature, is a monoclinic crystal. It decomposes into Na ₂ SO ₄ and Na ₂ S ₅ at 223 °C. It is an ionic crystal with the formula [S ₂ O ₃ ] ^2- . S-O 1.47 Å, S-S 2.01 Å. Both the pentahydrate and the anhydrous form are colorless crystals. The pentahydrate effloresces in dry air and deliquesces in moist air. Soluble in water. A significant drop in temperature occurs when it dissolves. Its aqueous solution is nearly neutral. When the aqueous solution comes into contact with air it gradually oxidizes, and this is accelerated by heating, causing sulfur to precipitate. It reacts with I ₂ to form a tetrathionate salt.

I ₂ + 2Na ₂ S ₂ O ₃ → 2Na ₂ S ₄ O ₆ + 2NaI

It reacts with _Cl2 to form hydrogen sulfate.

4Cl ₂ + Na ₂ S ₂ O ₃ + 5H ₂ O → 2NaHSO ₄ + 8HCl

It dissolves silver halide by forming a complex ([Ag(S ₂ O ₃ )] ^－ ), so it is used in fixing solutions for silver halide photography. It also forms complexes with several other metals. When an aqueous solution containing an alkali is exposed to air, it is oxidized to produce SO ₄ ^2－ . When an acid is added to the aqueous solution,

Na ₂ S ₂ O ₃ + 2HCl → 2NaCl + H ₂ O + S + SO ₂

It is used to remove chlorine from aqueous solutions, to extract silver from ores, as a mordant for dyeing and printing textiles, as a reducing agent for chromate dyeing, as a bleach for bones, ivory, oils, etc., as an analytical reagent, and in medicines (antidote for cyanide). [CAS 7772-98-7]

Source: Morikita Publishing "Chemical Dictionary (2nd Edition)" Information about the Chemical Dictionary 2nd Edition

Na₂S₂O₃(158.11)．IUPACの認める慣用名．特性基置換命名法ではスルフロチオ酸ナトリウム(sodium sulfurothioate)．市販品は五水和物または無水塩．写真定着用のハイポ(hypo)は旧誤称の次亜硫酸ソーダ(sodium hyposulfite)から．Na₂SO₃水溶液に硫黄を加えて煮沸，濾過，濃縮すると五水和物の結晶が析出する．また，NaOH水溶液にSを反応させて得たNa₂S水溶液に，SO₂を通じても得られる．五水和物を加熱すると，48 ℃ で二水和物になり，68 ℃ で無水塩となる．五水和物は単斜晶系．無水塩は高温ではβ，γ形になるが，室温で安定なα形は単斜晶系．223 ℃ でNa₂SO₄とNa₂S₅に分解する．イオン結晶で，[S₂O₃]^2－ が存在する．S-O1.47 Å，S-S2.01 Å．五水和物，無水物ともに無色の結晶．五水和物は乾いた空気中では風解し，湿った空気中では潮解する．水に可溶．溶ける際にいちじるしい温度降下がみられる．水溶液はほぼ中性を示す．水溶液は空気に触れるとしだいに酸化され，加熱で促進され，硫黄を析出する．I₂ と反応して，テトラチオ酸塩になる．

I₂ ＋ 2Na₂S₂O₃ → 2Na₂S₄O₆ ＋ 2NaI

Cl₂ と反応して，硫酸水素塩になる．

　　　4Cl₂ ＋ Na₂S₂O₃ ＋ 5H₂O → 2NaHSO₄ ＋ 8HCl　　　

ハロゲン化銀を錯体にして溶かす([Ag(S₂O₃)]^－)ので，銀塩写真の定着液に使われる．そのほかのいくつかの金属とも錯体をつくる．アルカリを加えた水溶液は空気に触れると酸化されて，SO₄^2－を生じる．水溶液に酸を加えると，

　　　Na₂S₂O₃ ＋ 2HCl → 2NaCl ＋ H₂O ＋ S ＋ SO₂　　　

などの反応で分解する．水溶液の塩素除去処理用，鉱石から銀の抽出，繊維の染色やなせんの媒染剤，クロム酸塩染色の還元剤，骨，ゾウゲ，油脂などの漂白剤，分析試薬，医薬品(シアンの解毒剤)などに用いられる．[CAS 7772-98-7]

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