Why is sodium carbonate used to make sodium thiosulfate solution?

Sodium carbonate is used in sodium thiosulfate solutions because it is a stabilizing element that neutralizes an acidic environment and prevents the thiosulfate from breaking down and releasing toxic sulfur dioxide.

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Sodium thiosulfate is a useful product, employed in many areas, from medicine (it can treat cyanide poisoning) to developing photographs to mining for gold to eliminating chlorine from water. But the problem with thiosulfate is that it becomes unstable in acidic environments and breaks down into sulfur, sulfur dioxide, and...

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Sodium thiosulfate is a useful product, employed in many areas, from medicine (it can treat cyanide poisoning) to developing photographs to mining for gold to eliminating chlorine from water. But the problem with thiosulfate is that it becomes unstable in acidic environments and breaks down into sulfur, sulfur dioxide, and water. This isn’t good, because sulfur dioxide is a toxic gas that is highly irritating to the lungs, eyes, nose, and throat. In high concentrations, it can trigger asthma attacks, cause lung troubles, and even make heart conditions worse.

We can easily see, then, the need for something to balance out the instability of thiosulfate, and sodium carbonate can do this. Sodium carbonate is also called soda ash or washing ash, and in solution, it is alkaline, the opposite of acidic. Acidic environments contain many positively charged hydrogen ions. The alkaline substance of sodium carbonate balances out the acidity of the environment by introducing negatively charged hydroxide ions. Since the positively charged hydrogen ions lead to the breakdown of the sodium thiosulfate, when the negatively charged hydroxide ions neutralize them, they thereby prevent the thiosulfate from breaking down and letting loose the sulfur dioxide. The sodium carbonate blocks the acid from reacting with the thiosulfate and allows the thiosulfate to retain its stability and remain safe for human interaction.

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