Why Does Sodium Thiosulfate Become Cloudy? Chemical Insights

Sodium thiosulfate turns cloudy when it encounters acids, like hydrochloric acid. This reaction generates solid sulfur as a product, and sulfur doesn’t dissolve in water. As a result, the solution becomes cloudy due to the tiny solid sulfur particles suspended in it. The reaction can be represented as follows:

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

This cloudiness is a helpful sign in various chemical processes, including titrations and analyses. Sodium thiosulfate’s ability to create this visual change is frequently used in laboratory settings and in photography to indicate specific chemical reactions or the endpoint of a reaction.

What is Sodium Thiosulfate?

What is Sodium Thiosulfate

Chemical Structure and Formula

Sodium thiosulfate has the chemical formula Na2S2O3, which reflects its composition. It consists of two sodium (Na) atoms, two sulfur (S) atoms, and three oxygen (O) atoms. The arrangement of these atoms creates a distinct crystalline structure. The “2” subscript in the chemical formula indicates that there are two sodium atoms for every molecule of sodium thiosulfate.

Common Names

Sodium thiosulfate is known by various names depending on its use and context. One of the most common nicknames for this compound is “hypo,” which is primarily used in the field of photography. In the context of medicine and safety, it may also be referred to as “sodium hyposulfite.”


Photography: Sodium thiosulfate has played a crucial role in photography for over a century. It is used as a fixing agent, which means it is employed to remove unexposed silver halide from photographic emulsions. This step is essential in developing photographs because it prevents further darkening of the image and makes it permanent.

Water Treatment: In water treatment, sodium thiosulfate is used to dechlorinate tap water and wastewater. Chlorine is often added to water supplies to disinfect them, and sodium thiosulfate is applied to remove the excess chlorine, ensuring that the water is safe for consumption and does not have adverse effects on aquatic life when discharged into the environment.

Medical Use: Sodium thiosulfate is used as an antidote for cyanide poisoning. It works by forming a less toxic compound with cyanide, which the body can more easily eliminate. This property makes it a lifesaving substance in emergency medical situations involving cyanide exposure.

Chemical Reactions: Sodium thiosulfate is often involved in various chemical reactions. One notable reaction is the one discussed in the original question, where it becomes cloudy when reacting with certain acids. This property makes it valuable in analytical chemistry for titrations and experiments.

Industrial Processes: Sodium thiosulfate finds applications in a range of industrial processes. In the gold extraction process, it is used to recover gold from ores and solutions. In the textile industry, it is used to remove excess dye and fix colors in fabrics.

Chemical Properties:

Sodium thiosulfate is a reducing agent, meaning it has the ability to reduce the oxidation state of other substances. This property is especially relevant in its applications in photography, where it helps to reduce silver halide compounds. Additionally, it can undergo various chemical reactions, depending on the substances it encounters, leading to the formation of new compounds.


One of the key features of sodium thiosulfate is its high solubility in water. This solubility is a critical aspect of its practical applications, as it can be readily dissolved in water to create solutions for its various uses, whether in photography, water treatment, or chemical reactions.

What Causes the Cloudiness?

The cloudiness phenomenon is a remarkable change that occurs in solutions containing sodium thiosulfate. When sodium thiosulfate meets specific substances, particularly acids, it reacts in a way that causes the solution to go from crystal clear to cloudy.

This cloudiness is more than just a visual spectacle; it signifies the occurrence of a chemical reaction. It’s like a signal from the world of chemistry that something significant is happening.

The Chemistry Behind Cloudiness

When sodium thiosulfate encounters an acid, such as hydrochloric acid, the following reaction occurs

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

In this equation, Na2S2O3 represents sodium thiosulfate, and HCl stands for hydrochloric acid. When these two substances come together, they give birth to a series of products, including sodium chloride (NaCl), water (H2O), sulfur dioxide (SO2), and solid sulfur (S).

The Role of Sulfur

Solid sulfur, or S, is a critical player in the cloudiness phenomenon. Unlike sodium thiosulfate, sulfur isn’t soluble in water. So, when it’s formed in the reaction, it turns into tiny particles that make the solution cloudy. This transformation is the key to why sodium thiosulfate becomes cloudy.

Why It Matters

You might be wondering, “Why is this cloudiness even important?” Well, there’s more to it than meets the eye.

  • In the Lab: Scientists and chemists use this cloudiness as a valuable tool in their experiments. It helps them determine the endpoint of a reaction, making their measurements and analyses more accurate.
  • In Photography: Sodium thiosulfate is a superstar in the photography world. It’s essential for fixing photographs during development. When you see that cloudiness, it tells you the fixing process is complete and your photos are ready to stand the test of time.

How does temperature affect sodium thiosulphate?

Temperature can significantly affect the behavior of sodium thiosulfate (Na2S2O3) in chemical reactions. Specifically, it can influence the rate and extent of reactions involving sodium thiosulfate. 

Reaction Rate

  • Higher Temperatures: An increase in temperature typically accelerates chemical reactions, including those involving sodium thiosulfate. This means that at higher temperatures, reactions that include sodium thiosulfate will proceed more quickly.
  • Lower Temperatures: Conversely, lower temperatures slow down reactions. Chemical reactions involving sodium thiosulfate will progress more slowly at lower temperatures.


Temperature can also affect the solubility of sodium thiosulfate. In general, most solid substances, including sodium thiosulfate, dissolve more readily in warm or hot solutions than in cold ones. So, increasing the temperature of a solution can increase the solubility of sodium thiosulfate.

Practical Applications

In laboratory settings, controlling the temperature can be crucial for experiments involving sodium thiosulfate. Adjusting the temperature can impact reaction rates, which is vital for precise measurements and reaction endpoints.

In the field of photography, temperature plays a role in the development process. The temperature at which photographic solutions, including those containing sodium thiosulfate, are used can affect the speed and quality of image development.

How to prevent cloudiness

Preventing cloudiness in a solution of sodium thiosulfate involves controlling the conditions and factors that lead to the formation of solid sulfur particles, which make the solution appear cloudy.

Dilution: One effective way to prevent cloudiness is to dilute the sodium thiosulfate solution with water. By reducing the concentration of reactants, you can minimize the cloudiness that occurs when sodium thiosulfate reacts with acids.

Lower Temperature: Keeping the solution at a lower temperature can slow down the reaction with acids, reducing the cloudiness. Cold solutions are less prone to cloudiness than hot ones, so controlling the temperature can be key.

Control Concentrations: Maintaining lower concentrations of both the acid and sodium thiosulfate can help prevent cloudiness. Lower concentration reactions are less likely to result in significant cloudiness.

Use of Inhibitors: In some cases, the addition of specific chemical inhibitors can be employed to slow down or prevent the cloudiness-causing reaction. These inhibitors are compounds that interfere with the reaction process.

Precise Measurement: Accurate measurements when preparing solutions are essential to maintaining the desired concentrations and preventing unexpected cloudiness.

Stirring or Agitation: Uniform mixing and consistent stirring of the solution can help maintain a consistent distribution of reactants, reducing the likelihood of localized cloudiness.

Filtering: If cloudiness does occur, you can filter the solution to remove solid sulfur particles and obtain a clear solution. This can be especially useful when a clear solution is necessary for a specific application.

Contaminant Avoidance: Ensure that the solution and equipment used are free from contaminants or impurities that could initiate unwanted reactions and cloudiness.

pH Control: Monitoring and controlling the pH of the solution can also influence the cloudiness. Adjusting the acidity or alkalinity can impact the reaction rate.

Testing and Experimentation: For specific applications or experiments, it may be necessary to conduct tests to determine the ideal conditions and concentrations for preventing cloudiness.

Medical Uses of Sodium Thiosulfate

Medical Uses of Sodium Thiosulfate

Sodium thiosulfate (Na2S2O3) has several medical uses, primarily in the field of medicine and toxicology.

Cyanide Poisoning Treatment

Sodium thiosulfate is a key component in the treatment of cyanide poisoning. Cyanide is a highly toxic chemical that can interfere with the body’s ability to use oxygen, leading to severe health effects. 

Sodium thiosulfate works by reacting with cyanide to form a less toxic compound, thiocyanate, which can be safely excreted by the body. This treatment is particularly important in cases of accidental or intentional cyanide ingestion.

Calciphylaxis Treatment

Calciphylaxis, also known as calcific uremic arteriolopathy, is a rare and life-threatening condition often seen in patients with kidney disease. Sodium thiosulfate is used in the treatment of calciphylaxis by chelating calcium and helping to reduce the calcification of small blood vessels. This can improve blood flow and alleviate the condition.

Radiation-Induced Dermatitis

Sodium thiosulfate has been explored as a potential treatment for radiation-induced dermatitis, a common side effect of radiation therapy in cancer treatment. It is believed to have anti-inflammatory and antioxidant properties that may help relieve skin damage caused by radiation.

Metal Chelation

In some cases, sodium thiosulfate is used as a chelating agent to remove heavy metals from the body. Chelation therapy with sodium thiosulfate is employed to treat heavy metal poisoning, such as lead poisoning.

Antifungal Properties

Sodium thiosulfate has been studied for its potential antifungal properties and is sometimes used in the treatment of fungal skin infections or conditions like tinea versicolor.

Extracorporeal Membrane Oxygenation (ECMO)

In the context of ECMO, a medical procedure that provides cardiac and respiratory support for patients with severe respiratory failure, sodium thiosulfate may be used to help neutralize excess acid in the blood.


What causes a solution to turn cloudy?

A solution turns cloudy due to the formation of tiny solid particles in the solution, often the result of a chemical reaction.

Why does sodium thiosulfate and hydrochloric acid turn cloudy?

The cloudiness in this case is due to the formation of solid sulfur (S) when sodium thiosulfate reacts with hydrochloric acid.

What happens when sodium thiosulfate reacts with hydrochloric acid?

When sodium thiosulfate reacts with hydrochloric acid, it produces sodium chloride, water, sulfur dioxide, and solid sulfur, leading to cloudiness in the solution.

What species causes the cloudiness that masks the cross marked on paper?

The solid sulfur (S) particles formed during the reaction of sodium thiosulfate and hydrochloric acid are responsible for the cloudiness that obscures the cross on paper.

How does temperature affect sodium thiosulphate and hydrochloric acid?

Higher temperatures can accelerate the reaction between sodium thiosulfate and hydrochloric acid, resulting in quicker cloudiness. Lower temperatures slow down the reaction.

What happens when sodium thiosulfate is heated?

Heating sodium thiosulfate can lead to decomposition, producing sodium sulfate, sulfur dioxide, and oxygen as byproducts.

What happens when sodium thiosulphate reacts with concentrated Sulphuric acid?

The reaction with concentrated sulfuric acid results in the production of sulfur dioxide, sodium sulfate, and water, with no cloudiness observed.

Why does sodium thiosulfate make water cold?

Sodium thiosulfate dissolving in water is an endothermic process, meaning it absorbs heat from the surroundings, causing a cooling effect and making the water feel cold.

What happens when sodium thiosulfate is dissolved in water?

When sodium thiosulfate is dissolved in water, it forms a clear solution. The dissolution process is endothermic and cools the water.

Why must the water used to prepare sodium thiosulfate be boiled?

Boiling the water helps to remove impurities and dissolved gases, ensuring that the sodium thiosulfate solution is free from contaminants that could affect its reactions or properties.

Final words

Finally, the cloudy transformation of sodium thiosulfate is like a hidden secret in the world of chemistry. It’s caused by a special reaction with acids, which turns a clear solution into a cloudy one. This phenomenon isn’t just fascinating; it’s super useful too!

Scientists use it to know when their experiments are finished. In photography, it tells us our pictures are ready to last a lifetime. And by understanding how things like concentration and temperature affect the cloudiness, we can control and use this chemistry magic in many different ways.

So, next time you see sodium thiosulfate turning cloudy, remember there’s a whole world of science and practical wonders behind it. Chemistry is full of amazing secrets waiting to be discovered!