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Chromic Acid: Solution Preparation, Disposal & Hazards

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  • 0:04 Strong Acid
  • 0:37 Preparation
  • 1:56 Disposal
  • 3:50 Chromic Acid Hazards
  • 4:18 Lesson Summary
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Lesson Transcript
Instructor: Matthew Bergstresser

Matthew has a Master of Arts degree in Physics Education. He has taught high school chemistry and physics for 14 years.

Chromic acid is a strong acid that can be prepared in a few steps. There are more steps involved in the proper disposal of chromic acid. In this lesson, we will discuss how to prepare a solution of chromic acid, how to safely dispose of it and hazards associated with it.

Strong Acid

Acids are very beneficial substances. They are widely used in industry in the production of many different products. There is a drawback though. Many acids are toxic and chromic acid is one of them. Chromic acid (H2 Cr2 O7 (aq)) is a strong acid because hydrogen ions disassociate and stay in solution. This is why (aq), meaning ''aqueous,'' is written after its formula. Let's discuss how to make chromic acid, what do do with it when it needs to be discarded, and hazards associated with it.

Preparation

Because chromic acid is toxic, it is not recommend to make large batches of it that have to be stored for extended periods of time. The amount of reagents (chemicals involved in the reactions) can be modified based on the needs of the scientist. Before working with any reagent, it is important to wear appropriate safety equipment, such as chemical resistant goggles, gloves and lab coat. The room where the reagents are being prepared should also be ventilated and preferably done in a fume hood.

The first step in preparing a chromic acid solution is to obtain a clean liter flask or beaker and fill it with 20 grams of sodium dichromate (Na2 Cr2 O7) or potassium dichromate (K2 Cr2 O7).

Next, add a small amount of water to the flask or beaker and stir with a glass stirring rod to make a paste. While continuously stirring, carefully pour 300 milliliters of concentrated sulfuric acid (H2 SO4 (aq)) to the graduated cylinder or beaker.

Finally, pour the chromic acid into a glass storage container with a stopper. Be sure to put a label on the glass storage container indicating it contains concentrated chromic acid and the date it was made. After the chromic acid has served its purpose or it has been in storage for a specific amount of time, it can be disposed of. Let's discuss how to safely dispose of chromic acid.

Disposal

It is common for people to think that chemicals can just be poured down the drain. In some cases this is true, but not with chromic acid. There are special procedures that must be followed when toxic chemicals need to be disposed of. Let's learn how to dispose of chromic acid responsibly.

Just like when making chromic acid, the procedure of disposing of chromic acid should be done in a ventilated room and preferably in a fume hood. The appropriate safety equipment should be worn as well.

The disposal of chromic acid should be done in small batches. The first step is to add 100 mL chromic acid slowly to an equal amount of water that is already in a large beaker. Put a pH meter in the solution and add 3 molar (3 moles/L concentration) sulfuric acid or calcium carbonate to the solution until a pH of 1 has been obtained. Add around 13.5 grams of solid (more if necessary) sodium thiosulfate (Na2 S2 O3) while stirring until the solution turns cloudy and blue. You should see a flocculent precipitate forming, which looks like wool in water. Let the solution sit for a week or longer to allow all of the precipitate to settle.

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