Grease sticking inside a burette will not wash off with water alone. The lab has three cleaners for it, and the best one oxidises the dirt away. Once it turns green, it is finished.
1. Three cleaners
Glassware is washed with one of three cleaning mixtures: Na₂CO₃ solution, the detergent Decon-90 and the chromic acid mixture.
Put a 10% dilute Na₂CO₃ solution in a large plastic bowl. Soak the dirty glassware for a while, rub it with a brush, then rinse it in water.
Decon-90 cleans almost all the glassware in a chemistry lab. It removes grease, tar-like matter, silicone oil and polymeric residue. It is an environment-friendly detergent: it hardly pollutes water, it is 100% biodegradable by microorganisms, and it is phosphate free.
2. Chromic acid: why it is the best
The chromic acid mixture is the best cleaner for laboratory glassware. It is very effective at removing grease or oil sticking to glassware, especially in a burette.
The reason is that chromic acid is a strong oxidising agent. In reaction it gives nascent oxygen [O], and that [O] oxidises oily dirt and removes it.
K₂Cr₂O₇ + 4H₂SO₄ → K₂SO₄ + Cr₂(SO₄)₃ + 4H₂O + 3[O] (3[O] is the nascent oxygen)
3. Making the mixture
The mixture has three ingredients: potassium dichromate K₂Cr₂O₇ 4.0 g, distilled water 40.0 mL and concentrated H₂SO₄ (2M) 40.0 mL.
Make the recipe yourself on the bench; break the order and the bench stops you. Once it is made, keep using it and watch the colour.
the benchstep by step
Making chromic acid mixture
How many uses it takes to turn green is made up, for the drawing.
In a 250 mL pyrex beaker, dissolve 4 g of K₂Cr₂O₇ powder in 40 mL of distilled water. Mix in 40 mL of 2M H₂SO₄ a little at a time. Walk through the steps one by one.
step by stepdichromate first, bottle last
Preparing the chromic acid mixture
Step 1. Take 4 g of K₂Cr₂O₇ powder in a 250 mL pyrex beaker.
Step 2. Dissolve it in 40 mL of distilled water.
Step 3. Add 40 mL of 2M H₂SO₄ a little at a time. The reaction is exothermic and the temperature rises to 70°-80°C.
Step 4. Cool it to room temperature.
Step 5. Fill it into a glass-stoppered reagent bottle.
Mixing the acid gives out heat, which is why it goes in a little at a time. Only when the mixture has cooled does it go into the bottle.
A freshly made mixture holds several higher chromic acids of chromic anhydride, CrO₃:
The mixture is a clear orange-red when made but turns green after many uses, because green Cr₂(SO₄)₃ has formed. It then has no oxidising power, so it is not used any more.
Wear latex gloves. Fill the dirty pipette, burette or conical flask with the mixture and leave it for some hours or a day. Then pour the mixture back into the glass-stoppered bottle and wash the glassware well with water.
Some oily matter oxidises very slowly in this mixture, so the glassware is kept immersed for several days. The grease inside a burette stopcock is cleaned this way.
Now put the facts about the three cleaners in their right boxes.
sort thempick a chip, then a bin
Which cleaner is it?
Pick up a fact, then drop it in the cleaner it describes.
Right. Next one.
Not yet. Think: bowl and brush, the environment, or oxidation and the green colour?
All in place. Oxidant, burette grease and turning green all belong to the chromic acid mixture. Biodegradable and phosphate free belong to Decon-90.
5. Precautions
Concentrated H₂SO₄ is dehydrating and corrosive. So when using it, and when using the chromic acid mixture, wear safety glasses or goggles on the eyes and latex or vinyl gloves on the hands.
If the chromic acid mixture touches the skin, wash with plenty of water and finally wash well with 5% NaHCO₃ solution.
To remove oil and grease from glassware, acetone (CH₃COCH₃) solvent is used when needed.
Now match the six substances of this topic to their roles.
match the pairsmatch the six pairs
Who does what?
Pick up a substance, then drop it in the box for its role.
Right. Next one.
Not yet. Ask: is it an ingredient, the oxidant, an end product, or a safety wash?
All in place. K₂Cr₂O₇ and H₂SO₄ are the ingredients, [O] removes the dirt, green Cr₂(SO₄)₃ spoils the mixture, and 5% NaHCO₃ is for the skin.
Three short cases. The first is about protecting the hands.
precaution checkrecall the book's precautions
Which is the safe act?
What do you wear on your hands to use the chromic acid mixture?
Right. Concentrated H₂SO₄ is corrosive, so latex or vinyl gloves on the hands and goggles on the eyes.
Not yet. The mixture contains concentrated H₂SO₄, which is corrosive; it must not be handled bare-handed.
Not yet. The book names latex or vinyl gloves, not paper ones.
In the second, the mixture has touched the skin.
precaution checkrecall the book's precautions
Which is the safe act?
The acid mixture touches your skin. What do you do?
Right. Plenty of water first, and finally a good wash with 5% NaHCO₃ solution.
Not yet. Wiping leaves the acid on the skin; the book's rule is to wash with plenty of water.
Not yet. The book says 5% NaHCO₃, not NaOH. NaOH is itself a strong alkali.
The third is about an old mixture.
precaution checkrecall the book's precautions
Which is the safe act?
The mixture has turned green. What now?
Right. Green means Cr₂(SO₄)₃ has formed and the oxidising power is gone.
Not yet. A mixture with no oxidising power will not oxidise dirt however often it is used.
Not yet. The green comes from Cr₂(SO₄)₃ forming, not from heat; cooling changes nothing.
6. Check yourself
The first question is in the exam's style.
check yourselfpick one answer
Question 1
Which is used as the best cleaner of laboratory glass apparatus?
Soap lifts ordinary dirt, but the book names the chromic acid mixture as the best cleaner.
A detergent (Decon-90) is good but not the best; chromic acid works better on grease and oil.
Right. This is the book's answer: the chromic acid mixture is the best cleaner of glassware.
Soda (Na₂CO₃) is the third cleaning mixture but not the best.
Think a little on the second.
check yourselfpick one answer
Question 2
Used to clean laboratory glass apparatus:
i. H₂SO₄ ii. K₂Cr₂O₇ iii. CHCl₃
Right. H₂SO₄ and K₂Cr₂O₇ together make the chromic acid mixture; both are right.
CHCl₃ is not a glass cleaner, so iii is wrong.
iii is wrong; and i is also right. Statement iii reads: "CHCl₃".
CHCl₃ is not among the book's cleaners. The right choice is i and ii.
For the last, recall the book's facts.
check yourselfpick one answer
Question 3
A lab has no chromic acid. Which solution is put in a plastic bowl to soak and scrub dirty glassware?
Right. This is the book's method: soak in a plastic bowl, scrub with a brush and rinse in water.
Conc. H₂SO₄ is corrosive and is not used so in a plastic bowl.
This is the wash for acid on skin, not a glass cleaner.
Acetone is only for removing oil and grease.
The whole note on seven cards.
cut from the notetap to turn over
Seven cards
QuestionWhich is the best cleaner for glassware?
AnswerThe chromic acid mixture
QuestionWrite the reaction that gives nascent oxygen from the chromic acid mixture.