Percent Carbonate

This method describes how to determine the percent carbonate of your sample. It is based on simply adding acid to a known-weight sample until all of the carbonate has been digested. Note, this method originated from a longer rock-preparation method where the goal is to remove carbonate for an organic-carbon analysis. This particular method assumes you only want percent carbonate and you don't want to save the sample. As such, less care is taken in this method to ensure minimal organic carbon contamination (e.g. test tubes are not muffled, DI water instead of 18 M ohm water, discarding sample when finished). The reaction that takes place is:

2 HCl(aq) + CaCO₃(s) ---> CaCl₂(s) + H₂O(aq) + CO₂(g)

The hydrochloric acid (HCl) steps in this method ensure complete digestion of the calcium carbonate (CaCO₃). The water rinsing steps ensure removal of the calcium chloride (CaCl₂). The desiccation steps ensure final removal of water.

Consider reading this post for a more exhaustive understanding of what acid pretreatment does to your samples.

Risks associated with this method are exposure to HCl and broken glass.

Appropriate precautions should be taken to protect yourself against hydrochloric acid exposure. This method should be conducted in a fume hood. Wear closed-toe shoes, pants, lab coat, gloves, and eye protection. Be extra careful if you are using concentrated HCl to make the dilute solutions. Make certain to dispose of waste appropriately.

Glass test tubes may break while in the centrifuge. Don gloves to protect against spilled acid and broken glass. Use tools or remove the tipping bucket test tube holders to avoid lacerations.

1. Label and weigh as many clean test tubes as you have samples. This is the "Test Tube Weight" in the equation below.
2. Weigh approximately 1 g of sample material into a test tube. If you are sample limited, use less material. You may be able to find smaller reaction vessels and use the microbalances instead of the coarse balance.
3. In a fume hood, add 10-20% HCl drops at a time while stirring with a glass rod. Don't let the reaction boil over.
4. Continue step 3 until test tube is about half full.
5. Label rack with the date and your name and push the rack of test tubes to the back of the hood and leave overnight.
6. Add a few drops of acid the next day and stir with a glass stirring rod. If you see a reaction, you are not finished and should repeat steps 3-5. If the test tube is too full, proceed through step 8, then go back to step 3.
7. Once all the carbonate has been removed (no reaction, no fizzing), place test tubes in a centrifuge for 5 minutes at speed 5, ensuring that the centrifuge is balanced. An unbalanced centrifuge will vibrate, shake, and otherwise make a lot of noise.
8. Pour the HCl supernatant into an appropriate waste container or if you are rinsing with water, pour the supernatant down the drain.
9. Add DI water to fill the test tube about half full and stir.
10. Repeat steps 7 – 9 two more times so that you have rinsed your samples three times with DI water.
11. Pour off final supernatant and place uncapped test tubes in the rack and in the sample drying oven (60 – 70 °C) to dry overnight.
12. Once dry, remove from oven and allow to cool (~30 minutes).
13. Once at room temperature, weigh each test tube plus decarbonated dry sample powder.


14. The percent carbonate is: (Initial Dry Weight - Final Dry Weight) / Initial Dry Weight
    Initial Dry Weight = sample powder only without the weight of the test tube.
    Final Dry Weight = Final Weight - Test Tube Weight.
    


15. If you find your final weight is heavier than your initial weight, you have probably not rinsed the CaCl₂ away. Try rinsing a few more times.