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Online literature reports the melting point of $\ce{NaHCO3}$ as ~50 deg. C.

Using a standard Mel-Temp II melting-point apparatus and a standard alcohol thermometer, I didn't observe any melting of any sample lower than 150 deg. C. I don't think hydration of the salt can even explain this. Is there some other material in commercial baking soda that would alter the melting point this much?

As a raw calibration, I found that some Bayer Aspirin powder (possibly containing cellulose or other binding agent) had a melting point of 120 deg. C., a good deal below the reported ~135 deg. C.

Does anyone know what may be present in commercial Arm-and-Hammer baking soda besides sodium bicarbonate? The box reports 100% purity but I have trouble believing it now.

I've heard rumors that some baking-soda products contain aluminum and this has raised some alarm about ingestion hazards. Is this valid?

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See the MSDS for Arm and Hammer Baking Soda here, from the manufacturer (Church & Dwight Co.) (it will link to a PDF) in which it is stated:

CONDITIONS TO AVOID: Temperatures above 65°C (150°F).

INCOMPATIBILITY WITH OTHER MATERIALS: Reacts with acids to yield carbon dioxide. May also yield free caustic in presence of lime dust (CaO) and moisture (i.e., water, perspiration). Dangerous reaction with monoammonium phosphate or a sodium-potassium alloy may occur.

HAZARDOUS DECOMPOSITION PRODUCTS: Heating above 100°C may cause dangerous levels of carbon dioxide gas to be present in confined spaces. Yields sodium oxide if exposed to temperatures above 850°C. Avoid inhalation, eye and skin contact with sodium oxide.

The ingredients are listed as 100% sodium bicarbonate with CAS number 144-55-8.

Your choice of thermometer is questionable in the present case. See this Wikipedia entry which reliably reports that the upper limit on temperature measurement from an alcohol-based thermometer is 78 $^{\circ}$C. A mercury thermometer has a higher upper temperature range (see this Wikipedia article) and is preferable to the alcohol thermometer. Alternatively, use a gallium thermometer or a digital model with an operating temperature range that suits your needs.

You ask:

I've heard rumors that some baking-soda products contain aluminum and this has raised some alarm about ingestion hazards. Is this valid?

Probably not in the case of the specific brand in question here, based on the MSDS.

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  • $\begingroup$ It may not be an alcohol thermometer, but it is accurate up to 150 deg C. Its the typical red-fluid kind. Thanks for your response. Regardless, the samples of baking soda I had simply did not melt at >150 deg. C. $\endgroup$ – khaverim Aug 15 '15 at 4:26
  • $\begingroup$ It has much less to do with the thermometer than the fact that I heat the sample substantially and it doesn't actually melt. $\endgroup$ – khaverim Aug 15 '15 at 4:29
  • $\begingroup$ Interesting - maybe you're rapidly driving off the $\ce{CO2}$ and have something that's not exactly baking soda leftover, which exhibits the high melting point. $\endgroup$ – Todd Minehardt Aug 15 '15 at 14:20
  • $\begingroup$ Found this. $\ce{2 NaHCO3 → Na2CO3 + H2O + CO2}$, upon heating >50 deg. C $\endgroup$ – khaverim Aug 16 '15 at 19:19
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Online literature reports the melting point of $\ce{NaHCO3}$ as ~50 deg. C.

No!!!

On-line literature reports that $\ce{NaHCO3}$ decomposes at ~50 deg. C. to $\ce{NaCO3} + \ce{H2O}$. Both are solids. So under magnification you'd see the grains change shape and some water droplettes above, but you'd still have a white solid, not a clear melt.

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