For a long time, I have been using store bought baking soda (with sodium bicarbonate as the sole ingredient listed) as the sodium bicarbonate in my (qualitative) home experiments, but recently I noticed that a 100mL solution with 3.6g baking soda developed light-yellow precipitates after air exposure for two days. I was pretty sure the baking soda had all dissolved initially.
Also, I started to worry about this because of the weird smell associated with baking soda when heated.
Anyways, what are the main impurities in baking soda? I especially want to know about the source of the yellow precipitate and smell, but a full list or some articles on this topic would be most appreciated.
Searching in search engines and online databases using terms like "baking soda smell" gives results about using baking soda as a deodorant; "impurity in baking soda" does not yield an answer; "baking soda composition" only shows sodium bicarbonate. The vendors I asked persisted that their product contained nothing but sodium bicarbonate, "100% pure with no other substance." (It's unlikely that they admit the presence of impurities, as they think I would refuse to buy "impure" baking soda!)
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2$\begingroup$ Why not to ask the vendor? The third parties cannot know what minor impurities, possibly intentional additives for baking context, are present or added. // The SE site expects you search, read and think about the topic. Then, if you still need to ask, provide the explicit summary of partial answers/ideas/thoughts you have got. That prevents us to tell you what you already know or what you can easily find yourself. Effort not shown may be considered as effort not done and the question may get closed. How do I ask a good question?. $\endgroup$– PoutnikCommented Jan 28, 2022 at 9:37
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$\begingroup$ Thanks for commenting. I have asked a few vendors just now, and will edit the question and show my personal search results. $\endgroup$– anemia0Commented Jan 28, 2022 at 12:49
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$\begingroup$ Do you have older, well behaving lot ( or from different vendor) of baking soda to compare, to eliminate possibility it is because of the used water? $\endgroup$– PoutnikCommented Jan 28, 2022 at 13:13
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$\begingroup$ What was the smell like ? Rather organic or inorganic ? Weird is rather vaque. $\endgroup$– PoutnikCommented Jan 28, 2022 at 14:56
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1$\begingroup$ What was the source of water and how did you store the solution? $\endgroup$– Buck Thorn ♦Commented Jan 28, 2022 at 15:50
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1 Answer
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The appearance of light brown precipitates over time in tap water is a sign of (dissolved) iron impurities in a solution. Iron can come anywhere, iron pipes, water, baking soda itself.
For example, the now famous Hamaliyan pink salt, which comes from Khewra mines from Pakistan rather is slightly pink. If you dissolve the salt in water and let water sit for a few minutes, you will see rust colored soilds sitting at bottom. That is iron oxides.
Simply change the baking soda brand, and if you still see yellow ppt in water aftter letting it sit for days. Make a conc. solution to accelerate precipitation.
Heating food-grade baking soda should not generate any smell. The so-called weird smell must be coming from heating the metallic vessel and volatilization of adsorbed organic stuff. Most likely the vessel was a kitchen item (?).
If your baking soda was sitting for a long time in a fridge or outside, it does tend to adsorb smells.
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$\begingroup$ That's a great point. Ferric ions do precipitate under alkaline conditions, and the smell could the baking soda releasing previously adsorbed chemicals in the air. I was using a beaker for the heating though, so I felt the smell should still be from the baking soda. But after all, has nobody done a composition test for common baking soda? $\endgroup$– anemia0Commented Jan 29, 2022 at 7:45
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$\begingroup$ Just a sanity check: You are not using baking powder, right? Food grade baking soda from a reliable company should be pure. $\endgroup$– ACRCommented Jan 29, 2022 at 8:18
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$\begingroup$ No. I'm sure it was baking soda; it has "sodium bicarbonate" as the sole ingredient. $\endgroup$– anemia0Commented Jan 29, 2022 at 8:43
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1$\begingroup$ @anemia0 with respect to "nobody has done a composition test for common baking soda" there is no such thing as "common baking soda" of fixed composition. The impurities will vary depending on the method and facility (and even batch) of preparation. For each grade (ie food, industrial, etc), there are limits for specified impurities, and every batch will have an associated certificate of analysis (such as this one here: ishtarcompany.com/wp-content/uploads/2018/12/…), but those generally aren't available to household consumers $\endgroup$– AndrewCommented Jan 29, 2022 at 14:07
