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A project log for Coin Cell Challenge...

Lithium coin cells are convenient energy stores for projects. Not safe if swallowed though. Can we do better?

Simon MerrettSimon Merrett 04/02/2019 at 15:460 Comments

In which we try to answer:

Can saliva be a proxy for "tissue fluids" in battery injury tests?

Warning, this log is not going to look nice if you are squeamish about spit

I ran a basic experiment to answer this. From the previous log, we found out that hydrolysis of tissue fluids was believed to be the main mechanism of harm to people who swallow coin cell batteries. If we could use saliva as a proxy for the "tissue fluids" found in the throat/oesophagus, we could expose them to the sources of current provided by the coin cells and set a baseline against which to try and improve our energy stores. I don't know why we didn't do this experiment in school - it was rather fun.

Equipment:

Method

  1. Take two jars and spit in them to approximately equal levels. One will receive the coin cell and the other will be our control sample.
  2. Measure your coin cell voltage with the voltmeter/DMM. Nominally they are 3V but open circuit they can be 3.3V for a new one.

  3. Add your coin cell with the anode (negative terminal) facing down. Bear in mind that anode and cathode can get switched around when people describe coin cell electrolysis. We're going to refer the anode (smaller face) and cathode (positive, often marked with an etched or stamped "+" and wraps round the sides of the cell) as they are marked on the cell. 

  4. Start your stopwatch/timer and put the lids on the jars. I did this experiment at around 18°C although the reaction in the case of the battery being actually swallowed is going to be much nearer 37°C.

  5. After 2 hours have elapsed, put on whatever protective equipment or clothing you deem necessary. Put a drop of bleach in the remaining jar and open the lids of the two saliva jars.

  6. Get your litmus paper and colour chart ready for comparison. Dip a piece of litmus paper in the control sample of saliva and note the colour score from the colour chart. Then repeat for the coin cell saliva sample and the bleach. Be quick making your comparison with bleach - the litmus colour starts to quickly fade as it is bleached... ...by the bleach.

  7. Use the tweezers to remove the coin cell from the saliva. Wipe it down with the absorbent material and measure the voltage. Dispose of battery, bleach and saliva samples safely and wash containers.

Results

So, bleach is around pH 12, (my) saliva is around pH 7 and saliva-hydrolised-by-CR2032 for two hours seems to be between pH 10-13. From a rough comparison of the open circuit voltage between the start (3V) and end (2.8V) of the 2 hour test, using the chart in this analysis of coin cell capacity, we may have used ~150mAh or, at 2.7V (the voltage when the current was flowing),  ~1500 J.

Conclusion

Given the effect of the hydrolysis on the saliva, I conclude that we can use saliva as a proxy for oesophageal fluids in future tests. As a baseline, it appears that a 2hr exposure of a small quantity of saliva to a new CR2032 cell makes something almost-as or as caustic as bleach. 

Don't swallow your coin cells kids. And if you ever need a disinfectant in a pinch and you have a current source to hand...

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