DIY CO2 system question

Folks,

The calculations I did in the previous mail were wrong, I just double
checked them...

Here are the original mail with the right figures...

/Morten


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The addition of baking soda (2NaHCO3) will not lower the amount of alcohol
(CH3CH2OH) on the solution unless you put so much if in there that it lovers
the pH too much below 5 effectually killing the yeast before it gets a
chance to convert the sugar (C6H12O6) to CO2

The chemical reaction (or at least the end result of the yeast's reduction
of sugar goes like this:

C6H12O6(aq) = 2(CH3CH2OH(aq)) + 2(CO2(g)) + energy (used by yeast)

so, for each mol of sugar you get 2 mol of CO2 and because the molar weight
of sugar is 180.1572 g/mol and the molar weight of CO2 is 44.0098 g/mol, you
will get (2 * 44.0098 g/mol) / 180.1572 g/mol = 0.4886 g CO2 for each g og
sugar.

This is max you can get but because some of the CO2 will stay in the water
in the yeast reactor you will get a little less than the 0.4886 g CO2 / g
sugar mentioned above.


Since the molar weight for alcohol (CH3CH2OH) is 46.0688 g/mol you will get
(2 * 46.0688 g/mol) / (180.1572 g/mol) = 0.5114 g alcohol for each g sugar
added to the reactor.


But, because the alcohol will kill the yeast if the concentration gets to
high (depending on which type of yeast is used, common baking yeast will
start to die arround (7% - 8%) we need to make sure that the concentration
of alcohol is low enough to make sure that all the sugar is consumed before
the alcohol kills the yeast.


If using 2.0L of water (H2O) = 2000g and 220g og sugar the end concentration
will be (0.4886 * 220 g ) / (2000 g + 04886 * 220 g) = 0.0533 = 5.33 % which
will alow the yeast to consume all the sugar and still survive the alcohol.

Using 220 g of sugar this will yield 0.5114 * 220g = 112.5143 g alcohol and
0.4886 * 220g = 107.4856 g CO2


When all the sugar is gone the yeast will 'floculate' out of solution and
sink to the bottom of the yeast container but will not die, so we can actual
ly re-use the yeast every time we set up another batch of sugar if we
carefully drains the water / alcohol solution from the container and don't
rinse the container with very hot water killing the yeast in the process.

In the long run it will benefit the yeast if we used a yeast nutrient (can
be bought from home brewers shops) to feed the yeast the nutrients that the
yeast needs to stay healthy for prolonged use.


Because some of the CO2 stays in the water we have a buffer solution like
this:

CO2 + H2O = H+(aq) + HCO3-(aq)

This process is dependent on temperature and the specific wapor pressure of
CO2 in the container and the water, but i can't remember how to calculate
the exact ammount of CO2 in solition given the pressure / temperature of the
solution, so I'll ignore this bit :-)

When adding baking soda you effectively increases the kH of the solution
which loveres the pH making the environment a little more hostile to the
yeast. If i remember correctly yeast prefers a pH of 5 to 6, but will
survive 4 to 9, so there are some room for pH swings before the yeast gives
up.

This reaction goes like this:

2NaHCO3 + H2O = 2Na+(aq) + 2HCO3-(aq)


Baking soda can be added to stabilise the pH in the reactor if the water
used have a very lov Kh, but for normal (higher that 4 Kh) water baking soda
actually makes the conditions for the yeast worse but might prolong the
reaction somewhat purely because some of the yeast will die because of the
higner pH in the reactor.



I hope this came through as i thought it, if not please dont hesitate to ask
further questions :-)


Regards

Morten
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