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DIY CO2 system question (Correct calculations)
On Thu, 4 Mar 2004 18:35:05 -0000, "Morten"
wrote: snip 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 Wow... thanks Morten!... This is definately a keeper and I think your explanation is easier to read than the one that was provided in that other link...which I remember reading during my investigations of going DIY CO2... I guess I just didn't pay close enough attention to the chemistry details. My suggestion regarding adding the baking soda came from reading that suggestion at 'the Krib' in the DIY CO2 notes etc... guess it was just an old fishkeepers tale. I have always added it, just a little to each mixture... now, I can save that extra step. I was, apparently falsely, under the impression that the baking soda would adsorb the alcohol. Oh well... I never took chemistry... never made it past 10th grade, got kicked out of school for knocking down the principal when he started calling a female student names that I doubt you would hear in a prison! Anyway, education since has been non graduate level, independant studies in college. IE, no class requirements, just going for the knowledge, not the credits to get a degree and since I have no practical use for chemistry, I never bothered to take those sorts of courses... I may have to add some basic chemistry in over the next couple semesters.... |
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