extendedforecast Posted January 31, 2012 Share Posted January 31, 2012 We are working on the Story of Science Quest Guide lesson 5, which requires bromothymol blue. I do not have any, and I would like to find something to substitute for it, so that I do not have to order online or drive 30+ miles to buy some. Please help! Quote Link to comment Share on other sites More sharing options...
TheHomeScientist Posted January 31, 2012 Share Posted January 31, 2012 It depends on what you're using it for. Bromothymol blue is a pH indicator with a relatively sharp transition. At pH 6.0 or lower (very, very slightly acidic) it's yellow. At pH 7.6 or above (even more slightly basic), it's blue. In the range of 6.0 to 7.6 it goes through greenish shades from yellowish green at the lower pH to bluish green at the upper pH. Red cabbage juice functions more like universal indicator, with a wide range of colors over a broad range of pH ranges, from red in very acid solutions to purplish as pH approaches neutral to blues, greens, and at high pH yellow. The most common use of bromothymol blue in home science is as an indicator for photosynthesis experiments. You blow bubbles through a straw into a solution tinted with BTB. As the carbon dioxide in your breath dissolves in the water, it forms carbonic acid and turns the BTB yellow. As a plant immersed in the water does photosynthesis, it consumes the carbon dioxide and produces oxygen. The pH of the solution increases, and the BTB turns green or even blue. If that's what you're doing, the red cabbage juice should work. Just blow bubbles in the water until the color doesn't change any more. Keep some of that solution in a small bottle, filled to the brim and capped. That'll serve as a color reference. Then after the plant has done its thing you can compare the color of the solution in that container with your sample. Quote Link to comment Share on other sites More sharing options...
extendedforecast Posted January 31, 2012 Author Share Posted January 31, 2012 The experiment involves a candle, a dish of water with bromothymol blue, and a jar. I believe we are supposed to observe the color change of the water as the candle burns out, then again watch the changes as air is blown through a straw. Quote Link to comment Share on other sites More sharing options...
KristenR Posted January 31, 2012 Share Posted January 31, 2012 It depends on what you're using it for. Bromothymol blue is a pH indicator with a relatively sharp transition. At pH 6.0 or lower (very, very slightly acidic) it's yellow. At pH 7.6 or above (even more slightly basic), it's blue. In the range of 6.0 to 7.6 it goes through greenish shades from yellowish green at the lower pH to bluish green at the upper pH. Red cabbage juice functions more like universal indicator, with a wide range of colors over a broad range of pH ranges, from red in very acid solutions to purplish as pH approaches neutral to blues, greens, and at high pH yellow. The most common use of bromothymol blue in home science is as an indicator for photosynthesis experiments. You blow bubbles through a straw into a solution tinted with BTB. As the carbon dioxide in your breath dissolves in the water, it forms carbonic acid and turns the BTB yellow. As a plant immersed in the water does photosynthesis, it consumes the carbon dioxide and produces oxygen. The pH of the solution increases, and the BTB turns green or even blue. If that's what you're doing, the red cabbage juice should work. Just blow bubbles in the water until the color doesn't change any more. Keep some of that solution in a small bottle, filled to the brim and capped. That'll serve as a color reference. Then after the plant has done its thing you can compare the color of the solution in that container with your sample. :iagree: HAHAHA I actually have no idea. I just wanted to sound smart. Quote Link to comment Share on other sites More sharing options...
TheHomeScientist Posted January 31, 2012 Share Posted January 31, 2012 The experiment involves a candle, a dish of water with bromothymol blue, and a jar. I believe we are supposed to observe the color change of the water as the candle burns out, then again watch the changes as air is blown through a straw. Same idea. It should work, but be aware that the color transition is going to be much, much more subtle with the RCI than with the BTB. With the latter, you go from a distinct yellow to a green-blue or blue. With the former, you go from pinkish-purple to a bluish-purple. Here's an image of RCI at different pH values: http://www.erowid.org/archive/rhodium/chemistry/equipment/pictures/ph-cabbage.jpg You'll be in the 5 to 8 range. Quote Link to comment Share on other sites More sharing options...
extendedforecast Posted January 31, 2012 Author Share Posted January 31, 2012 :iagree: HAHAHA I actually have no idea. I just wanted to sound smart. :lol: I hear you on that, lol! Quote Link to comment Share on other sites More sharing options...
extendedforecast Posted January 31, 2012 Author Share Posted January 31, 2012 (edited) Same idea. It should work, but be aware that the color transition is going to be much, much more subtle with the RCI than with the BTB. With the latter, you go from a distinct yellow to a green-blue or blue. With the former, you go from pinkish-purple to a bluish-purple. Here's an image of RCI at different pH values: http://www.erowid.org/archive/rhodium/chemistry/equipment/pictures/ph-cabbage.jpg You'll be in the 5 to 8 range. Thank you so much! Edited January 31, 2012 by extendedforecast Quote Link to comment Share on other sites More sharing options...
extendedforecast Posted January 31, 2012 Author Share Posted January 31, 2012 Same idea. It should work, but be aware that the color transition is going to be much, much more subtle with the RCI than with the BTB. With the latter, you go from a distinct yellow to a green-blue or blue. With the former, you go from pinkish-purple to a bluish-purple. Here's an image of RCI at different pH values: http://www.erowid.org/archive/rhodium/chemistry/equipment/pictures/ph-cabbage.jpg You'll be in the 5 to 8 range. I forgot to ask one question: should I add more cabbage juice to compensate for using a weaker indicator? Quote Link to comment Share on other sites More sharing options...
TheHomeScientist Posted January 31, 2012 Share Posted January 31, 2012 I forgot to ask one question: should I add more cabbage juice to compensate for using a weaker indicator? Just use sufficient to give the water a clearly visible tint. The amount isn't critical. If you mean "more" relative to the amount of BTB specified, the answer is yes. Even a drop or two of standard strength BTB (0.04%) is sufficient to tint quite a bit of water. You'll need more RCI, probably much more. Incidentally, the RCI doesn't keep. It'll probably last several days if you refrigerate it, but eventually you'll end up with a moldy mess. Quote Link to comment Share on other sites More sharing options...
extendedforecast Posted February 1, 2012 Author Share Posted February 1, 2012 Just use sufficient to give the water a clearly visible tint. The amount isn't critical. If you mean "more" relative to the amount of BTB specified, the answer is yes. Even a drop or two of standard strength BTB (0.04%) is sufficient to tint quite a bit of water. You'll need more RCI, probably much more.Incidentally, the RCI doesn't keep. It'll probably last several days if you refrigerate it, but eventually you'll end up with a moldy mess. That's what I figured. Thanks again! Quote Link to comment Share on other sites More sharing options...
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