How to use Bromothymol Blue

[student22]

Hello,

I am conducting an experiment found from this site, albeit a few small changes, that can be found below.

https://www.sciencebuddies.org/science- ... p003.shtml

Basically I am following the same instructions, but having volunteers from different age groups partake in the experiment. I want to use bromothymol blue, a pH indicator, to measure the amount of carbon dioxide a person exhales, but don't know how to properly use it. The instructions say to have a volunteer blow into the indicator using a straw, but how much of it do I use for a single person? Does it change into a lighter or darker shade of yellow depending on the the amount of carbon dioxide? Do I mix it with water? I plan to have 24-30 people partake in the expreriment so how much bromothymol blue should I buy? I would greatly appreciate any help or additional tips to make my experiment a success.

Thanks

[yvetteds]

As a biology teacher, I often used Bromothymol blue to show the effect of exercise on the amount of time it took the BB to turn from blue to a yellow/green.
There are many variables that have to be controlled for that I often found challenging.
1. The volume and concentration of the BB in each flask when starting. You need to find out what dilution to use and it needs to be exactly the same for each of your flasks.
2. I'm not sure if you can really determine the 'amount' of carbon dioxide that will turn the BB from blue to green/yellow - it is an indicator that will turn from blue to yellow when the pH changes. (Carbon dioxide changes the pH of the BB solution) - If you wanted to measure carbon dioxide specifically, then you would need to use a carbon dioxide probe.
3. Make sure you use distilled water. I learned this the hard way by using tap water that is slightly acidic and turns the BB greenish without anyone blowing into it at all.

So, what I'm suggesting is maybe you can see the differences in the amounts of Carbon dioxide for your experiment by determining the amount of time it takes to turn the BB from blue to yellow. The more carbon dioxide coming out in each breath the faster it should turn yellow. The less carbon dioxide coming out the longer it will take.

Also, you are using volunteers from different age groups. Do you think you need to control for sex? and weight? If you remember, carbon dioxide is a waste product of cell metabolism. Is age a variable that controls for other factors such as size? activity of the subject?

You might want to review these questions with your science teacher.

I hope some of this has been helpful.

[student22]

Thank you so much for your quick reply. Your post was indeed very helpful and cleared up a lot of my confusion. I plan to use your advice by determining the amount of time it takes to turn from blue to yellow. Also thanks for the additionally tips, and I will review them with my teacher.

[yvetteds]

You are welcome! It's always been a challenge for us in class when we did this to figure out exactly when to say 'stop' on the clock when watching when the BB changed from blue to yellow.. since time is the dependent variable (the one we are measuring and will change with the experimental group).
by the way
1. what is your control group? (the group you are going to compare to see if there are any changes?)
2. ask your teacher if 'age' is a good 'experimental group' - example - you can be 13 and be 6 ft tall and weigh 200 pounds or 13 and be 5 ft tall and weigh 100 pounds. If there is such a variation, do you think that age is a good 'isolated' variable to test in this experiment?
Your teacher should be able to direct you here - the most important thing is to first isolate that ONE variable you are testing -- if you don't - your results will be confusing and hard to analyze.

[student22]

You have brought up several points that hadn't come to my attention yet. Like you said, how exactly do I know when to stop the clock when watching the color change? Could you please explain to me what you have done in your previous classes?

Looks like I have a good amount of work ahead of me to make sure that I am only testing for one variable. I may have to rethink several things to make my results more accurate. Suppose I gather volunteers of relatively the same body frame and condition (height,weight,health etc.). Would that specify it enough to perform a justified experiment, or am I still exposed to too many alternate variables.

Like you said, I will consult my teacher tomorrow when I see her in school. At this point of the project (having already submitted my objective and a brief research paper), I may not be able to completely rework my experiment, however I'm pretty sure she can direct me to a solution.

Thanks again for your advice.

[yvetteds]

In my class, we were using the BB experiment as a way to compare the change in metabolism ( by the amount of carbon dioxide the body produced as a waste product) comparing a group at rest (the control) to the experimental group after exercise. It was not an in depth data analysis - but we did find that after exercise the average amount of time it took to change the BB from blue to yellow was shorter (more carbon dioxide being exhaled from each breath when subjects stopped exercising.) But then again, this was a class demonstration - and there were probably more variables we needed to control if this was to be a reliable science fair project.
I am hoping your science teacher can give you more specific directions since I don't know the level of work she expects - or the requirements you're expected to achieve.

The hardest part of any science project is as you are finding out - how to control for just one variable - so that when you do get data to analyze, that the change you see is due to just the one thing you changed and not to some interfering variable.

Even the food the person ate before your experiment will change the results.

So, when using human subjects it's really a challenge to control for all the variables necessary. That is not to say this is impossible -
you might look up information under Sports Medicine - since that is where a lot of research is being done on how the body reacts and uses fuel - I am assuming you have had some background on basic cell respiration? If so, then you will probably find exercise research interesting as far as lung capacity and what you are trying to show.

Some questions to think about -
Does a person's 'fitness' affect how fast they exhale carbon dioxide after exercise?

That was one question that came up in our classes when we were trying to analyze the results.

But all the questions you think of along the way - once you start your basic project can be incorporated into your discussion and analysis and 'further investigation'. That's the purpose of research - to uncover more questions - one step at a time.

[inventor_1022]

I have been following instructions for the bromothymol blue a I even tried distilled water and the solutions always turns yellow. I have tried using salt in the distilled water but it still ends up yellow. What am I doing wrong?
The experiment calls for the solution to turn blue when added to tap water or distilled water or salt water. I have tried all three and the result is always yellow. We had some success in getting the solution to turn light green when we added salt to distilled water. Otherwise, the solution always turns yellow when added to water.
Please advise.
Thank you

[yvetteds]

I am not sure why you are having this problem. Have you followed directions in making a solution of BB and DISTILLED water in a CLEAN glass flask that can be sealed until you are ready to use it?
BB turns from blue to green-yellow when an acid is added to it as when you breath into it and your carbon dioxide in your breath reacts with the BB. Thinking about this, try to determine what acid might be contaminating your BB solution.