Home-made pH indicator paper

[science_spot]

pH indicators are widely used.  You may have some indicator paper in your house for use with your swimming pool or fish tank, or possibly even kitchen use.  Laboratories often use a pH meter with digital readout for higher accuracy, but many also have indicator paper when a quick test of general pH is required.  Sometimes liquid indicators such as phenol red are added to products that could suffer from pH shift because it allows the user to have an estimate of the ph just by looking at the color of the solution.

You don't have to purchase pH strips - you can make your own, very easily and safely, in your kitchen (lab).  I am not sure how long the strips will last, but if you decide to store them for later use, I suggest making sure you dry them completely, then place in a plastic ziploc bag and store in a cool, dry place IN THE DARK.Darkness can be achieved by placing in a drawer that is rarely opened, in a box, or by wrapping the bag in tin foil.  Make sure you label the contents and make sure no one tries to eat them.  You can use your home-made strips to teach your kids about pH, as well as to test solutions you use in your home. 

 

The science of cabbage color:

Red cabbage contains high quantities of a reddish-purple dye called flavin, which is a type of anthocyanin (see my post about leaf colors, since this is also found in autumn leaves).  This same pigment is also found in plums, apple skins and grapes.  Changes in pH will change the color of anthocyanins, which is a useful tool for determining the approximate pH of a solution.  For information on the chemistry of pH, check out Wikipedia.

 

What you will need:

1 head red cabbage

chopping knife

Pot to boil water in - this MIGHT get stained, so do not use your favorite one!

drinking glass

gloves

glass pan

1 coffee filter or thick absorbant paper towel

 

What to do:

1.  Chop up 2-3 cups of red cabbage into small pieces and place them in the pot.

2.  Put enough water in the pot to cover the cabbage, bring to a boil, and boil for about 15 minutes.

3.  Remove the cabbage pieces and throw in the trash.  The water should be a good purple color.

4.  Now you want to boil off alot of the water, which will make the purple color darker as the solution becomes more concentrated.  The more concentrated you canget the solution, the better your strips will work.  Try to boil it down to about 1/2 - 1 cup of water.

5.  Also the solution to cool to near room temperature so no one gets burned.

6.  Cut up strips of paper towel and sprinkle them around in the glass pan.  They don't have to be in a single layer, but you don't want them all stuck together.

7.  Pour the purple solution over the paper strips and stir them around a bit so they all get colored well.

8.  Remove the excess solution (or make even more strips).  This can go down the drain with alot of water (so you don't stain the sink).

9.  Allow the strips to air dry.  This might take overnight.  Now they are ready to use for pH testing!

 

How to use your test strips:

Gather some various liquids from around your home.  You can use both edible solutions as well as cleaning solutions, but be very careful with cleaning products around the kids.  NEVER mix together any cleaning solutions as this can cause a serious reaction between strong acids and strong bases!  Here are some recommendations:  milk, lemon juice, orange juice, water, vinegar, ammonia, human saliva,  seltzer water and apple juice.  You can also use a little water to dissolve the following to make solutions:  baking soda, sugar, salt, and antacids.

Lay out a dry test strip on a plate.  Using a dropper, syringe, orjust your finger, place 1-2 drops of your liquid on the test strip and watch the color change.  Repeat for each solution you are using, making sure the plate and dropper is clean and dry between each use.  Your results should be in line with this:

Acids: lemon juice, vinegar, orange juice, apple juice, seltzer water

Neutrals: milk, water, human saliva

Bases: baking soda solution,  ammonia, antacid solution

 

Find some pH values here. What color/pH would you expect to get for sour milk?  grapefruit juice?  milk of magnesia?  dog saliva?

 

The picture below is from ehow.com, and it shows the expected color progression from acidic (red, on the left), to basic (yellow, on the right). 

 

 

How it teaches your kids:

Younger kids will enjoy seeing the color changes, and they can understand the basics of acids and bases by taste.  They can also help prepare the pH strips by cutting the paper towels, pouring the concentrated juice and testing some solutions.  Allow them to taste an acidic solution such as fresh lemon juice or vinegar.  Show them how the dye in the cabbage juice changes color when an acidic solution touches it.  You can do the same with a neutral solution such as water and basic solution such as that made from the antacids.  Only let them taste something that would be safe to ingest!!  If you are using this taste-test I STRONGLY recommend you do not use ANY other solutions so there is not a mix-up!!!!  SAFETY FIRST!!  Once they see the color change, they can make a chart comparing the color on the strip (the giant box of crayons works great) to the taste of the solution.

Older kids can do everything the younger kids can do, and may also be able to place the solutions in order by increasing pH, or make a comparison between the home-made pH indicator strips and those from a test kit. 

The oldest kids will be able to do everything the younger kids have done, as well as understand the chemistry of an pH changes.  They may also be able to determine if one of the indicators on the store-bought strips is also a flavin like the one in the cabbage juice!

 

I hope you enjoy this activity.  An expanded version will be available in my book, planned for completion in 2010!

 

Tonya

AKA sunmoonstars

The Science Spot

 

[Nixshix]

Oct. 2, 2009 at 2:26 PM

COOL! I am definitely doing this with my kids!

[Momforhealth]

Oct. 2, 2009 at 9:05 PM

wow totally cool.  I will have to try this.  My kids will have a blast dressing up like scientists, and testing everything in my house. Pretty much everything we have wont explode.   Would beets have the same reaction as cabbage?

[science_spot]

Oct. 2, 2009 at 9:17 PM

good question momforhealth!  I had to look it up, but it seems that beets contain the pigment betacyanin, which is very stable at a wide range of pH's.  This means it will not work as a pH indicator.  Awesome idea though!

So, you could make a preparation of both vergatbles (separately), and challange your kids to figure which one is which.  They should test a variety of solutions for color changes and be able to pick the one that is changing color as the cabbage juice!

 

(Original Poster)

[You_Are_Invited]

Oct. 3, 2009 at 3:37 AM

Very Cool!

[ERNAProud]

Oct. 3, 2009 at 4:58 AM

This is awesome!

 

[tyheamma]

Oct. 3, 2009 at 11:46 AM

This is pretty much AWESOME. I'm definitely going to have to save this information and do this with DD when she's just a little bit older.

[heydooney]

Oct. 3, 2009 at 12:04 PM

This is really cool. When my kids are a little older, I will definitely do this with them!

But there's gotta be something you can do with the boiled cabbage so it's not wasted...

[science_spot]

Oct. 3, 2009 at 5:15 PM

heydooney - I was thinking that, but I don't like cabbage..... any suggestions?

(Original Poster)

[RoseWall]

Oct. 3, 2009 at 7:12 PM

exciting i would like to try this!

[aenima49]

Oct. 5, 2009 at 9:26 AM

Very cool!!