"Red cabbage indicator" experiment

How to make a pH indicator at home

Here’s an­oth­er ex­per­i­ment to add to your bag of life­hacks. In this col­or­ful ex­per­i­ment, we learn to make a uni­ver­sal pH in­di­ca­tor.

Reagents and equip­ment:

  • con­cen­trat­ed sul­fu­ric acid;
  • 9% acetic acid;
  • fizzy wa­ter;
  • dis­tilled wa­ter;
  • a sat­u­rat­ed so­lu­tion of sodi­um car­bon­ate;
  • 10% am­mo­ni­um;
  • sat­u­rat­ed so­lu­tion of sodi­um hy­drox­ide;
  • 150 ml glass­es (x70;
  • 50 ml glass­es (x6);
  • 1-liter glass;
  • fun­nel;
  • 1-liter con­ic flask;
  • cot­ton wool;
  • red cab­bage.

Step-by-step in­struc­tions

Cut the red cab­bage, put it in the 1-liter glass, pour hot wa­ter over it and let it cool slight­ly. Fil­ter the mix­ture in a fun­nel through cot­ton wool. Pour the ob­tained pur­ple so­lu­tion into 7 150 ml glass­es, and grad­u­al­ly add so­lu­tions with dif­fer­ent pH to our in­di­ca­tor for cal­i­bra­tion: con­cen­trat­ed sul­fu­ric acid, acetic acid, fizzy wa­ter (car­bon­ic acid), am­mo­ni­um car­bon­ate so­lu­tion, am­mo­ni­um so­lu­tion and sodi­um hy­drox­ide so­lu­tion. Ob­serve the change in col­or from red to yel­low.

Pro­cess­es de­scrip­tion

Aque­ous so­lu­tions can be neu­tral, al­ka­line or acidic. This means that they have po­ten­tial of hy­dro­gen, or pH. The po­ten­tial of hy­dro­gen, or pH, is the mea­sure of ac­tiv­i­ty (or con­cen­tra­tion) of hy­dro­gen ions in a so­lu­tion. If pH is < 7, the so­lu­tion is acidic, but if рН = 7, then you can re­lax, it’s neu­tral. But this gra­da­tion is only ap­pli­ca­ble to aque­ous so­lu­tion; if you take any oth­er sol­vent, ev­ery­thing can change dras­ti­cal­ly. Po­ten­tial of hy­dro­gen can be mea­sured by two meth­ods: with a spe­cial pH mea­sur­ing de­vice, or with in­di­ca­tor pa­per or so­lu­tion. To­day, we’ve made an in­di­ca­tor from red cab­bage, and cal­i­brat­ed it. Col­ored veg­eta­bles and fruits con­tain var­i­ous col­orants with the gen­er­al name of an­tho­cyans. De­pend­ing on the pH, they can have dif­fer­ent col­ors. When we pour so­lu­tions with dif­fer­ent pH val­ues into our pur­ple so­lu­tion, we re­ceive a gra­da­tion of col­or from red to yel­low. Sul­fu­ric acid, as a strong min­er­al acid, gives us the val­ue of pH ≈ 0-1, acetic acid gives рН ≈ 3-4, fizzy wa­ter (car­bon­ic acid so­lu­tion) gives рН ≈ 5-6, a so­lu­tion of sodi­um car­bon­ate gives рН ≈ 8-9, so­lu­tion of am­mo­nia gives рН ≈ 10, and a so­lu­tion of sodi­um hy­drox­ide gives рН ≈ 14. We can also evap­o­rate the so­lu­tion in­com­plete­ly and ap­ply the re­main­ing mix­ture to pa­per, cre­at­ing in­di­ca­tor pa­per which can be stored for a long time.

Safe­ty pre­cau­tions

Wear pro­tec­tive glass­es and gloves. Con­cen­trat­ed so­lu­tions of acids and bases are very caus­tic sub­stances.

Warn­ing! Sub­stances of this ex­per­i­ment are tox­ic and high­ly dan­ger­ous for your health. Do not try this at home. Only un­der pro­fes­sion­al su­per­vi­sion.

Try it