How to make indicators at home

Colors and chemistry of indicators

Safe­ty pre­cau­tions

Wear pro­tec­tive gloves, eye­wear, and a mask. Work in a well-ven­ti­lat­ed area.

Reagents and equip­ment

  • 0.1% al­co­hol so­lu­tion of bro­moth­y­mol blue;
  • 10% sodi­um hy­drox­ide so­lu­tion;
  • 10% hy­drochlo­ric acid so­lu­tion;
  • 1% methy­lene blue so­lu­tion;
  • 10 g glu­cose;
  • 10% cobalt(II) chlo­ride so­lu­tion;
  • fil­ter pa­per;
  • dis­tilled wa­ter;
  • 1% starch so­lu­tion;
  • 3% aque­ous so­lu­tion of io­dine in potas­si­um io­dide;
  • red cab­bage;
  • 9% vine­gar so­lu­tion;
  • or­ange juice;
  • 10% bak­ing soda so­lu­tion;
  • drain clean­er (usu­al­ly 10% sodi­um hy­drox­ide so­lu­tion);
  • black tea;
  • lemon;
  • beakers;
  • fresh­ly-boiled wa­ter;
  • con­i­cal flask with rub­ber stop­per;
  • cup.

Step-by-step in­struc­tions

  1. Add a few drops of the 0.1% al­co­hol so­lu­tion of bro­moth­y­mol blue to 30 mL of dis­tilled wa­ter. Watch as the so­lu­tion turns green. Add 1 mL of 10% hy­drochlo­ric acid so­lu­tion – the liq­uid should turn yel­low. Next, add 5 mL of 10% sodi­um hy­drox­ide so­lu­tion, and watch the so­lu­tion turn blue.
  2. Pour 10 g of glu­cose into your con­i­cal flask. Add 50 mL of 10% sodi­um hy­drox­ide so­lu­tion, and a few drops of 1% methy­lene blue so­lu­tion. The so­lu­tion should turn col­or­less. When shak­en, the so­lu­tion turns blue.
  3. Ap­ply some 10% cobalt(II) chlo­ride so­lu­tion to a fil­ter pa­per. Use a blow dry­er at max­i­mum strength to dry the fil­ter pa­per un­til it turns blue. Add a few drops of wa­ter and watch as it turns pink.
  4. Add a few drops of a 3% aque­ous so­lu­tion of io­dine in potas­si­um io­dide to 70 mL of a 1% starch so­lu­tion. Watch as the so­lu­tion turns dark blue.
  5. Dice 300 g of red cab­bage. Trans­fer the cab­bage to your heat-re­sis­tant glass and add 250 mL of hot wa­ter. Let stand for 30 min­utes. Drain the so­lu­tion into the sec­ond glass through the fun­nel with cot­ton wool to fil­ter out the cab­bage slices. Use the pipette to ap­ply the so­lu­tion to a cof­fee fil­ter, then let the fil­ter dry at room tem­per­a­ture for 30 min­utes. Cut into strips. Cal­i­brate us­ing reagents you like­ly have on hand: vine­gar cre­ates an acidic medi­um, which turns the strip red; wa­ter cre­ates a neu­tral medi­um, so the strip stays pur­ple; a 10% bak­ing soda so­lu­tion cre­ates a ba­sic en­vi­ron­ment, which caus­es the stip to turn blue; drain clean­er (usu­al­ly a 10% so­lu­tion of sodi­um hy­drox­ide) cre­ates a strong­ly ba­sic medi­um, which turns the strip first green, then yel­low.
  6. Brew a cup of strong black tea. Add a lemon slice and watch as the tea light­ens in col­or.

Process de­scrip­tion

  1. Bro­moth­y­mol blue is an acid-base in­di­ca­tor. Its struc­ture changes de­pend­ing on the acid­i­ty of its en­vi­ron­ment, which caus­es its col­or to change too. It is green in a neu­tral en­vi­ron­ment, yel­low in an acidic en­vi­ron­ment, and blue in a ba­sic en­vi­ron­ment.
  2. Methy­lene blue is an ox­i­da­tion-re­duc­tion in­di­ca­tor: it is eas­i­ly re­duced to a col­or­less state. In a ba­sic en­vi­ron­ment, glu­cose is a good re­duc­ing agent, and it vis­i­bly re­duces methy­lene blue to its col­or­less form. If the so­lu­tion is shak­en, the oxy­gen in the air ox­i­dizes the col­or­less form back to blue.
  3. An­hy­drous cobalt(II) chlo­ride is a sky-blue salt, but turns pink when ex­posed to mois­ture, form­ing cobalt(II) chlo­ride hex­ahy­drate. This char­ac­ter­is­tic is of­ten used in hu­mid­i­ty in­di­ca­tors, such as those in smart­phones.
  4. Starch can act as an in­di­ca­tor for molec­u­lar io­dine. The io­dine em­beds in the molec­u­lar struc­ture of the starch to form a dark-blue starch-io­dine com­plex.
  5. Red cab­bage con­tains pig­ments known as an­tho­cyanins, which change col­ors in ac­cor­dance with the acid­i­ty of their en­vi­ron­ment – a prop­er­ty that can help you de­ter­mine the pH of var­i­ous sub­stances around you! They turn red in acidic medi­ums such as vine­gar, pur­ple in weak­ly acidic and neu­tral medi­ums such as wa­ter, blue in weak­ly ba­sic medi­ums such as a so­lu­tion of bak­ing soda, and green, then yel­low in strong­ly ba­sic so­lu­tions such as drain clean­er.
  6. Tea leaves con­tain a pig­ment known as thearu­bi­gin, which in­flu­ences the col­or of the drink. Adding a lemon slice to tea cre­ates an acidic en­vi­ron­ment, which changes the struc­ture of the thearu­bi­gin and caus­es the tea to light­en.