Nitrazine or phenaphthazine is a pH indicator

PH indicator

A pH indicator is a halochromic chemical compound that is added in small amounts to a solution so that the pH of the solution can be determined visually. Hence a pH indicator is a chemical detector for hydronium ions or hydrogen ions in the Arrhenius model. Normally, the indicator causes the...

 dye.

Nitrazine test

As the disodium salt, it is used in pH testing strips for medical applications. It is also used in civil engineering

Civil engineering

Civil engineering is a professional engineering discipline that deals with the design, construction, and maintenance of the physical and naturally built environment, including works like roads, bridges, canals, dams, and buildings...

 to determine the carbonation

Carbonation

Carbonation is the process of dissolving carbon dioxide in water. The process usually involves carbon dioxide under high pressure. When the pressure is reduced, the carbon dioxide is released from the solution as small bubbles, which cause the solution to "fizz." This effect is seen in carbonated...

 spread in concrete

Concrete

Concrete is a composite construction material, composed of cement and other cementitious materials such as fly ash and slag cement, aggregate , water and chemical admixtures.The word concrete comes from the Latin word...

 structures and therefore assess the state of the rebar

Rebar

A rebar , also known as reinforcing steel, reinforcement steel, rerod, or a deformed bar, is a common steel bar, and is commonly used as a tensioning device in reinforced concrete and reinforced masonry structures holding the concrete in compression...

's passivation

Passivation

Passivation is the process of making a material "passive", and thus less reactive with surrounding air, water, or other gases or liquids. The goal is to inhibit corrosion, whether for structural or cosmetic reasons. Passivation of metals is usually achieved by the deposition of a layer of oxide...

 film. More sensitive than litmus paper, nitrazine indicates pH in the range of 4.5 to 7.5.

• The test is used to test vaginal pH during late pregnancy to determine the breakage of the amniotic sac
  Amniotic sac
  The amniotic sac is the sac in which the fetus develops in amniotes. It is a tough but thin transparent pair of membranes, which hold a developing embryo until shortly before birth. The inner membrane, the amnion, contains the amniotic fluid and the fetus. The outer membrane, the Chorion,...
  
  . While vaginal pH is normally acidic, a pH above 7.0 can indicate that the amniotic sac has ruptured. (However, elevated pH can also be associated with bacterial vaginosis
  Bacterial vaginosis
  Bacterial vaginosis or less commonly vaginal bacteriosis is a disease of the vagina caused by bacteria. The U.S. Centers for Disease Control and Prevention states it is not clear what role sexual activity plays in the development. However, it is known that BV is associated with having a new sex...
  
  .)

• Nitrazine papers can also be used to perform a fecal pH test
  Fecal pH test
  A fecal pH test is one where a specimen of feces is tested for acidity in order to diagnose a medical condition. Human feces is normally alkaline. An acidic stool can indicate a digestive problem such as lactose intolerance or a contagion such as E. coli or Rotavirus.-Test procedure:The test is...
  
  to diagnose intestinal infections or other digestive problems.

• They can also be used to test the pH of the human eye.