In this method, blood sample is mixed with a weak ammonia solution. Absorbance of this solution is measured in a spectrophotometer at 540 nm or in a photometer using a yellow-green filter. Absorbance of the test
sample is compared with that of the standard solution.
This method is rapid and simple. However, no stable standard solution is available, color of oxyhemoglobin solution rapidly fades, and hemoglobin derivatives other than oxyhemoglobin are not measured.