This looks like an alkaline reaction. What is the indicator and what is the pH of the boric acid solution?

Indicators are methyl red and bromocresol green and i dont know about the pH but i added 10g of boric acid in 1 liter of distilled water to make a boric acid solution.

Hi,

The colour you have correspond to bromcesol in basic conditions. Under basic form it's yellow, under acid condition it's blue, and between approximatively pH 4 to 5 it's green. Visibly you the colour problem is linked to the pH value.

If the pH is not at the neutral point (to which you will titrate the captured NH3) add sufficient dilute strong acid to take it to that pH, i.e., the use the same acid you intend to use for the NH3 titration. If you are using the indicator to detect the endpoint make sure that it is a shade that you can reproduce. Test this by trial and error.

You may wish to mix another indicator with the bromocresol to make the colour change more distinct blue/green is difficult for many men to distinguish. Look at 'screened' indicators

Make sure that your boric acid solution is buffered at pH 5.0. The pH of the boric acid can drift a bit because it is a weak acid. When making up a batch of boric acid, adjust the pH to pH 5.0 and let it sit for a day, then check the pH again. if it drifts from pH 5 then rebuffer and wait another day and check again. You may want also want to do this periodicially if you have a large batch of boric acid that may sit a few weeks between usage. This will help maintain a consistent pH between the times you use it.

However, the most important step is to run blanks at least daily, titrate them to find the background level of acid used and then try to titrate your samples to the same color end point. Different people see colors differently, but if an operator is titrating the samples to the same color as the blanks, the error in titration will be very small.

Another factor affecting the color of your solution is the quality of lighting in your laboratory.