At work, I have to dilute 300 mL of 6 N HCl to a pH of 5.5. How much water should I add?
Thanks.
(It has been a long time since I had chemistry.)
I work in a laboratory that tests limestone for use in building roads. In some circumstances, the limestone must contain 10% quartz. We test for this by dissolving the limestone in acid, then weighing the residue and estimating the amount of quartz in it. This is not a general chemistry laboratory. We only do specific tests and only have the equipment for those tests. I do not have a pH meter.
According to ASTM Procedure D 3042, which must be followed, some naturally occurring carbonate minerals will not dissolve in acid at room temperature. To dissolve these, the Procdure requires that, after dissolving as much as possible at room temperature, 300 mL of 6 N HCl must be added to the residue and that this must be heated to 95-deg C for 90 minutes.
Since the amount of these low-solubility carbonates is small, I estimate that at the end of the 90 minutes the concentration of the acid is still 6 N. I know that some amount of HCl gas is lost during heating. I don't have a way of measuring this, so I assume the concentration after heating is 6 N. This assumption will result in overestimating the amount of water to add and therefore result in a pH higher than 5.5. This is an acceptable error.
After heating for 90 minutes, the acid must be decanted through a metal sieve. The purpose of the sieve is to retain any +200 mesh (75 micron) material that goes out with the supernatent liquid. Since this is a quantitative test, the sieve must be accurate, but concentrated acid will dissolve the metal from which the screen is made. The Procedure directs to dilute the acid to pH 5.5 for the sake of reducing the corrosion of the sieve. It does not tell how much water to add to achieve this.
The necessary water must only be determined once because the test must be performed identically according to the Procedure each time. Once I know how much water to use, I will use that amount every time.
The man who worked this lab for 23 years died suddenly and I am filling in for him while we hire someone new. No one else knows how he did it and we have found no written records. I would appreciate it if someone who is familiar with these calculations would tell me how much water to use. As I recall from my long-ago chemistry class, the calculation is a simple one, but I just do not know how to do it.
Thanks for your attention.