Making

purp · May 29, 2020

Hello,

I'm trying to make my own dilute acid to do alkalinity titrations. Per the method I'm using, I'm trying to create a solution that has a pH of 3.6 to 3.9 when the acid is diluted at 100:1.

For my initial calculations, i used 12M (37%) lab grade HCl. at a ratio of 2.464 g HCl per L of RODI. I'm relatively confident that the scale I used is accurate, as well as the liquid volumes.

I made a batch of solution 18L in size, so I added 44.352 g of HCl to the batch. However, after time was allowed for mixing, the pH of the solution was 4.0. I'm very confident in the precision of the pH probe. I'd like to continue to adjust the pH down with HCl such that I sit in the middle of that pH range. So I would like to adjust the pH down by 0.25 using HCl.

Can a chemistry guru here help me determine how much to add so I dont ruin this batch of reagent?

Randy Holmes-Farley · May 30, 2020

purp said:
Hello,

I'm trying to make my own dilute acid to do alkalinity titrations. Per the method I'm using, I'm trying to create a solution that has a pH of 3.6 to 3.9 when the acid is diluted at 100:1.

For my initial calculations, i used 12M (37%) lab grade HCl. at a ratio of 2.464 g HCl per L of RODI. I'm relatively confident that the scale I used is accurate, as well as the liquid volumes.

I made a batch of solution 18L in size, so I added 44.352 g of HCl to the batch. However, after time was allowed for mixing, the pH of the solution was 4.0. I'm very confident in the precision of the pH probe. I'd like to continue to adjust the pH down with HCl such that I sit in the middle of that pH range. So I would like to adjust the pH down by 0.25 using HCl.

Can a chemistry guru here help me determine how much to add so I dont ruin this batch of reagent?

I do not understand what sort of titration needs pH at a certain level.

An alk titration is VERY well served by pH 1 (0.1 N HCL). That is how I recommend my DIY, and it is easy to buy.

A DIY Alkalinity Test: By Randy Holmes-Farley - REEFEDITION

The difference in acid concentration between pH 3.6 and pH 3.9 is a factor of two. Hopefully you are not using ph to standardize the acid concentration.

purp · May 31, 2020

Its just a check suggested by the manufacturer of the reagent to ensure that the provided reagent (which is mixed with user provided RODI) is in spec. I believe the issue of not directly testing the mixed reagent stems from the device pH probe not being accurate and well calibrated near the low end of the pH scale, thus the dilution.

Randy Holmes-Farley · May 31, 2020

purp said:
Its just a check suggested by the manufacturer of the reagent to ensure that the provided reagent (which is mixed with user provided RODI) is in spec. I believe the issue of not directly testing the mixed reagent stems from the device pH probe not being accurate and well calibrated near the low end of the pH scale, thus the dilution.

Do you have a link to this product?