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What is new pH value of solution

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multivits:
Hi Forum,
I have done lots of research on this and used Equation 1 M1V1 = M2V2

I have 5ml of a pH 11.5 cleaning fluid and I am adding 1 litre of water what is my new pH?

I do the negative log of pH11.5 to get 3.16 x10^-12  when I plug this into Equation 1

3.16 x10^-12 * (5ml)/ 1000ml = M2, = but I get 3.16x10^-15 which when converted to pH (-log[h+]) = -14

Therefore pH is 14 but that not right since the pH 11.5 should surely move towards a pH of 7 since I added water.

TIA,

Multivits

AWK:
To calculate the pH of your cleaning fluid, it is necessary to know the content of the substances in this solution and their concentrations. Dilution of the solution should be done before calculating the pH.
Dilution of the solution always gives a pH closer to 7.
For very dilute solution the calculation of pH is more complicated.

Borek:
1. Cleaning fluids contain substances that will work as a buffer, making calculations more difficult. Knowing just the initial pH is not enough, you don't have enough information to calculate the answer.

2. Assuming initial solution contains just a base which gives pH of 11.5 you should take into account autodissociation of water, this will shift your final pH in the right direction.

AWK:

--- Quote from: Borek on May 14, 2019, 02:49:03 AM ---2. Assuming initial solution contains just a base which gives pH of 11.5 you should take into account autodissociation of water, this will shift your final pH in the right direction.

--- End quote ---

For 200 times diluted pure base, the autodissociation of water still may be neglected.

multivits:
Thank you so much for these replies, I checked the cleaning chemical's MSDS and put below:

CHEMICAL ENTITY                              CAS NO                 PROPORTION
Sodium metasilicate pentahydrate       10213-79-3           10 - 30 %
Ingredients determined to be non-hazardous                    Balance
                                                                                    100%

With a 200:1 ratio of mains water:above cleaning chemical (Water  1000ml : Cleaning chemical 5ml) @ 65°C I am hoping to get at or below pH 9,

This is to comply with the requirements for the machine parts I am cleaning (PTFE coated anodized aluminium), I am not a chemist (electronics engineer) and am not fully across some of the terms that have been used above e.g. dilution (I thought I was diluting the cleaning chemical with water) and autodisassociation (I think this is the break up of the chemical compounds ).

I think I still need your help and am interested in learning more about this.

TIA multivits

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