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Topic: What is new pH value of solution  (Read 812 times)

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Offline multivits

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What is new pH value of solution
« on: May 13, 2019, 09:50:42 PM »
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

Offline AWK

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Re: What is new pH value of solution
« Reply #1 on: May 14, 2019, 02:36:30 AM »
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.
AWK

Offline Borek

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Re: What is new pH value of solution
« Reply #2 on: May 14, 2019, 02:49:03 AM »
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.
Chembuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

Offline AWK

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Re: What is new pH value of solution
« Reply #3 on: May 14, 2019, 11:54:53 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.

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

Offline multivits

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Re: What is new pH value of solution
« Reply #4 on: May 14, 2019, 09:04:22 PM »
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

Offline AWK

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Re: What is new pH value of solution
« Reply #5 on: May 15, 2019, 02:03:23 AM »
For your cleaning solution, it is not possible to calculate the pH of the solution after dilution. Above all, the so-called sodium metasilicate is not a pure compound - it is a mixture of several low-molecular silicates (see wikipedia - sodium metasilicate). The pH calculation can only be performed if you know the exact composition of the solution. Also, due to the composition of the solution, pH measurement is problematic. However, you can try to estimate the pH change of the solution after 200 fold dilution. For a pure NaOH solution, the pH will drop by 2.3 pH units.
For a clean Na2SiO3 solution, the pH of the diluted solution will drop by approximately 1.2 pH units. If after dilution of the solution your solution is clear, you may expect a pH about 10.3 or slightly higher. If the solution is opalescent, the pH will be slightly less than 10.3.
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Offline multivits

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Re: What is new pH value of solution
« Reply #6 on: May 15, 2019, 06:12:54 PM »
Thank you for the insights. It seems to ambiguous to call mathematically, so I propose to mix up the dilution with 65°C water and test the pH with paper test strips.

But how accurate are paper test strips? We have no lab equipment like a pH meter.

TIA multivits

Offline AWK

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Re: What is new pH value of solution
« Reply #7 on: May 16, 2019, 12:32:20 AM »
Read the MSDS carefully. The producer usually gives the pH for 1 or 2% solution, not the concentrated solution.
For a typical application, simply ask the manufacturer.

And ignore the use of indicator strips.
AWK

Offline multivits

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Re: What is new pH value of solution
« Reply #8 on: May 16, 2019, 09:57:18 PM »
Thanks, unfortunately the MSDS doesn't state the pH at any dilution.

I have asked the manufacturers to provide the pH at the 200:1 dilution,

their last communications stated it will be "much less than 10 pH in the tank, use test strips to check it"

Oh well, thanks any way.

multivits

Offline AWK

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Re: What is new pH value of solution
« Reply #9 on: May 17, 2019, 02:50:47 AM »
From the manufacturer's response and MSDS's, it can be seen that this manufacturer is rather unreliable (no one in their right mind determines the content of the most expensive component in the product as 10-30% - for this substance 2-3 % error would be accepted). As indicated by Borek, and I gave the numbers - the pH of pure NaOH solution with 200-fold dilution will change from 11.5 to 9.2. In the case of any hydrolyzing salt, and such is sodium metasilicate, the pH will decrease less. My estimate at pH 10.3 (~half-way) is probably better than the pH value obtained using even the most accurate indicator papers by a very experienced chemist due to subtle changes in the color shades.
1% solution of non-stabilized sodium silicate shows pH ~12.5. Stabilization is achieved by the addition of a few percents of sodium carbonate.
AWK

Offline DrCMS

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Re: What is new pH value of solution
« Reply #10 on: May 17, 2019, 05:46:58 AM »
From the manufacturer's response and MSDS's, it can be seen that this manufacturer is rather unreliable (no one in their right mind determines the content of the most expensive component in the product as 10-30% - for this substance 2-3 % error would be accepted).

NO!  That the SDS gives a range of 10-30% does NOT mean that is the accuracy the manufacturer works to.  The SDS must list all the hazardous components but the manufacturer does not have to print the exact formulation level they are allowed to list a range for each component.  The correct formulation, along with physical testing, is used to determine the full hazard classification for the formulation.  The SDS then lists the correct classification for both supply and transport but it does not need to detail the precise formulation levels.  The product specification may give more details of the component levels.

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