# [Wet Thumb Forum]-Titration of a solution of two acids, humic and carbonic (long)



## gsmollin (Feb 3, 2003)

Hypothesis:

A titration can be performed on dilute peat-water extract, and dilute carbonic acid, H2CO3, separately, then as a solution of two acids in water. The results for the separate titrations will add together to give the result for the combined titration. Assume that the peat-water extract contains equilibrium levels of CO2 as carbonic acid.

Equipment:

Bromothymol blue pH kit, AP
Phenol red pH kit, Rainbow Lifeguard
KH kit, AP
CO2 titration kit, LaMotte PCO-DR
Wide range pH kit, LaMotte, Model P-5085
Air pump, tubing, and airstone
CO2 cylinder, regulator, needle valve, tubing, and airstone
1 litre beaker
1 gallon of bottled spring water
Prepared peat-water extract

Procedure:

1. Measure the KH, pH, and acidity as equivalent CO2 concentration of the peat-water extract. Data: pH<5.0, KH<=1, CO2=62 mg/l.

2. Aerate the gallon of spring water in the bottle for 30 minutes. Pour out 500 ml of water into the 500ml beaker. Measure pH, KH, and CO2 concentration. 
Data: pH = 7.6, KH = 4, CO2 = 6 mg/l.

3. Pour out water until 340 ml remains in the 500 ml beaker. Add 160 ml of peat-water extract to the beaker. Measure pH, KH, and acidity as equivalent CO2. 
Data: pH=6.4, KH=3, C02=21 mg/l.

4. Aerate peat-water solution for ten minutes. Measure pH, KH, acidity as CO2. 
Data: pH=7.6, CO2=9 mg/l, KH=3.

5. Put 500 ml fresh water into beaker. Aerate with CO2. Measure pH, KH, and CO2 concentration. Data: pH=6.4, KH=4, CO2=20 mg/l.

6. Reduce volume of water in beaker to 340 ml. Add 160 ml peat-water extract. Measure pH, KH, acidity as equivalent CO2. 
Data: pH=6.0, KH=3, CO2=35 mg/l.

7. Re-measure acidity of peat-water extract as equivalent CO2 concentration. 
Data: CO2=66 mg/l.

Calculations:

From Step 3.

6 mg/l (CO2) x 340 ml (water) = 2.04 mg CO2

62 mg/l (acidity) x 160 ml (water) = 9.92 mg CO2 equivalent

(2.04 mg + 9.92 mg)/500 ml (water) = 23.9 mg/l CO2 equivalent.

From Step 6.

20 mg/l (CO2) x 340 ml (water) = 6.8 mg CO2

66 mg/l (acidity) x 160 ml (water) = 10.6 mg CO2 equivalent

(6.8 mg + 10.6 mg)/500 ml (water) = 34.8 mg/l CO2 equivalent

Conclusion:

The measurements of step 3 are in agreement with the hypothesis, with a error of +10% in the theory. The measurements of step 6 are in agreement with the hypothesis within the error bounds of the equipment. The most important calculation is the result which will allow us to subtract out the acid in the water from a total titration result to calculate the CO2 concentration: 35 mg/l (total measurement) - 21 mg/l (peat-water solution measurement) + 6 mg/l (equilibrium level of CO2 measurement) = 20 mg/l (CO2 concentration, alone). The equilibrium level must be added back in, since it was part of the stand-alone CO2 measurement of step 5, but is subtracted out by the peat-water-solution term of the equation. This result, 20 mg/l, is the same as the measurement of CO2 concentration taken in step 5. The second titration of the peat-water extract was done because the bottom of the bottle was noticably darker than the top, and I wanted to be sure there wasn't some gross error being caused by peat moss particles settling in the bottom of the bottle. There was a small, measurable difference between the first titration done at the top of the bottle, and the second, nearer the bottom. The second measurement was used in step 6. The acidity of the peat-water solution was easily removed from solution by aeration for 10 minutes in step 4. This indicates that the acidic components are unstable, similar to the results with acetic acid, and carbonic acid.

Discussion:

The 10% error in the prediction of dilution strength of the peat-water solution has been troubling for the results of this experiement. All other results are in excellent agreement. Particularly gratifying is the final result, where the CO2 concentration can be calculated by subtracting the CO2 equivalent measurement from the total CO2 measurement. That result is the reason for pursuing this whole train of events, and is the enabling technology to allow CO2 concentrations to be calculated in aquariums with other acids present in the water. I now believe that in aquariums with peat substrates, or those like mine, where the substrate is loaded with decaying organic matter, the injected CO2 concentration can be determined by the following method:

1) Measure the total CO2 equivalent using a CO2 titrator, or the pH/KH method.

2)Stop CO2 injection long enough for the CO2 concentration to return to equilibrium levels.

3)Subtract those two readings, and the difference is what you were injecting. This result seems pretty obvious, but proving it turned out to be quite a chore.

[This message was edited by gsmollin on Sun August 17 2003 at 06:36 AM.]


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## gsmollin (Feb 3, 2003)

Hypothesis:

A titration can be performed on dilute peat-water extract, and dilute carbonic acid, H2CO3, separately, then as a solution of two acids in water. The results for the separate titrations will add together to give the result for the combined titration. Assume that the peat-water extract contains equilibrium levels of CO2 as carbonic acid.

Equipment:

Bromothymol blue pH kit, AP
Phenol red pH kit, Rainbow Lifeguard
KH kit, AP
CO2 titration kit, LaMotte PCO-DR
Wide range pH kit, LaMotte, Model P-5085
Air pump, tubing, and airstone
CO2 cylinder, regulator, needle valve, tubing, and airstone
1 litre beaker
1 gallon of bottled spring water
Prepared peat-water extract

Procedure:

1. Measure the KH, pH, and acidity as equivalent CO2 concentration of the peat-water extract. Data: pH<5.0, KH<=1, CO2=62 mg/l.

2. Aerate the gallon of spring water in the bottle for 30 minutes. Pour out 500 ml of water into the 500ml beaker. Measure pH, KH, and CO2 concentration. 
Data: pH = 7.6, KH = 4, CO2 = 6 mg/l.

3. Pour out water until 340 ml remains in the 500 ml beaker. Add 160 ml of peat-water extract to the beaker. Measure pH, KH, and acidity as equivalent CO2. 
Data: pH=6.4, KH=3, C02=21 mg/l.

4. Aerate peat-water solution for ten minutes. Measure pH, KH, acidity as CO2. 
Data: pH=7.6, CO2=9 mg/l, KH=3.

5. Put 500 ml fresh water into beaker. Aerate with CO2. Measure pH, KH, and CO2 concentration. Data: pH=6.4, KH=4, CO2=20 mg/l.

6. Reduce volume of water in beaker to 340 ml. Add 160 ml peat-water extract. Measure pH, KH, acidity as equivalent CO2. 
Data: pH=6.0, KH=3, CO2=35 mg/l.

7. Re-measure acidity of peat-water extract as equivalent CO2 concentration. 
Data: CO2=66 mg/l.

Calculations:

From Step 3.

6 mg/l (CO2) x 340 ml (water) = 2.04 mg CO2

62 mg/l (acidity) x 160 ml (water) = 9.92 mg CO2 equivalent

(2.04 mg + 9.92 mg)/500 ml (water) = 23.9 mg/l CO2 equivalent.

From Step 6.

20 mg/l (CO2) x 340 ml (water) = 6.8 mg CO2

66 mg/l (acidity) x 160 ml (water) = 10.6 mg CO2 equivalent

(6.8 mg + 10.6 mg)/500 ml (water) = 34.8 mg/l CO2 equivalent

Conclusion:

The measurements of step 3 are in agreement with the hypothesis, with a error of +10% in the theory. The measurements of step 6 are in agreement with the hypothesis within the error bounds of the equipment. The most important calculation is the result which will allow us to subtract out the acid in the water from a total titration result to calculate the CO2 concentration: 35 mg/l (total measurement) - 21 mg/l (peat-water solution measurement) + 6 mg/l (equilibrium level of CO2 measurement) = 20 mg/l (CO2 concentration, alone). The equilibrium level must be added back in, since it was part of the stand-alone CO2 measurement of step 5, but is subtracted out by the peat-water-solution term of the equation. This result, 20 mg/l, is the same as the measurement of CO2 concentration taken in step 5. The second titration of the peat-water extract was done because the bottom of the bottle was noticably darker than the top, and I wanted to be sure there wasn't some gross error being caused by peat moss particles settling in the bottom of the bottle. There was a small, measurable difference between the first titration done at the top of the bottle, and the second, nearer the bottom. The second measurement was used in step 6. The acidity of the peat-water solution was easily removed from solution by aeration for 10 minutes in step 4. This indicates that the acidic components are unstable, similar to the results with acetic acid, and carbonic acid.

Discussion:

The 10% error in the prediction of dilution strength of the peat-water solution has been troubling for the results of this experiement. All other results are in excellent agreement. Particularly gratifying is the final result, where the CO2 concentration can be calculated by subtracting the CO2 equivalent measurement from the total CO2 measurement. That result is the reason for pursuing this whole train of events, and is the enabling technology to allow CO2 concentrations to be calculated in aquariums with other acids present in the water. I now believe that in aquariums with peat substrates, or those like mine, where the substrate is loaded with decaying organic matter, the injected CO2 concentration can be determined by the following method:

1) Measure the total CO2 equivalent using a CO2 titrator, or the pH/KH method.

2)Stop CO2 injection long enough for the CO2 concentration to return to equilibrium levels.

3)Subtract those two readings, and the difference is what you were injecting. This result seems pretty obvious, but proving it turned out to be quite a chore.

[This message was edited by gsmollin on Sun August 17 2003 at 06:36 AM.]


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## Roger Miller (Jun 19, 2004)

gsmolin,

I'm still studying this, but I have a couple questions:



> quote:
> 
> 6.20 mg/l (CO2) x 340 ml (water) = 6.8 mg CO2


Shouldn't this be 20 mg/l on the left rather than 6.20 mg/l?

Similarly, in the first line of the calculations, shouldn't 3.6 on the left side be 6?

Elsewhere you use the term "acidity." Are you referring to the results from the LaMotte CO2 kit?

Roger Miller

[This message was edited by Roger Miller on Fri August 15 2003 at 09:26 PM.]


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## gsmollin (Feb 3, 2003)

> quote:
> 
> Originally posted by Roger Miller:
> gsmolin,
> ...


Yes, the "6." is supposed to have a space between it and the "20 mg/l". It means "Calculation from step 6." I had serious formatting problems with this, because it was written in Microsoft Word, and the news server took it a unformatted ASCII; not even a line break survived the upload. I had to re-format the whole thing, and I mistook that 6. as decimal number.

Yes, see above. I have added "Calculation from step 3" in the text of the post.

Yes, this means "acidity as equivalent CO2". So in step 3, where the LaMotte test kit reports 21 mg/l CO2, we are getting a result that is a solution of the natural acidity from the spring water and the humic acid from the peat water, all reported as "CO2" by the LaMotte titration kit. This is perfectly normal, since all the LaMotte kit does is measure acidity using a classic KOH titration with a phenolpthalein end-point indicator. They do a dis-service to aquarists when they call this a "CO2 Test Kit", since that test can't tell the difference between CO2 and your (ex)friend's stomach acid when he vomited into your aquarium at your New Year's Eve party.:O

Thank's for the catch. I'm glad you are actually reading this. It was a lot of effort, and few people are interested in the work.


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## Roger Miller (Jun 19, 2004)

gsmolin, thanks. I just wanted to make sure I was on the right page.

You did an excellent job documenting your experiment. I think you demonstrated your hypothesis very well, showing that both alkalinity and acidity were conserved when two acids are mixed. That result could vary a bit depending on the acids, but it worked here.

That said, I don't think that the test method you describe at the end of your letter is supported by your experiment.

First, when you mix the acidic peat extract with the spring water there is an acid-base reaction; acid in the peat extract reacts with bicarbonate in the spring water to form CO2. As a result, the CO2 content in the mixed water should always be higher than the CO2 content from your mixing calculations.

Second, in your test method you measure CO2 in a CO2 injected water, then allow that water to come to equilibrium with air and measure CO2 again. Your result is based on the difference between those measurements. In your experiment you measure CO2 in water injected with a known amount of CO2 (step 6) and compare the measured CO2 with water with the same composition that is not injected with CO2 (step 3).

The problem is that -- unlike in your test method -- the water in step 3 is *not* brought into equilibrium with air. In step 4 you aerate the mix that you tested in step 3 and the pH and acidity both change. The mixed water was not in equlibrium with air because the acidic peat extract combined with bicarbonate in the spring water to generate CO2.

If you compare the measurement in step 6 with the measurement in step 4 then that experiment would be more comparable to your test procedure. Calculations based on step 4, however don't support your method.

Experimental method aside, there are still problems. The test procedure that you describe makes common sense, but it ignores too much chemistry. I can calculate the conditions for a model system and in the model system there is a big difference between the results from your method and the conditions specified to the model system.

In the model system the peat extract is treated like a simple monoprotic acid - call it "HR"

The two acids in solution give us the simultaneous equilibrium conditions:

HR <--> H+ + R - and
H2O + CO2 <--> H+ + HCO3-

We have four formulas to define the system, the two equibrium expressions, the charge balance and the mass balance for R. We cannot balance C because the system is open with respect to CO2.

from equilibrium:
Kr={H+]{R-]/[HR] = unknown experimental variable
Kc=[H+][HCO3-]/[CO2] = 4.16e-7

from mass balance:
[R-] + [HR] = R where R is constant

from the charge balance:
D[H+]=D[R-]+D[HCO3-] where "D" denotes the change between two states

These four equations along with starting compositions can be used to calculate the pH, acidity and alkalinity of the system under different CO2 concentrations. The formulas combine to give a cubic equation. I used a spreadsheet program to solve the system.

Use Kr=5.01e-7 and start with a system that has the following conditions:

[CO2]=.455 m/liter = 20 ppm
R=1 m/liter
HCO3-=1.1 m/liter=67.1 mg/l bicarb or 3 dKH

The formulas above then define the remaining terms:
pH=6.76
[R-]=.745 moles/liter
{HR]=.255 moles/liter

Acidity is the sum of [HR] and [CO2] and is .71 meq/liter or about 31 apparent ppm of CO2.

Alkalinity is the sum of [HCO3-] and [R-] and is 1.845 meq/l or about 5 dKH; along with pH of 6.76, this also estimates 31 apparent ppm of CO2.

Now assume that the system is aerated and the actual CO2 concentration drops to 1 ppm. The formulae give:

[CO2]=0.0227 m/liter = 1 ppm
R=1 m/liter
[HCO3-]=0.866 m/liter=52.8 mg/l or 2.4 dKH
pH=7.96
[R-]=0.979 m/liter
[HR]=0.021 m/liter

Acidity as the sum of [HR] and [CO2] is 0.044 meq/liter or about 2 apparent ppm of CO2.

Alkalinity as the sum of [HCO3-] and [R-] is unchanged at 1.845 meq/l or about 5 dKH; along with pH of 7.96 this estimates 2 apparent ppm of CO2.

Your method would lead me to believe that I injected 29 ppm of CO2. In fact I was only adding 19 ppm of CO2.

I have run into similar problems before. Check this article that I wrote to APD, now at thekrib. I came up with a method for estimating CO2 without measuring alkalinity. It was intended to work as long as bicarbonate content remained the same between two states. Alkalinity doesn't change, so why not assume that bicarbonate (part of alkalinity) is also constant? The reason is obvious in the example above. Even though alkalinity may stay the same between two states, the concentration of bicarbonate changes.

Roger Miller


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## gsmollin (Feb 3, 2003)

Ok, now I may have something to study. However, I still don't think we are both on the same page, yet.

STEP 4 IS A RED HERRING! I'm sorry I actually put it in the post.

Step 1, 2, and 3 are logically connected. Step 4 is a dead end. I had completed this experiment with acetic acid + CO2 before using peat-water + CO2, and originally had planned to be able to reduce the CO2 with aeration, while not affecting the acetic acid. It didn't work for actic acid, or peat-water either, as you may have said. So the results of step 4 don't enter into the experiment. I apologize for the confusion, but that step is in there just to see if the aeration reduces peat-water acidity. It does.

In step 5 I'm using fresh water again, not the solution from step 4. That gets thrown out. Does this help any, or are am I still not explaining it well enough?

[This message was edited by gsmollin on Sun August 17 2003 at 07:14 PM.]


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## Roger Miller (Jun 19, 2004)

I understand that step 4 was not used in your experiment. I didn't know why it was there but it was useful just the same. I also understand that step 5 starts with fresh water, not with the results from step 4. No clarification was needed.


Roger Miller


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## Phil Edwards (Jan 22, 2004)

This is really interesting, but I need more education to understand it.









To simplify it almost to the point of stupidity can I assume that adding Bicarbonate to peat filtered water I'll get "natural" CO2 production?

What about adding a small chunk of coral in a peat filter? Would that have the same effect?

Thanks,
Phil


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## gsmollin (Feb 3, 2003)

Alright, then do you agree here is what I did in the procedure:

1. Measure CO2 of peat water.
2. Measure CO2 of spring water.
3. Mix 340 ml spring water and 160 ml peat water and measure CO2 in .
(Ignore step 4.)
5. Bubble CO2 into new sample of spring water and measure CO2 as in step 2.
6. Mix 340 ml CO2-spring-water from step 5 with 160 ml peat water and measure CO2 of the resultant.

(Step 7 was a check that the acidity of the peat water was constant throughout the sample, and was close enough to the value obtained in step 1 to be valid.)

In a Nutshell:
I measured CO2 with and without peat in the water. I was able to show that the acidity of the CO2 added to the acidity of the spring water and the acidity of the peat water. Since that is true, the acidity of the CO2 is equal to the acidity of all three minus the acidity of spring water plus peat water. That is to say: The total acidity is equal to the sum of its parts.

This should be perfectly obvious, since conservation of charge and conservation of mass are fundamental to chemistry.

Of course, I make no special claims as to the nature of the acids involved, save one. I had done this experiment previously with acetic acid replacing peat-water, and got the same result. I can't tell you if the peat water contained CO2 or not. I think that it did not contain more than equilibrium levels, since I had filtered it about 20 times to remove fine particles. It is also likely that the peat-water reacted with the bicarbonate in the spring water to evolve some CO2. 

The whole point of the experiment was that these are the reactions that are happening in an aquarium, but nevertheless, we can measure the "CO2" in the water with and without injecting CO2 into the water, and the difference between the two readings is the CO2 that we injected. Do we agree on this last point?


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## Roger Miller (Jun 19, 2004)

> quote:
> 
> To simplify it almost to the point of stupidity can I assume that adding Bicarbonate to peat filtered water I'll get "natural" CO2 production?
> 
> What about adding a small chunk of coral in a peat filter? Would that have the same effect?


That works, for a while anyway. The acidity of the peat is usually neutralized fairly quickly then the CO2 production stops. In the long term the peat might produce more CO2 through it's own degradation than it does through the acid reaction with bicarb or carbonate.

Roger Miller


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## Roger Miller (Jun 19, 2004)

I'll get to the main point first:


> quote:
> 
> we can measure the "CO2" in the water with and without injecting CO2 into the water, and the difference between the two readings is the CO2 that we injected. Do we agree on this last point?


No. I thought I was clear about that.

In the CO2-injected water the acidity is composed partly of CO2 and partly of acids from the peat. Several things happen when you let the CO2 level drop to atmospheric levels. The acidity drops, the pH rises and the ratio of dissociated acids to associated acids increases. This last point is what you are overlooking and it's what throws a wrench into your method.

When the acidity drops that drop is not *all* caused by the loss of CO2. Part of the drop in acidity is caused by the disassociation of peat acids. There is no way to tell how much of the drop in acidity is due to CO2 loss and how much is due to the loss of peat acids. Your method assumes that all of the drop in acidity is due to a loss of CO2 and so it always and inevitably overestimates the amount of CO2 loss. The method overestimates the amount of CO2 injected because the method equates the amount of CO2 injected with the amount of CO2 lost.



> quote:
> 
> n a Nutshell:
> I measured CO2 with and without peat in the water. I was able to show that the acidity of the CO2 added to the acidity of the spring water and the acidity of the peat water. Since that is true, the acidity of the CO2 is equal to the acidity of all three minus the acidity of spring water plus peat water. That is to say: The total acidity is equal to the sum of its parts.
> ...


You find your conclusions more obvious than I do.

The fact that acidity was conserved in mixing does not mean that the amount of CO2 in the mixed acidity stayed constant.

Roger Miller


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## gsmollin (Feb 3, 2003)

Roger,

Here is a real problem faced by an aquarist:

The aquarist has an aquarium with a peat substrate, or one that is many years old and full of decayed plant matter. Water parameters are: KH=6 degrees, pH=7.2, LaMotte CO2 kit reads 12 ppm CO2. The aquarium is not injected with CO2.

Now the aquarist injects 1 bubble/second CO2 into the aquarium water. Three days later the water parameters are: KH=6, pH=6.8, LaMotte kit reads 33 ppm CO2.

How much CO2 did the aquarist add to the aquarium water?


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## Roger Miller (Jun 19, 2004)

Based solely on theory that 12 ppm is two much CO2 for an uninjected tank I would suggest that there is a non-carbonate buffer present in the water. Neither the acidity or alkalinity results can be used to determine CO2. Neither can the difference between two tests be used to determine the amount of CO2 added.

More generally, since both the alkalinity and acidity tests will always overestimate CO2 in the presence of a non-carbonate buffer, I can say that the CO2 concentration in the tank was < 12 ppm before injection and < 33 ppm after injection. He added something < 21 ppm.


Roger Miller


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## gsmollin (Feb 3, 2003)

Only carbonate buffers are in the water. The 12 ppm CO2 reading is caused by the humic acids released by the peat or the decayed plant matter and fish excrement in the substrate, as indicated by aquarium history. Indeed, any disturbance of the substrate results in KH and pH rapidly dropping. Some of the CO2 reading may be real, from the bicarbonate reaction you described.

Your answer sheds no more light on this subject than any of the other expert answers I have read. I shall continue to use my method, since it gives answers that may be disregarding second-order effects, but it does give first-order estimates. When you have a better answer, you can post it and I will be glad to try it out.

Thank you for your time.


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## Roger Miller (Jun 19, 2004)

> quote:
> 
> Only carbonate buffers are in the water. The 12 ppm CO2 reading is caused by the humic acids released by the peat or the decayed plant matter and fish excrement in the substrate, as indicated by aquarium history


Humic acids are not carbonate buffers.

Roger Miller


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## gsmollin (Feb 3, 2003)

> quote:
> 
> Originally posted by Roger Miller:
> Based solely on theory that 12 ppm is two much CO2 for an uninjected tank I would suggest that there is a non-carbonate buffer present in the water....
> ...


Well since I flat-out told you there is a peat substrate, you don't have to "suggest" that could be the cause of the 12 ppm reading. It is the cause. I thought you were inferring that there was _something else_ that I hadn't told you about. There is nothing else. An yes, I am perfectly aware that humic acid, as well as a host of other acid-salt buffer systems that could possibly be dissolved in water are not carbonic acid-bicarbonate buffer sytems. I don't need, and never asked for a short course in freshman water chemistry. I was was hoping for some assistance about how aquarists might be able to measure their CO2 concentrations, in a simple manner, for at least the case where peat or decayed palnts and fish mulm have added other acids, and bases, and salts to the water, other than bicarb. I was hoping for more than I got.


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