# Making buffer solutions



## hoppycalif (Apr 7, 2005)

How do you make a 6.6 pH buffered water solution? I ask, because I want to make a drop checker with an added sealed compartment containing pH 6.6 water with the same amount of indicator reagent in it as in the drop checker bulb. This gives a color standard to improve the accuracy.

My chemistry knowledge is almost trivial - 2 inorganic chemistry classes early in my college education, most of which has been forgotten. I have looked for sources of such solutions, but the closest I can get is a 6.86 pH solution. Help!


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## dennis (Mar 1, 2004)

That is an interesting question for what suggests to be an interesting idea? I do not really know an answer though I would think the correct thing to do is use RO/DI water, something with absolutely no buffer, and adjust the pH using acid. Muratic acid, basically HCl may work well for this. A wikipedia search of hydrochloric acid yields a good bit of info.

Also, wikipedia tells the acid dissociation constant in the info table on the right side of the HCl page. Clicking "acid dissociation constant" explains what it means, a little anyway though its not in "english".


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## hoppycalif (Apr 7, 2005)

dennis said:


> That is an interesting question for what suggests to be an interesting idea? I do not really know an answer though I would think the correct thing to do is use RO/DI water, something with absolutely no buffer, and adjust the pH using acid. Muratic acid, basically HCl may work well for this. A wikipedia search of hydrochloric acid yields a good bit of info.
> 
> Also, wikipedia tells the acid dissociation constant in the info table on the right side of the HCl page. Clicking "acid dissociation constant" explains what it means, a little anyway though its not in "english".


A 6.6 pH is barely acidic, so it would take only slight contamination of such a solution to change the pH. For example bromothymol blue reagent is, as I recall, alkaline, so even that could change the pH, couldn't it? I did consider trying to mix a bit of an acid into distilled water to do this, but I have my doubts that the solution would remain at 6.6 pH. I will study that wikipedia entry some more.


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## dennis (Mar 1, 2004)

In that case, adding the indicator then using it as a visual for pH when adding the acid may be the way to go. One thing you would probably have to do manipulate DI water which had had an opportunity to equilibrate with the air otherwise it could change very quickly while you are working with it.

Don't really know though


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## hoppycalif (Apr 7, 2005)

This hobby should be a High School requirement - it teaches you chemistry, botany, and patience!


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## dennis (Mar 1, 2004)

Yeah, I know. It actually prompted me to go back to college. I also find it a great way to practice and solidify the things I learn in school. It gives me a base or an association to the material in school.


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## feistyfish (Aug 13, 2006)

I work at the Salk Research lab, and when we make buffers, it usually involves starting with say an acid and titrating slowly with a basic solution until the desired pH is reached. of course its easier to be precise with the pH probe.


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## SpaceBug (Jan 10, 2007)

If you are worried about the indicator affecting the pH, which seems like a reasonable assumption, then add the indicator and use a pH probe to check the pH as you add acid/base. Just keep track of what you add to the mix so you can recreate it. Of course, I suppose if you has the pH probe you wouldn't be working on this idea. Hmmmm ,,,,


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## dennis (Mar 1, 2004)

I am not sure that it will matter how picky you are with this. Seems like your trying to make a color standard for your checker that allows for an easy comparison between your tank conditions and "a real sample". Since the room for error you have in making the drop checker solution the right hardness, getting the standard the right pH and how it is affected by the indicator, I would think that the best way without a pH probe would be to use add the indicator to your water, then add muratic acid to the mix until the color matches what you consider to be 6.6 If you have a probe than spacebugs method seems the best.

This is another one of those cases where consistency seems way more important than accuracy.


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## hoppycalif (Apr 7, 2005)

My goal is to improve the accuracy of the drop checker determination that the color means 6.6 pH. So, I want a color standard located right beside the drop checker "bulb", which is the same geometric shape, viewed in the same light, thru the same amount of glass/acrylic. The major inaccuracy in using a drop checker is in deciding that the color is "green" (6.6pH) vs. yellowish or bluish green (6.5 or 6.7). This would be a way to improve that accuracy, but to do that, I need to know that my color standard is at 6.6pH and not something else. I do have a pH probe, so titrating isn't a major problem for me, but what chemicals to use to do so is. I have a small bottle of "pH down" for a spa, which I expected to be muriatic acid, but which is actually sulfuric acid. I propose to use that. What base is appropriate to use?


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## dennis (Mar 1, 2004)

Sulfuric acid should be fine to use. I did a bit of internet searching last night about your situation and found the the two best acids to use for this is HCl or H2SO4. Apparently, from a post by someone who worked for a public aquarium, these are the two chemicals the use to adjust the acidity of the aquariums. Some people consider H2SO4 better as it adds no additional Cl to the water but it was also pointed out the the Cl, as is comes from the acid, is a free ion so it will not bother the inhabitants.

Anyway, Base (chemistry - Wikipedia, the free encyclopedia) is a neat little thing about bases. It tells you what you probably already know though however, it mentions ammonia being a base since it readily attracts a H+ ion thus raising the pH of the solution. You may be able to use household ammonia if you go to far with the acid, or you could just add a bit more water as the water is more basic than.

Alternately, you could increase the pH with a aqueous sodium bicarbonate solution. Adding NaHCO3 to an acid solution should look something like this ( though I may be wrong), note that I have already accounted for the dissociation of NaHCO3 in the initial water.

Na+(aq) + H+(aq) + CO3-(aq) + H+(aq- when adding the NaHCO3 to an already acid solution with free H+ ions) --> Na+(aq) + H2CO3-> 2H2O

I took a few short cuts there but you can see that the addition of NaHCO3 to an already acid solution will lead to the eventual removal of free H+ by forming 2 waters from the formed H2CO3. You end up with free Na in water. Using H2SO4 to initially lower the pH would lead to a final solution of Na2SO4 in water, though it should not matter as , just like with table salt, the salt will be in dissolution in the water and not effect your situation one bit. Using muratic acid instead would ultimately lead to NaCl in water, though again the dissolved salt should not effect your situation at all.

Using ammonia to raise the pH would eventually give you ammonium in water which should again not matter to you.


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## hoppycalif (Apr 7, 2005)

Thank you Dennis. That gets me on the right track. Just to put things in perspective a bit more: when you add acid to a swimming pool, say a 20,000 gallon pool, a gallon of acid lowers the pH around 1 to 2 at most. So, thats a 1 in 20,000 dilution. Or one drop per liter! I sometimes lose track of just how dilute everything we work with is.


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## Left C (Jun 14, 2005)

hoppycalif said:


> How do you make a 6.6 pH buffered water solution? I ask, because I want to make a drop checker with an added sealed compartment containing pH 6.6 water with the same amount of indicator reagent in it as in the drop checker bulb. This gives a color standard to improve the accuracy.


Hi Hoppy and Dennis

Would it matter if you used a Sodium Phosphate Buffer in your sealed container that you want to use strictly for a color reference at a pH of 6.6?

My basic Biotechnology lab manual has the directions on how to make Phosphate Buffer solutions from a pH of 5.7 to 8.0. Here is the one for a pH of 6.6:

*To prepare 0.1 M Sodium Phosphate Buffer of a chosen pH of 6.6*

Step 1) *Prepare Solution A*, 0.2 _M_ NaH2PO4***H20 (Monobasic Sodium Phosphate Monohydrate, _FW_ = 138.0):
Dissolve *27.6 g NaH2PO4*H20* in purified water. Bring the total volume up to 1000 _mL_.

Step 2) *Prepare Solution B*, 0.2 _M_ Na2HPO4 (Dibasic Sodium Phosphate):
Use either: *28.4 g Na2HPO4* (Anhydrous Dibasic Sodium Phosphate, _FW_ = 142.0)
or *53.6 g Na2HPO4*7H20* (Dibasic Sodium Phosphate Heptahydrate, _FW_ = 268.1)
Dissolve in purified water. Bring the total volume up to 1000 _mL_.

Step 3) Combine Solution A + Solution B in the amounts listed below.

Step 4) Bring the mixture of Solution A + Solution B up to a total volume of 200 _mL_ by using purified water.

*pH 6.6
Volume of Solution A = 62.5 mL
Volume of Solution B = 37.5 mL*

{A Phosphate Buffer is typically prepared by mixing solutions of Dibasic Sodium Phosphate (Na2HPO4) and Monobasic Sodium Phosphate (NaH2PO4). At 25 degrees C, a 5% solution of Dibasic Sodium Phosphate has a pH of 9.1, whereas a 5% solution of Monobasic Sodium Phosphate has a pH of 4.2. Mixtures of these two solutions will make a buffer with a pH between the two extremes.

Phosphate based buffers aren't very sensitive to the effects of temperature like some other buffers, but they are more sensitive to dilution. Phosphate Buffers, therefore, should not be prepared as concentrated stock solutions. They should instead be prepared to the concentration at which the will be used.}


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## hoppycalif (Apr 7, 2005)

Left C, that is an interesting idea. I checked the Seachem website and find the ratios needed to buffer to a set pH by using a ratio of Seachem acid buffer and alkaline buffer. So, for about $10 I could get those chemicals, and be able to buffer hundreds of gallons of water! Then there are "pH up" and "pH down" for spas, which are easier to find and probably even more costly. But, I already have "pH down" so that may be the way I go. This seems to be a question with many answers.


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## Left C (Jun 14, 2005)

I've used Seachem's Alkaline and Acid Powder Buffers for years. I've tried mixing them according to the ratios listed on the bottles which is the same as on their website with very low KH tap water. I have found that the Acid Buffer is very strong and you get a much lower pH than they say it will be. Maybe you'll have better luck than I did. I was only using postal scales at the time to weigh them, so my accuracy may of not been very good.

Their Liquid Alkaline and Acid Buffers are very, very weak. Here's a post to Seachem about their Liquid Alkaline Buffer: PlantGeek :: View topic - Strength of Liquid vs. Powdered Alkaline Buffer

Two weeks ago or so, I was looking around for reference solutions. I found a site that offered certified pH reference solutions in many different pH levels. I've been looking for that site the past two days, but I haven't found it again.

I have two of the Sera brand drop checkers around here somewhere. If I can find them, I'm going to use them like you mentioned with an accurate KH solution with distilled water.

Good luck with your project. I've very much enjoyed reading about it.


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## hoppycalif (Apr 7, 2005)

I just finished making a 6.6 pH buffered (?) water solution. I added about a half tsp of baking soda to a half cup of distilled water and stirred it until all of it dissolved. Then I diluted that with more distilled water to get about a cup full. Next I added a bit of 10% sulfuric acid to another cup of water and stirred that. Then I began adding a few ml at a time of the acid water to the bicarbonate water. Eventually I reached pH 6.6. I tried diluting some of this with more distilled water to see if it held its pH and it held to 6.68. So, does that mean it is buffered?

Before I tried this I tried it with household ammonia, with little luck. The final 6.6 pH water I got that way would go to over 7.0 when I added a bit more distilled water to it. Chemistry is a great knowledge to have - I wish I did!


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