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48 votes

What is the pH of ice?

According to Martin Chaplin's Water Dissociation and pH: In ice, where the local hydrogen bonding rarely breaks to separate the constantly forming and re-associating ions, the dissociation constant ...
DavePhD's user avatar
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33 votes
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Sugar solutions have a neutral pH in themselves, but it makes your body acidic. Why?

It is not proven that "sugar makes your body acidic"! Your body's pH is very tightly regulated by the body's internal systems; it is also different in different parts of the body - the stomach is ...
Waylander's user avatar
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32 votes
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Why are some pH standard solutions 6.86 and 9.18?

The pH 6.86 and 9.18 values come from NIST standard buffer solutions for pH calibration, as described in, for example, NBS special publication 260-53 (1988) (pdf available here). It appears they chose ...
Andrew's user avatar
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20 votes

How do I calculate the isoelectric point of amino acids, each of which has more than two values of pKa?

Since the $\mathrm{pI}$ is the $\mathrm{pH}$ at which the amino acid has no overall net charge, you need to average the $\mathrm pK_\mathrm a$ values relevant to the protonation/deprotonation of the ...
orthocresol's user avatar
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How can benzaldehyde have a pKa of 14.9?

Significant amount of geminal diol of benzaldehyde exists in an aqueous solution of benzaldehyde at 25 °C because $\mathrm{p}K_{\text{hyd}} = 2$ (Ref. 1) The $\mathrm{p}K_{\mathrm a}$ of benzyl ...
Mathew Mahindaratne's user avatar
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Is the equation pH + pOH = 14 valid at different temperatures?

$\mathrm{pH + pOH = 14}$ This equation only holds true at around $25~^\circ\mathrm{C}$, where the water autodissociation constant $K_\mathrm{w} = 10^{-14}$. Mathematically, you can write $$\begin{...
orthocresol's user avatar
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17 votes

What is the pH of ice?

$\mathrm{pH}$ is the aqueous concentration of $\ce{H3O+}$ or $\ce{H+}$ ions in soution. I would not say that ice lacks $\ce{H3O+}$ and $\ce{OH-}$ ions as ice's structure would allow for such, however, ...
A.K.'s user avatar
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Why does pOH increase when pH decreases?

At a particular temperature, the $K_{\text{eq}}$ for the following reaction (yes, it's the auto-protolysis of water): $$\ce{H2O(l) <=> H+(aq) + OH-(aq)}$$ will be constant. Note that $K_\...
Gaurang Tandon's user avatar
16 votes
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Is it possible to measure pH without physically coming into contact with a solution?

Interesting analytical chemistry problem. Currently, it is not possible to accurately determine pH in a remote fashion. Spectroscopy is the technique of choice for remote sensing, but the H$^+$ is not ...
AChem's user avatar
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Why are almost all fruits acidic?

Fruits have culinary and botanical meaning. For the former, we call fruits those botanical fruits that are juicy and taste more or less sweet and sour. For the latter, e.g. nuts are non juicy fruits. ...
Poutnik's user avatar
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Why proton concentration is divided by 10⁻⁷?

The textbook is precisely correct. The equilibrium constant $K$ which the logarithm is taken of is dimensionless, and includes activities or fugacities, and not concentrations and pressures. In ...
andselisk's user avatar
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Why are synthetic pH indicators used over natural indicators?

In acid-base titrations, synthetic indicators are exclusively used to find accurate end-point determinations because they always have a highly defined color change at certain pHs. For example, ...
Mathew Mahindaratne's user avatar
15 votes

Why are some pH standard solutions 6.86 and 9.18?

Those strange $\text{pH}$ values belong to the primary $\mathrm{pH}$ buffer standard solutions, that have their $\ce{pH}$ value given by the definition of the practical $\mathrm{pH}$ scale. It means ...
Poutnik's user avatar
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14 votes
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pH probe bulb - what is happening within the glass?

My reference for all information and pictures is Harris' Quantitative Chemical Analysis, 9th ed., pp 347-9. I think it'll be worth your while to consult those pages, but I'll try to summarize the ...
a-cyclohexane-molecule's user avatar
14 votes
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Why measure both alkalinity and pH in pools if pH alone tells us how acidic or basic something is?

Alkalinity is typically reported in terms of either bicarbonate (the dominant carbonate species from around $\pu{pH} \simeq$ 6.5 to 8.3) or carbonate. You can have a high $\pu{pH}$ and alkalinity (...
Todd Minehardt's user avatar
13 votes
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Possible technique for measuring pH in solutions

Measuring pH with current flowing is not an appropriate idea. You would like to measure something without chemically disturbing the system. The moment you pass current in a solution, you start to ...
AChem's user avatar
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12 votes
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Combining acid dissociation constants to determine pH of diprotic acid

This should be broken into: \begin{align} \ce{H2A + H2O &-> HA- + H3O+}\\ \ce{HA- + H2O &-> A^{2-} + H3O+} \end{align} With equilibrium equations: \begin{align} K_\mathrm{a1}&=\frac{...
ringo's user avatar
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Why did the Rio 2016 pool water turn green?

Although not strictly alkalinity, this article quotes a changed pH level and an expert gives two possible explanations One: A low or high pH can bring out minerals. "Calcium would show itself as ...
mhchem's user avatar
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How is seawater alkaline?

The short answer to the title question is the ubiquitous presence of geologic and biogenic calcium carbonate $(\mathrm{p}K_\mathrm{a} = 9)$. The oceans as a whole can largely be thought of as ...
airhuff's user avatar
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12 votes
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Why are amino acids least soluble at isoelectric point?

In order for molecules to precipitate out of solution, they need to aggregate together. Amino acids that have zero net charge can aggregate together much more easily than those that are charged. ...
Andrew's user avatar
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12 votes

Can you meassure a negative pH value with a pH meter?

You are most likely getting an inaccurate value. As anticipated in Poutnik's comment above is difficult to reliably measure a $\mathrm{pH}$ of very acidic solution but the $\mathrm{pH}$ scale is ...
Alchimista's user avatar
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11 votes

Should the 'a' in the acid dissociation constant (Ka or pKa) be capitalised?

TL;DR: for the documents written in English use $\mathrm{p}K_\mathrm{a}$ (upright lowercase "a"). Current (2017Q3) chemmacros' documentation (p. 9) includes a ...
andselisk's user avatar
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Which salt will best be able to buffer the addition of HCl?

To pick the best salt in this case, we should pick the least acidic cation and the most basic anion. While sodium cannot dissociate into $\ce{H^+}$ ions, ammonium can, so the ammonium salts will be ...
ringo's user avatar
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Which definition of Acids and Bases conforms to the pH scale?

A few things first... Arrhenius Theory: An "acid" is a proton donor; a "base" is a hydroxide donor. So for example, $\ce{HCl}$ and $\ce{NaOH}$ (both in water) ionize to produce $\...
paracetamol's user avatar
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11 votes

How was pH measured back in the day if you had nothing to calibrate to?

"Hydronium" content can be easily determined by simple acid base titration. Keep in mind that there is nothing fundamental about pH. It is just a convenient scale for expressing hydrogen ion ...
AChem's user avatar
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Do solid phase pH buffers exist?

Just like wave-particle duality eventually ends up in meta-physics, you are extending the concept of pH too far where the concept of pH becomes murky. The simple answer to your query is ion-exchange ...
AChem's user avatar
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10 votes

How to calculate pH of a solution when H3PO4, NaH2PO4, Na2HPO4 and Na3PO4 are mixed together in certain amounts to form a solution?

Usually doing calculations of this kind is not hard. Roughly, you start from some idea of where the pH is going to end up. For example, if you are only adding these phosphates, count the number of ...
Rolfe Petschek's user avatar
10 votes
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pH of aqueous solution of HCl of low concentration

It is known that the equilibrium constant for the following reaction is $10^{-14}$. $$\ce{2H2O(l) <=> H3O+(aq) + OH-(aq)}$$ That means, $$\frac {[\ce{H3O+(aq)}][\ce{OH-(aq)}]} {[\ce{H2O(l)}][\...
DHMO's user avatar
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Calculation of the pH of a mixture of a strong acid and weak acid

Chemical thermodynamics is easy in that it always has enough equations to derive all the variables involved, and guarantees to produce a unique solution. The derivation itself may be not that easy, ...
Ivan Neretin's user avatar
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The effect of NaCl on the ability of an acid to protonate benzoate

Some salts added obviously will alter the pH value of the soda ($\ce{NaHCO3}$, for example), and others not (like the example provided by you, $\ce{NaCl}$). Changing the ionic strength by addition of ...
Buttonwood's user avatar
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