15 votes
Accepted

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
Accepted

Why isn't sodium hydrogen phthalate used instead of KHP?

For classical analytical problems you have to check at least 100 year old literature. It is easy in Google Scholar. Set the date limits to 1920s or 1950s whatever you wish to try. In older times, the ...
AChem's user avatar
  • 39.8k
12 votes
Accepted

Why is Borax used as a primary standard?

Yes borax is a hydrate, but the reason it is favored for titration is that it gives consistent results, which to most scientists, is far more important than accuracy. The $\ce{ .\!10H2O}$ hydration ...
A.K.'s user avatar
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11 votes
Accepted

Why does phosphoric acid mask the colour of iron(III) complex in water?

When phosphoric acid is added to an aqueous solution of iron (III), the ligands in the yellow complexes $\ce{[Fe(OH)(H2O)5]^2+}$ and $\ce{[Fe(OH)2(H2O)4]+}$ ( $\ce{[Fe(H2O)6]^3+}$ are weakly colored ...
Jannis Andreska's user avatar
11 votes

Best regression model for a sigmoidal pattern

It looks like your model is not complete. The current shape of your model makes the function go to zero for small values of $x$. There is nothing to indicate that the function should go to zero, ...
AccidentalTaylorExpansion's user avatar
10 votes

Best regression model for a sigmoidal pattern

Some of the examples on the page EdV's comment points to are programmatic (as in: knowledge of a scripting/programming language is beneficial/a requirement) in addition to statistics. If you are more ...
Buttonwood's user avatar
9 votes
Accepted

Use of pipette during experiments for determination of oxygen by Winkler's method

[...] the pipette tip should be well below the surface of the solution [...] Usually, one would indeed keep the pipette above the solution to avoid that a (minor) part of the solution forms a drop (...
Klaus-Dieter Warzecha's user avatar
9 votes
Accepted

How is the the law of chemical equivalence defined and what are its limitations?

Basically the law of equivalence wants you to balance the equivalents that are involved in your reaction. I'd like you to view you reactions in two parts, the oxidation and reduction halves. I see: $$...
Berry Holmes's user avatar
  • 4,124
9 votes
Accepted

How to (numerically) model a phosphoric acid titration curve

Much easier is to calculate the inverse function $[\ce{Na+}]=f([\ce{H+}],K_\mathrm{a1},K_\mathrm{a2},K_\mathrm{a3})$. Calculate fractions of respective phosphate forms as the function of $\ce{[H+]}$ ...
Poutnik's user avatar
  • 40.8k
8 votes
Accepted

What are the limitations of the Hendersson-Hasselbalch equation?

The key approximation made in deriving the Henderson-Hasselbalch equation is that the equilibrium constant can be written as $$K=\frac{c_{H^+}c_{A^-}}{c_{HA}}$$ that is, we assume activity ...
Buck Thorn's user avatar
  • 21.4k
8 votes

Why does the pH before the equivalence point of a titration depend on the initial concentration only when the acid is strong?

To understand what is happening with the two sets of titration curves, start by considering the titration of acetic acid ("$\ce{HOAc}$" for short) with sodium hydroxide solution. The ...
Ed V's user avatar
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8 votes

Best regression model for a sigmoidal pattern

I am answering to the comment that you made to the @Buttonwood's great answer. Clearly the $y$ values are the $\text{pH}$ of the solution and the $x$ values the volume of base added. If you don't mind,...
Metal Storm's user avatar
  • 2,751
7 votes

Shouldn't the pH at the equivalence point always be 7?

Let's consider your example where you are titrating acetic acid with sodium hydroxide. You are correct in saying that when equal amount of substance of acid and base is added, the acid will be ...
Nanoputian's user avatar
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7 votes
Accepted

How does an acid prevents or discourges oxidation of iron?

This is largely due to the extreme insolubility of iron (III) hydroxide, i.e. $\ce{Fe(OH)3}$. One can write the oxidation reaction, somewhat simplified, as: $$\ce{Fe^2+(aq) + 6 H2O(l) <=> Fe(...
Gert's user avatar
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7 votes

Mathematical explanation of blank titration in Mohr method for argentometry

$\ce{AgCl}$ ppts before $\ce{Ag2CrO4}$. But in order to visually detect the $\ce{Ag2CrO4}$ you must add a slight excess of $\ce{Ag^+}$. Hence you use a "blank" with no $\ce{Cl^-}$ to account for the ...
MaxW's user avatar
  • 22.3k
7 votes

Why are synthetic pH indicators used over natural indicators?

The main issue with natural indicators is their purity, long term chemical stability and shelf-life. Note that some natural indicators are pretty good for example the famous litmus paper is obtained ...
AChem's user avatar
  • 39.8k
7 votes
Accepted

Do both iodine and potassium iodide turn dark in the presence of starch?

Starch is a long ribbon or a long filament made of a great number of glucose units attached to one another like the wagons in a train. But this filament is wound in a helicoidal way. And the inner ...
Maurice's user avatar
  • 28k
7 votes

Procedure for measuring EC

I am conducting a school-based research project into the antioxidative properties of green tea. Part of the procedure is to determine the concentration of antioxidant in green tea. This antioxidant is ...
Mathew Mahindaratne's user avatar
6 votes
Accepted

Phosphoric Acid Titration

You might want to read about the pH of a neutralized solution. But in case you don't, one of my answers here on chem.se covers the calculation of titration curves with and without increasing volume. ...
pH13 - Yet another Philipp's user avatar
6 votes

Does titration usually always involve neutralization?

While neutralization of an acid (or a base) is a frequent example of titration, titration is more general. IUPAC's gold book mentions "Also contains definitions of: acidimetric titration, acid–base ...
Buttonwood's user avatar
6 votes
Accepted

EDTA Titrations

I posted the general solution for the degree of dissociation here: $$\ce{\alpha(H_{n-m}A^{m-})} = \frac{x^{n-m} \prod_{j=1}^{m}k_{j}}{\sum_{i=0}^{n}x^{n-i} \prod_{j=1}^i k_{j}}$$ with $x = [\ce{H+}]$, ...
pH13 - Yet another Philipp's user avatar
6 votes

How to quantify caffeine content in coffee?

According to a paper published named: "Chemical Test for Caffeine" for the "Hong Kong Chemistry Olympiad for Secondary Schools (2013-14)" (Hyperlink provided below). You can titrate caffeine, with the ...
Anny's user avatar
  • 111
6 votes
Accepted

How did early chemists measure concentrations and purity?

Not everything comes as a solution. Rather, the historical chemist would have started of with solid substances. These were purified according to ‘established procedures’ (distillation, ...
Jan's user avatar
  • 67.7k
6 votes
Accepted

How much additional hydrochloric acid should be added to arrive at ‘excess acid’?

The volume you've established seems correct, as well as your assumption that extra $\pu{40 mL}$ of $\ce{HCl}$ would be a good excess: $$\ce{NaHCO3 + HCl -> NaCl + H2O + CO2}$$ $$V(\ce{HCl}) = \...
andselisk's user avatar
  • 37.5k
6 votes
Accepted

Percent Dissociated from Titration Curve

I'm not sure I follow your logic. For a monobasic acid S $(\ce{HA})$ dissociation degree $α$ is $$α = \frac{[\ce{H+}]}{c_\mathrm{a}},$$ where $c_\mathrm{a}$ is the initial concentration of the acid ...
andselisk's user avatar
  • 37.5k
6 votes
Accepted

How to derive the conductivity titration curve which accounts for salt formation

I believe have found a answer. It's valid mathematically, make sense physically but I don't know if chemically is true. I posted to community appreciation. There we go! The reactions ...
grsousajunior's user avatar
6 votes

Is pK_{In} = pH at equivalence point still true for weak basic organic indicators?

It is true that acid-base indicators ($\mathrm{pH}$-indicators) are either weak acid or weak base. What I understood reading your question is that you have wrong impression about bases. To clear your ...
Mathew Mahindaratne's user avatar
6 votes

Conductometric titration curve plotting

Expressed in $\pu{\Omega ^{-1} cm^2 mol^{-1}}$, the ionic conductance of usual ions like $\ce{Na+, K+, Ca^{2+}, Cl-, SO4^{2-}}$ are between $50$ and $80$. But the two ions $\ce{H+}$ and $\ce{OH-}$ ...
Maurice's user avatar
  • 28k

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