15 votes
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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 ...
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  • 33k
14 votes
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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, ...
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12 votes
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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 ...
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11 votes
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Amount of Phenolphthalein required

Using a little extra phenolphthalein won't matter. You just need enough so that the color is visible once the pH becomes basic enough. If you were titrating a large volume, you might need more than a ...
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  • 10.1k
11 votes

Why must NaOH be kept dry during storage?

Without a proper closed container, sodium hydroxide readily attracts water and become a viscous solution (concentrated sodium hydroxide). This solution readily attracts carbon dioxide in air and ...
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  • 511
11 votes
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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 ...
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10 votes
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Why is a theoretical titration curve higher than a practical one?

One thing that that has already been mentioned in the comments is the impact of $\ce{CO2}$ on the equilibrium. If you look at the following figure you can see the pH diagram of carbonic acid. The ...
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10 votes
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How to analyze a acid and salt mixture using titrimetry?

Prologue The following will show you not only how to sketch a titration curve but how to produce an analytical form of a titration curve. So this might not be an easy solution even if I try to keep ...
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9 votes
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How do I calculate the concentration of sulphuric acid by a titration experiment with sodium hydroxide?

Molarity of diluted $\ce {H2SO4}$ (solution 2): $\pu{0.0013798 mol}/\pu{0.025 L} = \pu{0.054172 M}$ (I may be using the wrong volume, is it possible that I have to add the $\pu{25 ml}$ to the $\pu{...
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9 votes
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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 (...
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9 votes
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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: $$...
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  • 4,026
9 votes
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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 $\ce{[H+]}$ ...
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8 votes

Calculating Buffer Capacity

In addition to Martin's answer, there is at least an old recommendation by E. B. Sandell and T. S. West in Pure Appl. Chem., 1969, 18, 427-436 (DOI), which states: Buffer capacity or buffer index. ...
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8 votes

Is the Henderson–Hasselbalch equation volume independent?

For a much shorter and clearer explanation of this problem, look here. The Henderson-Hasselbalch (HH) equation is not to blame here. It is an approximate equation, with a certain region of validity. ...
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8 votes
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How to get a precise reading from pipette?

The bottom of the meniscus (in the middle) is your measurement point and should lie on the line that you are trying to read. This is completely dependent on the type of pipette that you are using. ...
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8 votes
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NaOH Normality Slowly Changing

I would be more worried about carbon dioxide (rather than water) being absorbed into the NaOH solution. A 1 molar NaOH solution is not going to be appreciably hygroscopic. But it will absorb CO2: two ...
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  • 9,036
8 votes
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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 ...
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  • 18.9k
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 ...
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  • 4,440
7 votes
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Find the initial volume of the weak acid given a titration graph

Outline There are three main points of interest in the titration graphic, and two you have found yourself. The three points are The start at $V_\text{titr}=0$ Halfway to the equivalence point at $V_\...
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7 votes
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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(...
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  • 1,869
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 ...
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  • 21.6k
7 votes
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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 ...
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  • 21.7k
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 ...
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6 votes
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What does 0.12 N mean on a bottle of sulfuric acid?

0.12N is indeed 0.12 normal. In short, there are 0.12 moles of hydrogen ions per liter. Since this is sulfuric acid, it is 0.06 moles of sulfuric acid per liter. 0.12 moles of hydrogen ions per liter ...
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  • 7,940
6 votes

No indicator is suitable when titrating weak acid with weak alkali

An indicator works when the solution's pH is in its range. It's not necessarily the equivalence point. The point is called the end point where it actually changes color. If the difference in volume ...
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  • 1,760
6 votes

pH Curves for Titrations

I'll try to answer your question firstly qualitatively, then quantitatively (using mathematical equation): Let's consider the case of the titration of strong acid (volume $V_a$ and concentration$C_a$...
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  • 8,573
6 votes
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How to calculate the concentration of conjugate acid from given pH and pKa values?

To answer your question about titrating without adding volume—you don’t. This problem is strange in many ways, but I will attempt to help you to understand. Since we are working with dissolved $\ce{B}...
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  • 23.4k
6 votes

What is the meaning of cmmol in chemistry?

Upon researching this, I found that this a measurement of charge: $$\mathrm{cmol=10~\mathrm{meq}}$$ and $$1~\mathrm{eq}= \mathrm{F}$$ where $\mathrm{F}$ is Faraday's constant and is: $$\mathrm{F}=...
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