# Tag Info

6

The Kjeldahl method has two versions, using $\ce{HCl}$ as trapping solution or a weak acid like boric acid. Depending which one is used, the nitrogen content is either determined by a back titration with $\ce{NaOH}$ (your case) or a direct titration with $\ce{HCl}$. The goal of the titration is to figure out how much ammonia (gas) has been trapped in the ...

2

There can be plenty. There is no definite list. I would suggest to look up the following terms in Skoog or Daniel Harris' Analytical Chemistry. Worth a trip to the library now :-) The following concepts give rise to systematic errors (i) buoyancy error (ii) temperature differences (iii) electrostatic charge build up on the sample (v) moisture (iv) above ...

2

I would imagine that you could just use one of the spectrophotometers you have available to determine the molar absorptivity constant experimentally. After all, the molar absortivity constant is simply the constant which equates the terms (those being Absorbance and Concentration) which are already proportional. So, using something like the beer-lambert law.....

2

A spike recovery value of >100% does not mean that you added too much or the recovery process is too good. All it means is that the analytical method which is being followed "in letter and spirit" gives a positive error. There may be a bias in the method or the precision of the method is poor. We cannot say just by looking at one number. At this stage it is ...

2

The link above which I used for reference mentions that for the test for sulfate anions, hydrochloric acid (HCl) is used to ensure that there is no presence of carbonate ions. For the test for halide ions (F−, Cl−, Br−, I−, etc), nitric acid (HNO3) is used instead, for the same purpose. Good question, but this is not a universal practice. In reality,...

2

The logic is the same used for a monoprotic weak acid, considering the two ionization steps, as indicated by @MaxW. For a diprotic weak acid we have: The first ionization \begin{align*} \ce{H2A + H2O &<=> H3O+ + HA-} &K_{\ce{a}1}=\frac{\ce{[H3O+][HA-]}}{\ce{[H2A]}} \end{align*} and the second ionization \begin{align*} \ce{HA- + H2O &&...

11

It might contain $\ce{NaNO3}$ as well as $\ce{KNO3}$. This paper by Foong et. al. cites some thermal measurements, such as melting point and heat capacity, that you could make with a thermometer and heater. Note that there are inflections in the heat capacity curve as the different components ($\ce{NaNO3}$, $\ce{KNO3}$ and the eutectic of the two) melt. If ...

3

I see two possible answers. Not (A) and (B) but rather (A) and (D). Chakravarthy Kalyan is responsible for reminding us that (D) can be an answer too (refer to comments). For option (D): $\ce{HC ≡ CH + 2AgNO3 + 2NH4OH→ AgC ≡ CAg↓ + 2NH4NO3 + 2H2O}$ The greyish precipitate of silver acetylide will readily form when ethyne is passed through freshly ...

0

Very simple answer: IR radiation (in very first approximation) interacts with the individual bond. While methane is unpolar, the bonds in it are polar. Not terribly polar, but still.

1

The key point is estimation of standard deviation of estimation of measurement standard deviation on Stats SE. The exact formula is complicated with extensive Gamma function involvement. The essential info is the relative uncertainty of $s$ decreases very slowly with number of measurements. It can be approximated by involving Stirling approximation for ...

0

Your instinct about not rounding early is good. Before I get to that, I wanted to address one thing in another answer: "> I learned that the absolute and relative error have only 1 significant figure That really isn't true. In your example you showed 5.34532g ± 0.001428g which could result from some kind of experiment. The point being that the digits 1428 ...

1

These parameters arise from non-dimensionalization of the Mathieu equations for ion motion in exogenous, periodically varying electric fields (i.e. oscillating potentials). You can't calculate $m/z$ directly from $a$ and $q$. For a fixed $U$ and $V$, you can regard $a$ and $q$ as non-dimensional parameters that correspond to $m/z$. (Conversely, at fixed $... 2 I found one study that examined cerium(III) polyoxometalate salts and concluded that at least under the conditions of their study,$\ce{Ce(III)}$did not oxidize under electrospray conditions. The study is Bray et al. 2011 in the International Journal of Mass Spectrometry. They say: The presence of the redox active$\ce{Ce}\$ metal ion introduces ...

0

I'm looking for some way to calculate how much base I will need to add to an unbuffered solvent consisting of a mixture of ethanol and water in order to basify it to a specific reading (10) on an a pH probe calibrated with an aqueous standard. I think you have summarized the main issue yourself. The reading of a pH probe in a basic alcoholic solution is ...

6

If your supervisor is unconvinced by Brand's guidance on this (as supplied by Loong in his answer), you could always test the effect of ultrasonic cleaning directly: Take, say, six volumetric flasks, and weigh the amount of water each holds when filled to the line. Then subject three to several cycles of ultrasonic cleaning (the other three can be left to ...

21

According to technical information for volumetric measurement provided by Brand, it is acceptable to use an ultrasonic bath. Both glass and plastic labware may be cleaned in an ultrasonic bath. However, direct contact with the sonic membranes must be avoided. Nevertheless, For gentle treatment of labware, clean immediately after use – at low ...

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