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  1. #111
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    Quote Originally Posted by tdeming
    At first I thought this was meant as a joke..

    Since the density of water is, by convention, 1.0 gr/ml, it really doesnt matter, eh?

    Since it's easier to measure out water volumetrically than gravimetrically, I'd prefer to go with the volume measurement.

    Cheers

    Tim
    Ummm, that is not exactly correct as it is temperature dependant.

    Also, the density of the emulsion is not 1.000 so it is important to keep track of the density of the emulsion so that I know the Kg/mole of silver for coating purposes.

    PE

  2. #112
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    Quote Originally Posted by Silverpixels5
    Water is only 1 g/ml at 20C, so it changes with temperature, and if there is anything dissolved in the water then the density will change slightly as well. So volumetric measurement really isn't that reliable...esp for QC stuff. Will it matter? Probably not...but there are less variables for error when you do everything by mass. Mass is always conserved (unless you're doing nuclear stuff)...volumes are not.
    Well, another point here should be mentioned.

    If you are weighing out 1 g of solid and 99 g of water, this is easier and more accurate than measuring 1 g of solid and 99 ml of water. Seeing the meniscus in the graduate cylinder is not easy. Using a volumetric flask is better but expensive. Using wt/wt measurement is much much easier.

    Most gravimetric measurement is one order of magnitude better than volumetric measurement. After all, how often do you see graduate cylinders marked in 0.1 or better still 0.001 ml divisions? I can buy scales and balances that can do this. It sometimes is quite useful in emulsion making.

    So, I find it handy.

    PE

  3. #113
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    Quote Originally Posted by Photo Engineer
    Most gravimetric measurement is one order of magnitude better than volumetric measurement. After all, how often do you see graduate cylinders marked in 0.1 or better still 0.001 ml divisions? I can buy scales and balances that can do this. It sometimes is quite useful in emulsion making.

    PE
    Hi PE,

    Not to be picky, and you are more than welcome to use any technique you like, but analytical chemists will always use volumetric glassware to measure out liquids with high accuracy and precision. You can always correct density for temperature effects if you are working at high (or low) temperature. I would imagine you will get much more error in your weight measurement due to evaporation and loss of material upon transfer than you would by delivering from a buret or pipet, but then again if you are using a graduated cylinder, you're never going to be very accurate in measuring volume. Volumetric glassware is also not that expensive compared to a good analytical balance. I tend to agree with silverpixels that the errors you get probably wont make much differrence as long as your scale is large enough.

    have fun mixing.

    Tim

  4. #114
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    Quote Originally Posted by tdeming
    Hi PE,

    Not to be picky, and you are more than welcome to use any technique you like, but analytical chemists will always use volumetric glassware to measure out liquids with high accuracy and precision. You can always correct density for temperature effects if you are working at high (or low) temperature. I would imagine you will get much more error in your weight measurement due to evaporation and loss of material upon transfer than you would by delivering from a buret or pipet, but then again if you are using a graduated cylinder, you're never going to be very accurate in measuring volume. Volumetric glassware is also not that expensive compared to a good analytical balance. I tend to agree with silverpixels that the errors you get probably wont make much differrence as long as your scale is large enough.

    have fun mixing.

    Tim
    Tim, holding an emulsion at 60 deg C for one hour does lead to a lot of evaporation, but measuring the volume of an emulsion at the end is virtually impossible in the dark and with an opaque liquid. These are additional considerations when it comes to weighing the emulsion rather than measuring volume.

    I guess when you get right down to it, after 30+ years of thinking of emulsions as X kg / mole or so many Kgs or Gs of emulsion, I never think in terms of liters of emulsion on a practical basis, and I have outlined all of my current thoughts here and above.

    Again, I repeat that volumetric measurment is not as precise or as accurate as gravimetric as any analytical chemist will tell you. Yes, a burette or a volumetric flask are very precise, but only at the ml level with the burette and only on premeasured volumes with the flasks. And, when you have clear liquids measured at one temperature in the light.

    My measurments have to hold true for a solid and a liquid (the gelled emulsion and the melted emulsion over a range from 10 deg C to 60 deg C) which will not work volumetrically. Sometimes I have to dump in hot water.

    This may seem odd to you, but it is an integral part of this branch of engineering. It is the only way to achive precise measurements.

    PE

  5. #115
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    PE,

    Please dont overinterpret what I've wrote. My original post only referred to measuring out *water*, and at a single temperature. Certainly if you are going to measure out solutions/suspensions/gels, etc, and then heat and cool your samples, it would be pretty much impossible to do this volumetrically and you'll have to make do with whatever you can.

    For the record, I have taught analytical chemistry in the past, and have many colleagues who do so now. I'm pretty sure all of us would agree to measure out water volumetrically if we needed a very accurate measure, and this is what we teach our students. I have burets in my lab that will deliver 50 mL with 0.1 mL accuracy. But bear in mind, I'm a scientist, not an engineer ;-)

    Tim

  6. #116
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    Quote Originally Posted by tdeming
    PE,

    Please dont overinterpret what I've wrote. My original post only referred to measuring out *water*, and at a single temperature. Certainly if you are going to measure out solutions/suspensions/gels, etc, and then heat and cool your samples, it would be pretty much impossible to do this volumetrically and you'll have to make do with whatever you can.

    For the record, I have taught analytical chemistry in the past, and have many colleagues who do so now. I'm pretty sure all of us would agree to measure out water volumetrically if we needed a very accurate measure, and this is what we teach our students. I have burets in my lab that will deliver 50 mL with 0.1 mL accuracy. But bear in mind, I'm a scientist, not an engineer ;-)

    Tim
    I think I was not overinterpreting as much as I was overexplaining.

    It was due to the lack of knowledge on the part of a lot of people as to why things work some ways in one field and another way in another field.

    And, too many myths about photograpic science abound on the internet.

    So, I'm compulsive about putting correct or complete information out there along with reasoning behind it sometimes.

    The bottom line is that we are both right but differ due to a materials handling problem that the readers might not appreciate and so I felt it might be useful to present the overexplanation.

    PE

  7. #117

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    Quote Originally Posted by tdeming
    I have burets in my lab that will deliver 50 mL with 0.1 mL accuracy. But bear in mind, I'm a scientist, not an engineer ;-)
    Another scientist here - but I would recommend for precise measurements that gravimetric weighing of water or other solvents can be more accurate. You often can weigh the liquid in an enclosed container which will decrease the error from evaporation that one can easily see if your balance is sensitive enough.

    I use this technique to check the calibration of adjustable pipettes where I have to measure down to 0.1 mg levels in order to calibrate a 1 mL pipette - to 0.01 mg for a 10uL pipette.

    Grad cylinders and even burettes can be pretty crude compared to making measurements by weight with a good balance. Especially when it is dark...

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