How many drops are in one milliliter?

[David Lyga]

In the case of aqueous solutions, droplet size is determined by the surface tension, not the viscosity. Wetting agents (surfactants) reduce the surface tension of water. The viscosity is unchanged.

If you are correct you have done me a favor. I thought that viscosity was determined, in part, by flow. Maybe I err, but I thought that if surface tension was reduced, the viscosity would also be reduced. Apparently, I stand corrected. - David Lyga

 

[Sirius Glass]

If you read the link in post 19, you will find that the word viscosity does not appear anywhere in the description of how droplets form.

No, no! Please do not take my viscosity away!

 

[RattyMouse]

If you are correct you have done me a favor. I thought that viscosity was determined, in part, by flow. Maybe I err, but I thought that if surface tension was reduced, the viscosity would also be reduced. Apparently, I stand corrected. - David Lyga

It seems to make intuitive sense that viscosity would affect droplet size, but the literature quoted above fails to mention that. Apparently resistance to flow is entirely different as to how droplets are formed. I learned something reading the posted link.

 

[RalphLambrecht]

Metric drops or Imperial drops?

Seriously, though, I've found droppers to be too inconsistent to be an accurate way of measuring anything, personally. Small syringes are my tiny measurement devices of choice.

it depends of course on the viscosity of the liquid but, for watery liquids, 12-16 drops equal roughly 1ml.

 

[darkroommike]

Why not use a transfer pipette? You can even buy bundles of disposable ones online. I'm going to add that the drop was eliminated as a measure for pharmaceutical "recipes" a long time ago in favor of first, minims and later milliliters.

 

[DREW WILEY]

The physics is a bit complicated. How many angels can stand on the head of a pin? How many mice fill a bushel? Big mice, small mice, dried mice? Small volumes can be measured with a pipette or special automated dispensers sold by lab suppliers.

 

[RalphLambrecht]

The physics is a bit complicated. How many angels can stand on the head of a pin? How many mice fill a bushel? Big mice, small mice, dried mice? Small volumes can be measured with a pipette or special automated dispensers sold by lab suppliers.

right on

 

[RattyMouse]

it depends of course on the viscosity of the liquid but, for watery liquids, 12-16 drops equal roughly 1ml.

Actually, viscosity has nothing to do with droplet size. Read the link posted above (19 I think).

 

[Doremus Scudder]

If you read the link in post 19, you will find that the word viscosity does not appear anywhere in the description of how droplets form.

Thanks Ratty. I was under the impression that surface tension and viscosity had a relationship; evidently they never got together.

I found it interesting that the size of a drop from a given diameter tube remains (fairly) constant. That would validate my practice of using only one type of eyedropper for measuring liquid amounts in drops, after having calibrated it, of course. Changing to a different-size dropper would change the number of drops/ml. I intuitively assumed this, but never really researched it.

Really, if measuring drops from a calibrated tube-size is good enough for IV infusions, then it really should also be accurate enough for measuring the less-critical components of many photographic solutions, e.g., wetting-agent baths, small amounts of ferricyanide bleach from stock solutions, etc. We just have to calibrate our eyedroppers first to find how many drops/ml they deliver and make sure to hold them exactly vertically when dispensing our drops.

Best,

Doremus

 

[BMbikerider]

How many drops are there in a milliliter? You may as well ask how long is a piece of string!

I have never heard of such a vague form of measurement being used. It calls into question the actual accuracy needed.

 

[RattyMouse]

Thanks Ratty. I was under the impression that surface tension and viscosity had a relationship; evidently they never got together.

Surface tension is an electrical property (charge) while viscosity is a physical one (molecules tangling up). When you think about it this way, it seems more logical that they are not related.

 

[RalphLambrecht]

Thanks Ratty. I was under the impression that surface tension and viscosity had a relationship; evidently they never got together.

I found it interesting that the size of a drop from a given diameter tube remains (fairly) constant. That would validate my practice of using only one type of eyedropper for measuring liquid amounts in drops, after having calibrated it, of course. Changing to a different-size dropper would change the number of drops/ml. I intuitively assumed this, but never really researched it.

Really, if measuring drops from a calibrated tube-size is good enough for IV infusions, then it really should also be accurate enough for measuring the less-critical components of many photographic solutions, e.g., wetting-agent baths, small amounts of ferricyanide bleach from stock solutions, etc. We just have to calibrate our eyedroppers first to find how many drops/ml they deliver and make sure to hold them exactly vertically when dispensing our drops.

Best,

Doremus

ising a lab micropipette could be a solution too.

 

[RalphLambrecht]

Don that's exactly what I found to be the answer, given the variables.

I'd go with that.