specific gravity hydrostatically, density determination kit

What is specific gravity?   Archimedes discovered that the weight of body in air minus its weight in water is equivalent to the weight of the water displaced by the body. Specific gravity is defined as "The weight of a body compared with the weight of an equal amount of pure water at 4°C (4°C is the temperature at which water is densest). When a body is placed in water, the volume of water displaced is equal to the volume of the body. When the body is placed in water that it undergoes an apparent loss of weight. This loss of weight is equal to the weight of the water displaced.

We'll substitute the word a "body" to a "gem stone" to explain specific gravity although you can really determine the specific gravity of any material.

When a gem stone is weighed in air and then weighed in water the loss of weight is equal to the weight of its volume in water displaced. The weight of the gem in air divided by the loss of weight in water gives the specific gravity of the stone.

Typically room temperature distilled water is used. It is best to use water that is boiled, because deionized water that has been sitting for a time to degas water and lessen the chance of fine air bubbles. A tiny drop of wetting agent, alcohol, or dishwashing liquid maybe added to the water to decrease surface tension of water which can adversely affect the results. One drop of detergent liquid the size of a pin head destroys surface tension in 1 liter of water.

The specific gravity of a diamond is 3.52. This mean the weight of diamond is 3.52 times that of an equal volume of water. There are a number of ways to write the equation for the specific gravity (SG). Here is the most common:

                 (Weight in Air)   x   (SG of Fluid)
SG = ---------------------------------------------
                         Loss of Weight in Water

Since the specific gravity for distilled water at 4°C is 1 the equation can be rewritten as

                         Weight in Air
SG = ---------------------------------------------
                  Loss of Weight in Water

The equation said another way still using distilled water at 4°C is

                                          Weight of Gem Stone in Air
SG = ---------------------------------------------------------------------
                     Weight of Gem Stone in Air - Weight of Gem Stone in Water

So from this equation, you need only first weigh the gem stone in air (which means the same as "weighing it"), then suspend the gem stone in water for the second weighing.

We know what you're saying - "Boy all that technical stuff has me confused, show me a real example."

Below is a Sartorius GD603 Balance with the specific gravity kit manufactured by Mineralab. Typical the specific gravity apparatus assembly slides under the balance, but the Sartorius GD603 has only two(2) adjustable feet in the front and the balance has a low profile. We used a calibration case to support the specific gravity post, but for many balances the stand is positioned under the balance. Also since the GD603 has a short Draft Shield Chamber Height measuring only 5.23 inches from pan to top of glass door we'll need to leave the top draftshield glass door open.

Your scale must have a minimum capacity of 50 grams and the weighing platform must accommodate the 3.25" diameter base of the kit's Gem Holder Support. The "tared weight" of that portion of the kit that sits on the weighing platform is about 26 grams, so this must be subtracted from your scale's listed capacity to derive available capacity for specific gravity measurements.

The specific gravity kit is installed and the balance is tared.

Sartorius with density determination kit

Below we measure the weight a slug of aluminum (the SG is 2.70) on the weighing platform of the balance.

After the balance is tared so the display shows zero the slug of aluminum is weighed on the Sartorius GD603 weighing pan. Above the display shows the weight in air for the slug of aluminum as 1.5202 grams

Now we remove the slug of aluminum from the weighing pan, zero the balance and then using tweezers place the slug of aluminum on the specific gravity kit that is submerged in water.

Above the display shows the weight in water for the slug of aluminum as 0.9558 grams

Now it is time to plug and chug and confirm that the aluminum slug is really aluminum.

                   Weight of Aluminum in Air
SG = ---------------------------------------------------------------------
                     Weight of Aluminum in Air - Weight of Aluminum in Water

                   1.5202 grams
SG = ---------------------------------------------------------------------
                     1.5202 grams - 0.9558 grams

                   1.5202 grams
SG = ---------------------------------------------------------------------
                     0.5644 grams

SG = 2.6935

The known specific gravity for Aluminum is 2.6989 (the slug of AL is marked 2.70 to round off the numbers to two decimal places). The slight difference or error of 0.0054 (2.6989 - 2.6925) could be surface tension or equipment error, but overall the results are excellent..

Maybe since we did not carefully cleaned the tweezers and slug of aluminum this attributed to the small error. Perhaps cleaning with toluene as a solvent with remove any oils from a person's finger might give us even better results. Any wax or grease on the sample must be removed. You can also use alcohol to clean surfaces.

When during specific gravity on gem stones large flaws or inclusions which might affect the accuracy of the results.

You can determine the specific gravity of a body a second method using just a balance and graduated cylinder to determine the specific gravity hydrostatically. This method is best used for larger samples.

1. Weigh specimen in grams.

2. Partly fill a graduated cylinder (cc) with water.

3. Drop in sample, note new volume.

4. Difference between volumes = volume of water displaced in grams since the weight of 1 cubic centimeter of water equals 1 gram

5. Use the following formula to determine specific gravity:

      Weight of Mineral (in grams)
SG = ------------------------------------------------
          Volume of Mineral (in cubic cm)

Why measure the specific gravity of a mineral?   Because specific gravity is one of the most important, constant, and quantifiable properties of a gem or mineral, and in your effort to identify an unknown stone, the specific gravity, if known, can narrow down your search to just a handful of possibilities. The specific gravity for each mineral is the same within narrow limits (except in species like topaz or garnet and zircon where subgroups with definite specific gravities occur).

Must my mineral specimen be pure? Yes and No. No, because you can theoretically measure the specific gravity of any solid using these devices, whether it be a chunk of plastic, a rock containing a varying amount of minerals, or a pure mineral itself in the form of a crystal. But yes, your mineral specimen must be pure if you wish to compare your measurement to standards of specific gravity for those minerals, which standards were developed using pure mineral specimens. In summary, the more pure your specimen, the more accurate will be your specific gravity measurement.

PAGES ON SPECIFIC GRAVITY KIT 1 2 3

All Acculab Products are warranted for two years from the date of purchase against defects in materials and/or workmanship. If the product fails on delivery or to operate to the specifications during this warranty period, when used under normal conditions, Acculab will repair or replace it at their option, without charge. You must obtain a Return Goods Authorization (RGA) directly from Acculab (800-656-4400) and not Precision Weighing Balances. You will be required to pay the shipping and insurance charges to Acculab's repair facility. Acculab strives for 24-48 hour turnaround for repair or replacement (of units under warranty.) Acculab will return the unit freight prepaid. Please refer to your Instruction Manual for additional warranty information and shipping instructions.

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