Refractometer bad?

[RexSJ]

I bought a refractometer a few years ago for different reasons but never used it. Today I dug it out. It is Brix 0-32. I made some sugar solutions to check it and the readings I see do not look right. Hoping someone can tell me I did something wrong or confirm the refractometer readings being bad.

First I zeroed it with water. I made three sugar solutions with supermarket cane sugar in tap water (same water used to zero). Solutions were 10%, 20% and 30% by weight. Using a triple beam balance I weighed 50 grams of water and added 5 g of sugar for 10%, then etc. for the other solutions. So I think I made these solutions accurately. (Tell me if I am wrong in my method.) A 10% sugar solution is 10 g sugar in 100 g water, isn't it? I think normal cane sugar is sucrose and good for this test.

I expected to see Brix readings of 10, 20, 30 but I measured much lower: 8.8, 16.5 and 23.0. They are not even off by a straight line percentage.

My understanding is that the Brix scale should be a direct measure of sugar % in solution by weight, and I have nothing but sugar and water in my samples.

Did I do something wrong making my solutions, or is this refractometer way off?

 

[DeafSmith]

I bought a refractometer a few years ago for different reasons but never used it. Today I dug it out. It is Brix 0-32. I made some sugar solutions to check it and the readings I see do not look right. Hoping someone can tell me I did something wrong or confirm the refractometer readings being bad.

First I zeroed it with water. I made three sugar solutions with supermarket cane sugar in tap water (same water used to zero). Solutions were 10%, 20% and 30% by weight. Using a triple beam balance I weighed 50 grams of water and added 5 g of sugar for 10%, then etc. for the other solutions. So I think I made these solutions accurately. (Tell me if I am wrong in my method.) A 10% sugar solution is 10 g sugar in 100 g water, isn't it? I think normal cane sugar is sucrose and good for this test.

I expected to see Brix readings of 10, 20, 30 but I measured much lower: 8.8, 16.5 and 23.0. They are not even off by a straight line percentage.

My understanding is that the Brix scale should be a direct measure of sugar % in solution by weight, and I have nothing but sugar and water in my samples.

Did I do something wrong making my solutions, or is this refractometer way off?

I would say that 5 grams sugar and 50 grams water is a 5/55 or 9.09% solution, 10 grams+50 grams is a 10/60 = 16.7% solution, and 15 grams+50 grams is a 15/65 = 23.1% solution. So maybe you are not so far off.

 

[jppostKW]

compare your readings to a hydrometer as well

 

[RexSJ]

I would say that 5 grams sugar and 50 grams water is a 5/55 or 9.09% solution, 10 grams+50 grams is a 10/60 = 16.7% solution, and 15 grams+50 grams is a 15/65 = 23.1% solution. So maybe you are not so far off.

I did a search to check percentage solution by weight before I made the solutions, so I think I did it right. Here's one link:
http://dl.clackamas.edu/ch105-04/weight.htm

The page is from a school, so I think I trust it, but the internet has lied before. Anyone know for sure if it is sugar/water or sugar/(sugar+water) for percent solution by weight?

The numbers vs. observed measurement do look close for the latter version.

 

[jppostKW]

yet again hydrometers and refractometers should be used together it will immediately tell you if the refrac is right

dont rely on the interwebs from the molar ratios needed to get the sugar solutions you need

 

[DeafSmith]

I did a search to check percentage solution by weight before I made the solutions, so I think I did it right. Here's one link:
http://dl.clackamas.edu/ch105-04/weight.htm

The page is from a school, so I think I trust it, but the internet has lied before. Anyone know for sure if it is sugar/water or sugar/(sugar+water) for percent solution by weight?

The numbers vs. observed measurement do look close for the latter version.

According to your link, it is weight of solute/weight of solution, where solution = "both the solute and the solvent together", so the correct formula is sugar/(sugar+water)

 

[RexSJ]

According to your link, it is weight of solute/weight of solution, where solution = "both the solute and the solvent together", so the correct formula is sugar/(sugar+water)

Arrgh! I am a poor reader, I guess. I only looked at the first formula and skipped the important details.

My Bad.

Thanks for setting me right. I'll double check with correct solutions, but it does look ok now.

 

[RexSJ]

DeafSmith, thanks again for setting me right. Here's one more message to finish it.

I carefully made 10, 20 and 30 percent sugar solutions *the correct way* and saw exactly 10, 20, and 30 Brix readings on the refractometer.

All is well.