strength of battery acid for Potassium Dichromate bleach

[rbrigham]

Hi

I am looking to make some Potassium Dichromate bleach for reversal of B/W
I am guessing that what I have managed to buy is about 38% as that is normal for battery acid
How can I work out the dilution I have ?

thanks

robin
London

 

[ME Super]

What does your recipe ask for? If it's asking for 100% sulfuric acid and all you have is a 38% solution, use about 2.63 times as much of your 38% solution, keeping in mind that you'll reduce the amount of water you add. So if your recipe calls for 100 mL of 100% sulfuric acid and 900 mL water to make 1L of part B of a reversal bleach, you'd use 263 mL of 38% sulfuric acid and 737 mL water to make 1L of part B. The math is simple: Cr/Cp * amt recipe calls for = amt. you should use, where Cr is the concentration the recipe calls for, Cp is the concentration you possess. (in my example above, Cr=100, Cp=38, recipe calls for 100 mL of Cr, so use 100/38 * 100, or 263 mL of Cp.

Does this help?

 

[jochen]

Hello,
the concentration of sulfuric acid is commonly given in weight-percent. This means 100 grams of 38 % H2SO4 contain 38 grams pure sulfuric acid.
Concentrated sulfuric acid has about 96 to 98 % H2SO4 and a density (specific gravity) of about 1,836 g/cm³. 38 % sulfuric acid has a density of about 1,188 g/cm³.
You have to calculate at first from, the original formula the weight you need. If you need 5 ml conc. sulfuric acid you have 5 x 1,836 = 9,18 g (the difference between 100 % and 96 - 98 % is here neglected).
To get these 9,18 g sulfuric acid from 38 % H2SO4, you have to take 100/38 x 9,18 = 2,63 x 9,18 = 24,16 ml of your battery acid.
Please be careful with sulfuric acid and with the dichromate, it is carcinogenic.
To dispose of the used bleach, you can mix it in the open air very carefully in small portions with used developer, the chromium (VI) is immediately reduced to nearly non poisonous green chromium (III) by the sulfite in the developer. Do it in open air as sulfur dioxide is formed.

 

[rbrigham]

thanks for the replays guys

i shall endeavour to weigh the stuff i have to work out the strength

robin

 

[jorj]

What a coincidence - a friend just asked me to dig up the same recipe, from when she and I had processed some reversal 16mm in a morse tank a few years back. I didn't have any sulfuric acid in the house at the time so we ran down to the automotive store.

700mL distilled water
9.5g potassium dichromate
32mL of 37% sulfuric acid
make up to 1L

We cleared it in a bath of 5% w/v sodium sulfite.

-- Jorj

 

[johnielvis]

you can also use sodium bisulfate instead of getting liquid acid. The granulated bisulfate is readily available over the counter at swimming pool stores and is much safer than the liquid as you add more to make a stronger solution instead of diluting a very strong solution--which you must also store in liquid form. Look for products that lower the pH "pH Lower" or similar. Very inexpensive also.

A point of clarity: dichromate has not proven to be carcenogenic but is only a suspected carcenogen. Anyways, just keep the stuff from going in your nose or eyes and you'll be fine is the main thing--just have water/sink nearby to wash any away that gets on you.

 

[jorj]

Uum, unless I'm missing something, you're mistaken; the MSDS shows that potassium dichromate is an A1 confirmed carcinogen in humans.

A1: The agent is carcinogenic to humans based on the weight of evidence from epidemiological studies of, or convincing clinical evidence in, exposed humans.

That's specifically not A2 ("suspected human carcinogen") or A3 ("animal carcinogen") - but A1, which is classified as "confirmed human carcinogen."

 

[johnielvis]

well whaddya know? it must have recently been "upgraded". the hazard sheet for this dichromate bleach concentrate I have says "suspected". Time to update the sheets!

My point was that you shouldn't be AFRAID of the stuff. If you have a fear, then you'll be more likely to make mistakes where you get hurt--shaking hands/spills, that kind of thing--all from being nervous. Just treat it with respect and you'll be fine--it is not to be feared. If you accidently get some on you, you're not going to get cancer. Just simply wash it away. The big hazard with this stuff is the oxidizing power--If you leave it on, you'll definitely get burned. If you get some inhaled, you'll feel it for sure. The same thing goes for the "safer" permanganate powder--it's every bit as nasty as the dichromate if it's mishandled.