Waters that have been discolored by dye, ink, smoke and the like are disqualified for netilla. Discoloration from mud and dust does not disqualify, firstly because it is natural, and secondly because they will ultimately settle. (ShA 1 – MB 3)
If the discolorant settles and the water clears, it is kasher. (MB 5)
Water that has been used for melocha is considered like dirty water that needs to be dumped and therefore disqualified for netilla. Examples:
Soaked bread or clothes in it.
Cooled a warm bottle of wine in it.
Rinsed with it dirty or smelly dishes or dirty vegetables.
Used to preserve vegetables from wilting. Some allow this.
Used to fill a weight.
Used to measure by displacement.
Used to locate a leak in a cup.
Rinsed the hands from dough.
Used by a blacksmith to cool the hot metal. (ShA 2,3 – MB 6,8,10,11,12,13,18)
If the water was used unintentionally, it is still okay. But if it was used on purpose even by a non-owner of the water, it could become disqualified. (MB 7,8)
Water from a fish tank is okay for netilla since it is used merely to preserve the fish’s life. (MB 9)
Water that has been wringed out from a cloth is disqualified. (MB 8)
If the water has been used to rinse clean dishes or vegetables it is still okay. If someone puts their clean hands into a container of water, the water is still kasher. (ShA 2 – MB 11)
If an animal or bird drinks from a container of water, the remainder is still kasher for netilla. If it was a dog or a pig, it is only to be used if you cannot find any other water. If it was a snake, do not use the water. (ShA 4 –MB 23)
Water left uncovered can be used. There is no worry nowadays that a snake drank from it, at least in regard to netillas yodayim. (MB23)
Disqualified water that got mixed with a majority of kasher water becomes bottol and the mixture may be used. (MB 23)
If melocha is done in a natural gathering of water, the water is kasher for netilla. Melacha disqualifies drawn water only (even if the water was poured back into the ground). (ShA 5)
If some water was drunk by a dog or a pig, the remainder is disqualified even if it was from a natural body of water. Even so, to use that water to toivel your hands would be fine. (MB 25)
Water warmed by fire is fine for netilla. If it was heated until the point that your hand recoils, there is an argument. It is best to wait a while until the water cools down. If you can’t wait, you may pour it while it’s hot. (ShA 6 – MB 27)
If the taste of the water is not drinkable for a person, it is still kasher for netilla as long as a dog would drink it. Sea water is not kasher for netilla – even a dog wouldn’t drink it. However you can use the sea for tevillas yodayim. (MB 37,38)
Since the mitzva of netillas yodayim is m’drabbonon, if you have a doubt about it and you cannot resolve it, you can consider your hands tahor. For example:
Was melocha done with the water ?
Did the water have the shiur r’viis ? (In this case don’t wash lechatchila.)
Was the k’li kasher (whole) ? (In this case don’t wash lechatchila.)
Did you already wash?
An undecided argument amongst the poskim. (ShA 11 – MB 47,50)
You should remove the doubt by washing again if you get more water before the end of the meal. (ShA 11)
Once your hands are tahor, we use chazakah and they remain so until you definitely touch filth. If you merely have a doubt if they touched filth, your hands are still tahor. (MB 53)
If your tomay hands touched water, the water is still kasher for netilla as long as you did not clean your hands in it. (Rama 11 – MB 54)
If you don’t have water, snow can be used for tevillas yodayim. The snow must be a depth to cover the hands and the volume of 40 se’ah all connected in one place. (MB 58)
If you don’t have water you can use fruit juice instead (but not wine). Consider it a sofek netilla – do not say a beracha and if you get water before finishing the meal, wash with the water. (MB 64 – ShaH 69)
Netillas yodayim requires that there be at least a reviis in the vessel before you begin. It makes no difference what size your hands are. (ShA 14,15)